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In describing the activities of the Engineers, we are carried to the front itself, into the zone beaten by enemy fire, where machine-gun bullet, bursting shell, and deadly gases have brought sudden death and painful wounds to many members of the technical services. A large proportion of the Engineers are combatant troops, constituting in the American Expeditionary Forces about 8 per cent of the total combatant troops engaged. These troops, trained and equipped to march and fight as Infantry, demonstrated their fighting qualities during the war on numerous occasions, both when used as Infantry to increase the rifle strength of that arm and when fighting as Engineers to obtain possession of terrain as a preliminary to the exercise of their technical art in its organization.

From the day the first sector was taken over by American troops in November, 1917, until the Meuse River was passed and the enemy, in flight, sought an armistice to save his armies from destruction, the combatant Engineers—the "sappeurs" of French soldier lore and song—- fought and bled in a manner never to be forgotten. Railroad engineers, nominally considered noncombatant, at Cambrai dropped their tools to take arms and stand stubbornly shoulder to shoulder with their British brothers with whom they were learning to work under the special conditions of the front. From Cantigny to Chateau Thierry, Engineer troops fought as well as worked, and often not only advanced with the Infantry under or through the barrage, but actually led the first wave, to demolish or remove the obstacles placed in its path. Through the days when from March 21, 1918, until July 18, 1918, the German army made its rapid plunges toward Paris until checked and thrown back across the Marne at Chateau Thierry, the sapper troops fought and worked with the Infantry of their divisions, enduring the same dangers, privations, and hardships, and winning equal honors and commendation.

In the drive at St. Mihiel and through the Argonne, the combatant Engineers played a conspicuous part. Advancing with the tanks, they made possible the passage of many difficult points for these lumbering monsters, against which was directed a particularly destructive fire. Using elongated torpedoes of high explosive, known as Bangalore torpedoes, they prepared passages for the Infantry through the broad barbed-wire entanglements, echeloned in depth by numerous separate lines, each to be breached and passed before the objective could be gained. In this work the Engineers reduced the machine-gun nests that hindered their operation, cleaned up the strong points that delayed the advance of the tanks they were assisting, and threw extemporized footbridges across the streams which barred the further advance of the Infantry.

The combatant Engineers did their part in the winning of the reconquered ground as well as the lion's share of its organization for the defense and the maintenance of the communications behind it. In this last respect alone, the Engineers, as combatant troops, opened across No Man's Land the first communications practicable for the light field artillery, which pressed forward immediately behind the Infantry troops to their support and protection.

Filling in trenches, removing wire entanglements, building trestles across wide mine craters, searching for and rendering inoperative treacherous mines and traps of extreme ingenuity and destructiveness, the sapper found a wide field for the exercise of his functions. Shattered and obliterated by four years of shelling and mining, trenching, and countermining, the "roads" across No Man's Land existed only on the map; and as they retreated the Germans demolished and obstructed the highways behind the old front from which they had been driven, with the thoroughness and attention to detail for which they are noted. As our Infantry advanced, upon their heels, literally speaking, came our Engineers, to attack the problem of providing for the Artillery and supply trains a means of following. From the standpoint of the road builder in civil life, their methods were crude in the extreme, but for the military purpose and the pressing immediate needs, their road building achievement was adequate. The Engineers sometimes reopened abandoned quarries, and sometimes started them where none had existed before, to obtain a supply of road metal, which supply was sometimes supplemented and in some cases replaced by the use of debris from ruined villages and shattered farmhouses. From demolished structures many useful materials were extracted and adapted to the military purpose by the Engineers. Where bridge and trestle timbers were lacking, deserted buildings—in one case the tower of a ruined church—filled the need. Where shell hole or crater yawned a remnant of a stable wall might be pulled down by ropes and man power, and broken up to fill the void.

Through the dense woods the soft forest floor offered no support even to the light artillery, and miles of corduroy and brush path were built to permit the guns to advance to the reinforcement of the attack. In many places the tactical situation admitted of insufficient time to build even the crudest paths, and then the Engineers fell to and assisted artillery and supply wagons to get through and over the bad spots, replacing guns on the road where they had run into the ditch, righting and reloading combat wagons when they had turned over in shell holes or deep ruts.

While thus engaged the sapper troops were subjected to the fire of enemy artillery seeking to prevent the advance of the supporting guns, and, further, they were working within the zone of combat of enemy aviators, the rattle of whose deadly machine guns, as they plunged at low altitude toward a busy working party, was as much to be dreaded as the high-explosive bombs which they dropped.

Behind the combatant Engineer troops, extending through the service of supply to the base ports and across the ocean to the United States was an organization of technical noncombatant supply and administration.

The work of these production, construction, and supply departments of the Engineer service in France was organized in the American Expeditionary Forces under the administration of three divisions of the office of the Chief Engineer. These were the division of military engineering and engineering supplies, the division of construction and forestry, and the division of light railways and roads.

ENGINEERING SUPPLIES.

The division of military engineering and engineering supplies was charged with the procurement, standardization, and distribution of all classes of supplies used by Engineer troops. During the 19 months of warfare this division handled 3,225,121 tons of supplies, storing them and distributing them from immense depots aggregating 25 acres of covered storage and over 756 acres of open storage. This service was further charged with the current investigations into new developments of the art of military engineering, and with the development, operation, and administration of certain technical branches of the American Expeditionary Forces, such as electrical and mechanical troops, water-supply troops, searchlight regiments, etc.

At its seven storage depots in the base, intermediate, and advance sections, this division had in service 23 locomotive cranes, mostly of 15 tons capacity and capable of handling an enormous amount of freight and material at warehouses and cars. The following table of principal items of engineer material shows the kinds and quantities of supplies which were received in France for issue through this division up to December 15, 1918:

Table of engineer supplies received in France.

FROM THE UNITED STATES.
Item.	Tonnage.
General machinery	45,454
Iron and steel products	242,226
Hardware and hand tools	26,780
Railway rolling stock	343,888
Railway motive power	144,066
Lumber	39,086
Track materials and fastenings	488,793
Automotive transportation, etc.	22,127
Horse-drawn transportation	7,967
Building materials and supplies	98,671
Liquids	7,067
Explosives and accessories	952
Unit accountability	7
Engineer supplies	52,106
Miscellaneous office supplies	2,239
Floating equipment and accessories	10,093
Material and tools for locomotive and car repair and erection shops	10,407
Total United States	1,541,929
FROM EUROPEAN SOURCES.
Tools and equipment	462,027
Machinery	13,146
Office supplies and equipment	1,781
Auto and track supplies	464
Track and ties	115,438
Locomotives and cars	8,649
Water supply, machinery	6,210
Water supply, supplies	48,416
Electric service, machinery	2,342
Electric service, supplies	4,083
Construction materials	188,830
Boats and barges	5,940
Motorcycles and bicycles	12
General engineer supplies	581,149
Cement from American Expeditionary Forces mills	54,860
Barracks	263,590
Bunks	9,892
Latrines	2,143
Miscellaneous	49,200
Total European sources	1,818,172
Total United States and European sources	3,360,101

To facilitate the procurement of supplies in the existing world markets, this division established in Paris a purchasing board, having branches in England, Switzerland, and Spain. When the war ended this board had accomplished the tremendous task of buying over 1,800,000 tons of engineer supplies, with a total value of $205,242,728. In addition to this material, our own country furnished over 1,500,000 tons, with a value of $248,993,322. France sold to us through this board 1,234,968 tons, valued at $134,393,870, and England 396,000 tons, valued at $56,145,818. In Switzerland, purchases consisting principally of sectional barracks and technical equipment, totaled 96,867 tons, with a value of $14,643,410. Purchases from Spain amounted to only 797 tons, with a value of $59,630.

Much work was done in standardizing supplies of all classes, so that quantity-production methods could be used in their fabrication, thus promoting economy and stimulating the rapidity of supply.

In the procurement of cement for the use of the American Expeditionary Forces, the Engineers dealt successfully with a problem of large magnitude and importance. By contract with English and French mills, by direct purchase for specific jobs from local mills, and by their own manufacturing operations, the Engineers secured enough cement to supply the demands for construction both at the front and in the S. O. S., as the service of supply was generally known. Three large cement mills were leased from the French owners and operated by special troops organized in the United States. To certain other French mills the Engineers furnished labor and materials in return for a certain proportion of their output. It is estimated that about 215,000 tons of cement were thus procured, representing a total cost of about $7,000,000.

The Engineers operated shops at various points near the front in which were manufactured standard material for dugout, trench, and emplacement construction, such as concrete beams, concrete slabs for overhead protection against high-angle shell fire, trench frames, revetment material, trench duck boards, mine and gallery timbers, knockdown bunk sets, etc.

CONSTRUCTION AND FORESTRY.

The division of construction and forestry was charged with all construction work in the service of supply, and also with the procuring of forest products for the American Expeditionary Forces. At the signing of the armistice its organization totaled 150,823 men, of whom about 127,000 were constantly engaged in production work. Using standardized building plans, this force performed a huge amount of construction work in France.

It was assumed that one-third of the American troops in France would have to be housed in new buildings erected specially for the purpose. Thus accommodations for about 750,000 men had to be built at the rate of 16 barracks, each 20 by 100 feet in size, for every 1,000 men. Contracts were let to British and French contractors for 23,000 demountable barracks, this order being based on the ultimate probable size of the Expeditionary Forces. During August, September, and October, 1918, these barracks were being received at the rate of 1,000 per month. To supplement a supply of even such magnitude, our own type of barrack was developed to be built with lumber furnished by the American forestry forces in France. One cantonment project involved the construction of 500 barracks, accommodating 55,000 men. A total of 11,862 barracks were erected for the American Expeditionary Forces in France, representing 225 miles of length, if all the barracks were placed singly end to end.

It was the policy of the American Expeditionary Forces to provide hospital room sufficient to give beds, if necessary, to 15 of every 100 American soldiers in France. On this basis the Engineers set out to provide hospitals with a total of 280,000 beds. Of these, 139,000 beds were in hospitals taken over from the French, 25,000 beds being added to this capacity by new construction. In entirely new base, camp, evacuation, and convalescent hospitals, 116,000 beds were ultimately made available for the casualties of the American Expeditionary Forces, requiring the erection of 7,700 hospital barracks of special type, all of which would have totaled 127 miles in length if placed singly end to end. As to the progress of this construction, on November 14, 1918, there were 190,356 beds occupied in American hospitals in France, but all 280,000 beds originally specified were ready and available.

The base hospital plants were complete municipalities in themselves, and had capacities varying from 1,000 to 6,000 beds. These units were built where nothing had existed before but little French rural communities, devoid of the improvements and modern conveniences with which we in this country are so familiar. To establish a modern military hospital, capable of caring for the varied casualties and illness arising from action and abnormal living conditions, it required the construction of roads, sidings, unloading platforms, sorting and classification buildings, operating rooms, surgical and medical wards, dormitories, morgues, cemeteries, complete water supplies, fire protection systems, sewage and garbage disposal plants, recreation buildings, electric light plants, and all that goes to make complete a modern installation for the care of the wounded and sick. Many of the camp and evacuation hospitals required construction of the same character, but differing in magnitude.

The Engineers developed port faculties at St. Nazaire, Bordeaux, La Pallice, Marseilles, Brest, and at less important harbors. In general, at these places the existing facilities were expanded to meet the needs for the debarkation of troops and the unloading and shipment of supplies. Originally 23 ship berths were placed at our disposal by the French. The Engineers expanded this equipment to a total of 89 berths, with authorized projects for 160 berths by June, 1919. Our overseas shipments grew from 20,000 tons in July, 1917, to 1,000,000 tons in October, 1918, but the port expansion kept abreast of this development. Fifty-eight 300-ton lighters were built by Engineer troops with French timber, and twenty-six 500-ton lighters with American timber. The Engineers constructed seven derrick barges with lifting capacities ranging from 30 to 100 tons.

The existing French railroads running from the base ports to the advanced zone were quite inadequate, so that it was necessary to supplement their facilities with many miles of new track and other construction, including important storage, classification, arrival, and departure yards, warehouse tracks, engine terminals, water points, and repair shops. At Bassens, St. Sulpice, Miramas, and Montoir, enormous storage depots were constructed to handle the supplies entering France for our forces. The American-built railroad yards at these points were comparable in magnitude and completeness to the important yard developments undertaken in this country in recent years by the large railroad systems, the yards at St. Sulpice having a trackage totaling 147 miles of single track. Those at Bassens and St. Sulpice were virtually completed during the war, while the construction at Miramas was well under way at the signing of the armistice. At St. Sulpice the project was designed on the basis of receiving, storing, and forwarding the supplies for 1,000,000 men for 30 days. The others were of like magnitude.

At Nevers, in the intermediate section, a condition existed requiring the construction of six miles of new double-track line, with a bridge over the Loire River 2,190 feet long. This piece of construction is known as the "Nevers Cut Off." It relieved the railroad congestion at this important point.

At Is-sur-Tille, in the advance section, was built a regulating station at which train loads of supplies and troops were dispatched to points where needed. Still farther toward the front, at Liffol-le-Grand, was another and smaller regulating station, controlling troop movements and the distribution of munitions and subsistence. Both of these projects were entirely new and were in useful operation when the war terminated.

In addition to the above projects, many storage yards, hospital tracks, ordnance depot yards, aviation center tracks, and construction tracks were laid out and built. In all 937 miles of single track were laid, thus fulfilling in the equivalent the prediction that to supply an American Army at the front we should have to build a double-track railroad from the French coast to the trenches.

Storage depots, remount depots, and veterinary hospitals erected by the Engineers proved entirely adequate for the needs of the American Expeditionary Forces at all times. A grand total of 536 acres of covered storage was built or acquired, of which about 482 acres was new construction. Space was provided in remount depots for 29,000 animals, and it was projected to accommodate 48,700 animals had it been necessary. Veterinary hospital space was provided for 17,250 sick animals. Each veterinary hospital required much special construction, such as concrete dipping tanks for the treatment of mange, operating rooms, exercising paddocks, hay sheds, living quarters for attendants and veterinary surgeons, and administration buildings.

At Gievres, in connection with the important storage depot built there, was constructed the third largest refrigerating plant in the world. This plant, built by the Engineers from plans prepared by experts, was capable of caring for 5,200 tons of meat at once, and of producing 250 tons of ice per day. Another similar plant at Bassens had a capacity for 4,000 tons of meat.

Miscellaneous construction work in France covered many fields of activity. The question of adequate water supply was ever present, and in most places where hospitals, depots, shops, or warehousing plants were built, a water supply development was incidentally necessary. Many systems were installed complete from the collection of the water at its source to its distribution to the points of consumption, while in some other cases only extensions and ameliorations of existing systems were undertaken. Water supply in the service of supply was placed under as fine and complete a system of bacteriological inspection and examination as is customary under more normal conditions. At Tours, Vierzon, St. Nazaire, and Dijon, where unfavorable bacteriological conditions existed, arrangements were entered into with these municipalities whereby the existing water supplies were chlorinated by the American water supply service.

At Is-sur-Tille was built a mechanical bakery at which 500,000 pounds of bread, fresh for immediate shipment to the troops at the front, could be produced in one day. Another such plant was built and put into service at Neufchateau, and at Liffol-le-Grand it was proposed, and plans had been prepared, to construct a third plant for 400,000 pounds of bread per day, but this project was canceled just after the armistice. In addition to these plants, bakery capacity for 240,000 pounds per day was provided at the base ports.

Oil storage was provided for 175,000 barrels of oil and gasoline. The large plants, with tanks having a capacity of 25,000 barrels each, built with enduring concrete foundations and equipped with connections and pumping plant for the loading of tank cars destined for the front, rivaled in size the installations at large refineries of this country.

For the operation of these many plants numerous power developments were undertaken, and a total of 5,000 kilowatts of new power, being provided for at the time of the armistice, was canceled. Plants of the capacity of 750 kilowatts each, providing 3,500 kilowatts of electric power in all, were in operation when the armistice was signed, not to mention numerous smaller units installed at various points where needed.

Ordnance repair shops were erected, as were also assembling plants for ordnance material, and heavy gun-mounting plants. Repair shops of enormous extent were established near the front, equipped with machine-tool equipment for the repair and maintenance of tank and motor transport material. Schools for the line and staff were constructed, the first and largest being at Gondrecourt and Longres. Laundry plants, salvage depots, aviation assembly plants, sewage disposal plants, refuse incinerators, mechanical repair shops, locomotive assembly plants and locomotive round-houses were placed at convenient points. At Chalmdray and at Colombey-les-Belles, both within a short day's automobile ride of the front, were the tank and air-service repair depots, each one covering many acres of ground and each provided with full equipment for any job of manufacture or repair in their respective fields.

The forestry work of the American Expeditionary Forces was developed to meet the heavy demands of our armies for forest products of all kinds. The first move in this direction was the dispatch to France of the Tenth Engineers, a forestry regiment of two battalions. This was in September, 1917. By the spring of 1918 we had recruited and trained the Twentieth Engineers, a forestry regiment of 10 battalions. Later additional forestry troops were sent across. Shortly before hostilities ceased all these troops were consolidated into a single regiment of 13,000 men, known as the Twentieth Engineers. To this force were added negro service troops to the number of 9,000, making 22,000 men engaged exclusively in the work of cutting down French forests and turning them into lumber required by our forces.

At first we had difficulty in supplying the necessary machinery. Until the sawmills came the forestry troops were engaged in building camps and hewing out railroad ties. In January, 1918, the machine equipment began to arrive. In February our troops cut about 3,500,000 feet of lumber; while in October the cut for the single month had reached the enormous figure of 50,000,000 feet. When the war ended we were expanding our forestry operations in France to produce 1,000,000,000 feet of lumber in a year.

The lumber produced by our sawmills in France up to November 30, 1918, would build completely enough barrack buildings 20 feet wide to stretch out to a distance of 600 miles if placed end to end, quarters enough for 3,107,600 men. In addition to this output the railroad ties produced would build 1,091 miles of standard-gauge railway and the small ties for the 24-inch track would build a double-track railroad behind 185 miles of trenches.

Just the posts and poles produced, if all cut into 6-foot posts, would be sufficient to support a wire fence, with posts one rod apart, reaching one-third of the distance around the earth. The piling, if stood end to end, would make a flagpole 362 miles high. The cord-wood produced would make a rack 1 yard wide, 1 yard high, and 600 miles long.

The sawmill machinery installed to accomplish such a production comprised 30 mills of 20,000 feet per day capacity, 56 mills of 10,000 feet per day capacity, and 92 smaller mills capable of producing ties and rough timber.

In the base and intermediate sections a large amount of work was necessary in the maintenance of the existing roads and highways, and in the construction of new roads in the vicinity of the various new projects. Experienced road engineers, drawn from civil life and commissioned as officers of the Army, were put in charge of this work, and specialist engineer troops and labor battalions were assigned to them. Quarrying the rock, grading the road, surfacing it, and maintaining it in good condition thereafter—all these duties fell within the province of the engineers.

LIGHT RAILWAYS AND ROADS.

The light railway and road regiments of engineers attached to the armies at the front, while their duties did not carry them so far or so much into the zone of enemy fire, may be considered as combatant units, since they operated with and in support of combatant troops in the field. To the light railway regiments were assigned the construction, operation, and maintenance of the light railroads of 60-centimeter gauge (about 24-inch gauge). A great quantity of such trackage was used during the war. These narrow-gauge railroads, capable of being operated under extreme conditions of grade and curvature, and powered with light steam and gasoline locomotives, were essential to the proper supply of a stabilized sector. They were the lines of communication between the railheads of the broad-gauge system and the dumps and depots within the front sectors. At the very front, sometimes within a few hundred meters of the German lines, these light railroads were operated by hand or animal traction, while further back the gasoline locomotive, less conspicuous than the steam engine, came well within range of the enemy's light field pieces. In periods of activity and during an advance these railroads did a tremendous service, not only in transporting troops, munitions, materials, and subsistence stores, but in affording a means of bringing up rapidly a certain class of railway artillery adapted for use upon 60-centimeter gauge trucks. Built of light rail and steel ties assembled in portable sections, this track was easily destroyed by shell fire, and such was often its fate, yet it was but short work for the engineers to replace broken sections with new material, a work frequently done under heavy fire. Engineer troops suffered many casualties in this service.

In cooperation with the Engineer Department in the United States, a practical, efficient, and standard type of narrow-gauge motive power and rolling stock was developed by American manufacturers. This material was shipped to France knocked down, and was assembled and set upon the rails at Gondrecourt, where a plant for this purpose had been established. Up to November 30, 1918, there had been built and placed in operation 538 miles of 60-centimeter track, with 347 steam and gasoline locomotives furnishing motive power for the operation of 3,281 cars of different types.

The road-building regiments in the zone of the armies built and maintained the roads immediately behind the front. Equipped with modern road-building machinery and motor trucks, these regiments maintained the roads in shape to handle the abnormally dense and heavy traffic incidental to operations at the front. The Army road troops were recruited from among men accustomed in civil life to road building, quarrying, and construction operations. They usually worked well within sound of the enemy guns, and frequently under their direct fire. During the advances made from the stabilized line of June, 1918, these regiments improved and perfected the hasty roads thrown across No Man's Land by the sapper regiments of the fighting divisions, so that transport of supplies and troops could be maintained to the advancing armies. To furnish materials for this construction many quarries were opened or taken over from the French road service. A total of 42,000 cubic meters of rock was quarried and prepared for use in quarries operated exclusively by American engineers, while in quarries jointly operated with French forces 75,000 cubic meters were produced.

MAP MAKING AND PRINTING.

A vitally important part of the work of Engineer troops was the making and reproduction of the many maps required for the conduct of tactical and strategic operations by the American Expeditionary Forces. A highly specialized regiment was organized to conduct the topographic surveying operations, map reproduction, and printing work in France. Many of the officers of this regiment had been formerly connected with the American Coast and Geodetic Survey and the Geological Survey, and they were well qualified for the work of war-map making. At Chateau Thierry a portion of this organization rapidly mapped to a large scale the new region in which the theater of operations suddenly found itself, thus supplementing the excellent small-scale map which was in existence for the whole of France, but which was not sufficiently precise for the conduct of our artillery fire. This work was done under pressure, but it contributed its share to the later American successes in that locality. These troops also were charged with furnishing to the Artillery the mathematical azimuths and coordinates, on the basis of which artillery indirect fire was executed.

The maps in use even on stabilized fronts were in a constant process of revision and change. The data and information on which these changes and revisions were based were constantly pouring in from the photographic branch of the air service, from the intelligence service, the Artillery, and from the sapper regiments at the front. Consequently new maps had to be prepared continually and furnished to all the organizations and officers concerned with their use. Then, too, an Army as large as ours required an impressive amount of field printing in order to distribute its orders and information.

As soon as our forces reached France it was apparent that the French map-production plant could not take care of our needs. The Chief of Engineers in the United States thereupon ordered the purchase of equipment for a base printing plant large enough to take care of all the map printing for an army of 1,000,000 men. The special machinery ordered in the United States for this plant did not arrive in France during 1917, and so the American Expeditionary Forces purchased abroad five large rotary lithographic presses, several type presses, and a number of linotype machines and other printing equipment.

The base printing plant was established at Langres, France. In the spring of 1918 the American equipment arrived, and thereafter the base printing plant was able to print not only the current maps required but also the base maps which the French had been supplying. In addition, during the heavy fighting in July and August, 1918, our printing plant supplied to the Seventh and Eighth French Armies the base maps of their fronts.

The demands for maps and printing steadily increased until the base printing plant grew to have a working force of 35 officers and 750 men. From July 15 until September 15, 1918, the plant worked continuously 24 hours a day to turn out the work required. By this time the shop had 10 rotary lithographic presses, 4 linotype machines, and several job presses, printing each month an average of over 1,200,000 lithographic impressions and 500,000 sheets of printed matter. In November the plant turned out 1,900,000 lithographic impressions and over 1,000,000 sheets of type work.

To supplement the base printing plant we had at each army headquarters an advanced printing shop to supply maps when they were needed within a few hours. At the base printing plant we had a department for making relief maps, which work had been done for us previously by the French Government.

The equipment for military map making was enriched during this period by an invention of Maj. James N. Bagley, United States Engineers, called the aerial cartograph, or map camera. The Bagley camera's three lenses at the height of 5,000 feet could photograph a strip of territory 3½ miles wide.

MILITARY BRIDGES.

The science of building military bridges is an old one. When war with Germany was declared the United States had developed its heavy ponton equipment, which was standard in design and yet which had changed but little since the Civil War. As soon as we formally took the step to send troops against Germany the Engineers ordered great quantities of this equipment and by the latter part of 1917 had plenty of it ready to go overseas. Our deliveries to France, however, were hindered by the shortage in ocean tonnage, particularly after we had begun to use every available ship for the transport of men.

Meanwhile the efforts of the Engineers were being directed to the development of standard ponton equipment strong enough to carry tanks and the ponderous artillery of the present day. The old ponton bridge was first strengthened to carry loads of 5 tons on each of two axles spaced 10 feet or more apart. The standard prewar equipment would support only 3 tons similarly spaced.

The next step was to develop a bridge that would hold up axle loads of 10 tons with a distance of 12 feet or more between axles, although in actual use this bridge showed itself capable of supporting a load of 15 tons concentrated on one axle. As soon as these developments were made, the plans were mailed to the American Expeditionary Forces, so that the Engineer Corps abroad could provide the beams and metal parts at its own mills and shops in France. When the fighting ceased, the Engineers were designing a raft capable of transporting the heaviest portable ordnance then under manufacture in the United States.

In 1917 the Engineer Department made designs for a standard sectional steel bridge, consisting of short latticed steel truss sections capable of being assembled to form trusses varying by increments of 11 feet up to a maximum span of about 90 feet. Two of these trusses with the span mentioned were capable of supporting a load of 30 tons, and they could be erected in a matter of hours over abutments prepared in advance or extemporized from the ruins of a demolished structure. These bridges had been manufactured in quantity in this country and were ready for shipment when the armistice was signed.

In the Argonne push Army bridge troops repaired and replaced the bridges destroyed by the retreating enemy as fast as material and labor could be provided at the points needed. For this work much heavy timber was utilized, and, in general, trestle structures were erected as best meeting the conditions of relatively soft crossings and soft river bottoms.

The fighting in the French terrain with its numerous narrow but deep streams and canals indicated to us the desirability of a portable floating footbridge. Such a bridge was designed and produced by the Engineers in France. Many of the crossings of the Meuse River and near-by canals under machine-gun and artillery fire from the high hills on the eastern side were made possible by the use of these bridges.

CAMOUFLAGE.

While camouflage has existed in nature since the beginning of time, its application to warfare on a grand, scientific scale was almost solely a development of the great war. Camouflage, due to the great developments of aerial observation and aerophotography, as well as of air bombing and indirect artillery fire, became a vital necessity for every branch of the service, far back in the rear as well as at the front. Any matÉriel or personnel the position of which was observed was at the mercy of the enemy, but, further, such observation might betray strategic plans. The need for camouflage became universal.

Camouflage organization was carefully developed for both the field and the factory, while one of the most important duties, that of instruction, was carried out in Army and corps schools and artillery camps, where thousands of officers and men were taught both the necessity and the methods of camouflage.

Our undertakings in this direction were based largely upon the methods developed by the French and British. In one respect camouflage was a matter of quantity production. This was in the manufacture of material used for concealing guns, roads, and other strategic locations which fall under the eyes of enemy observers on the ground or in the air.

In this work the British did nothing without the most careful scientific investigation, including the aerial photography of all their materials used, while the French relied more on their innate artistic sense of color and form. The camouflage material produced in quantity by the British consisted principally of burlap cut in strips about 1 inch wide and 12 inches long, colored in the desired hues with oil-emulsion paint. For artillery cover this was knotted in fish nets and chicken wire. The French for this purpose used raffia, a common product of Madagascar, whose natives use it largely for making their fantastic garments. The raffia was dyed and then knotted on nets and wire in the French camouflage factories.

After a careful study we adopted the British system and used burlap. Our Engineers made this decision because of the impossibility of finding permanent dyes for raffia and because raffia is more inflammable than burlap and scarcer and higher in price.

The first demand for camouflage material which we received embraced coverings for guns, sniper's suits, dummy heads, silhouettes, and some airplane hangar covers. In order to supply this material at the outset the engineering forces abroad leased a factory building in Paris and turned out a sufficient quantity with a working force of 30 enlisted soldiers and 100 French women.

But, as the American troops at the front increased in number, the demands for camouflage material became rapidly heavier. Battery positions of some types required about 4,000 square yards of camouflage cover. Aviation hangar covers were demanded in large numbers, and each one was a special order due to the varying conditions of terrain encountered. It became evident that we needed a vast increase in our camouflage-factory space.

In January, 1918, the Engineers secured about 20 acres of ground in Dijon, Haute Marne, a city on the main supply line north through the regulating station at Is-sur-Tille. They started to erect buildings immediately, and within 20 days this plant began turning out material. By November the Dijon factory numbered about 40 buildings, including blacksmith and machine shop, a sewing shop, a paint shop, laboratory, and a toy shop where dummies and silhouettes were made. The factory turned out artillery cover at the rate of 50,000 square yards per day.

The total output of camouflage cover for all purposes required about 3,000,000 square yards of burlap per month. When the fighting stopped the American Expeditionary Forces were using camouflage materials to the value of $1,500,000 monthly.

By new methods of manufacture, we succeeded in reducing the weight of fish-net covers. We designed two important field devices, one being an improved frame and set for mobile artillery protection, this equipment being later adopted by the British, and the other an umbrella machine-gun cover having special advantages. The central camouflage works of the American Expeditionary Forces at Dijon was declared by unprejudiced observers to be the best equipped and most efficient of any on the western front.

When the Dijon camouflage plant was projected it was expected that the American forces would require great quantities of camouflaged observation posts, silhouettes, dummy heads, snipers' suits, and other concealing devices. It was for this production that the toy shop at Dijon was erected, this shop being a kind of studio for the painters and sculptors connected with the Fortieth Engineers, which was the camouflage regiment. These various devices for deceiving the enemy, however, were used principally in the stagnant action of the trench warfare deadlock. By the time American forces came into the war in large numbers the struggle had become one of movement in which the trenches were left far behind. Also, American troops found themselves largely in sectors which were well wooded and therefore provided plenty of secure observation. The result was that there was never a great use on the part of American troops of these clever and interesting exploits in camouflage with which the public is familiar.

One of the best observation posts was the imitation of a tree trunk made of armor plate and set up in advanced positions during the night. Both the British and the French made considerable use of these. The British tree consisted of an oval shell of manganese steel. This was covered with tin, crimped in imitation of bark, and further camouflaged with paint and plaster and natural bark.

When it was desired to set up such a post a camouflage artist would surreptitiously make a faithful sketch of the tree trunk to be duplicated in armor plate. This sketch was then taken back to the workshop, where the spurious tree was built in exact duplication. The metal tree was built to rest on a base with hinges holding it down on one side. During the night two saps, or trenches, would be dug to the natural tree selected. Workers in one of these trenches would fell the branchless stub and carry it back out of the way. The armor plate tree would be drawn up in the other trench. The base would be set in place, and then the whole tree was raised on its hinges by means of ropes until it was upright and as life-like as artistry could make it. Inside the tree was a flight of iron steps leading to a seat in the upper part of the trunk. At this seat were peepholes and a stand for the phone which was connected up with the exchange at the adjoining trench. The sap, covered over, served as the gallery leading back to the trench.

The American camouflage force built only a single one of these trees, using it as a training device. Such objects were useless in an advancing movement, since, under such circumstances, they would play an important part only a short while and would then be left far in the rear.

The Dijon factory, however, turned out a number of small observation posts for use at the edges of shell holes. These were known to our troops as beehives and to the English soldiers as domes. Each one was built of light metal and covered with chicken wire and plaster. It was camouflaged with paint and bits of grass to simulate the appearance of the surrounding terrain, often being studded with tin cans or old shoes to make it appear to be an accumulation of rubbish. The favorite way of making the peephole for a beehive was to cover with gauze a hole cut in the bottom of an old shoe, which was then fastened to the observation post.

Another device built by the Dijon factory was the trench periscope. This was built and set up to look like an ordinary stick, thrown down casually upon the ground. For periscopes, too, we also used imitation stakes placed naturally in the barbed-wire entanglements. The British on occasions used imitation trench telephone poles to mask their periscopes.

The Dijon shop turned out large numbers of silhouettes and dummies. They were drawn from life by artists at Dijon and then cut out from ordinary wall board. Soldiers of the Fortieth Regiment posed as models for these silhouettes. All sorts of postures were employed, but nearly all of them represented soldiers in the act of climbing out of a trench or running, gun in hand, towards the enemy. The uniforms were painted in neutral shades, but the faces and hands were highly colored to be visible at considerable distances during the gray and mist of dawn, when silhouettes were usually employed.

The object of these dummy heads and silhouettes was to draw the fire of the enemy so as to make him reveal his strength and positions. The usual method of use was to place a number of silhouettes, possibly several dozen of them, in shell holes out in front of the trenches. The silhouettes were mounted so that they could be made to stand erect instantly whenever the ropes were pulled from the trenches. At the appointed moment the ropes would all be pulled at once, and the appearance to the enemy would be that of a raiding party starting out at top speed.

The British troops called this operation the Chinese attack. The Germans made no extensive employment of it. The silhouettes nearly always fooled the enemy, as indeed they would deceive anybody in such light and under such circumstances. The British were often amused to read in the German communiques that these Chinese attacks were regarded by the enemy as the real thing. More than one such "repulse" of silhouettes has gone down into the German records as a local success. On one occasion the Germans took a Chinese attack so seriously that they concentrated troops against it with the result that the British were able to gain considerable ground at the points weakened on both sides of the pseudo attack.

The Dijon factory made a thousand or so silhouettes, as well as a large number of dummy heads, these latter devices to draw the fire of snipers. These simulacra, however, had their principal use at the training schools in France, since they were peculiarly adapted to trench warfare, and by the time the American forces reached the front in strength the war of movement was in progress.

Several thousand sniper suits were turned out at Dijon. These suits were made of burlap, resembling in appearance the teddy-bear pajamas which little children wear. They were colored to match the terrain, either painted to resemble rocks or fitted with a grasslike covering. An adjunct to this suit was the cloth cover for the sniper's rifle.

Sniper suits were so deceptive that they would protect a man from observation even at short distances, and if exceptional care were used in the making of one, a man could conceal himself so effectually that an observer might step on him before seeing him. An American camouflage officer upon his return from France brought a sniper's suit with him and found a novel but practical use for it when he was invited to go duck hunting with a party of sportsmen. The other hunters stayed in their blinds, but the officer in the sniper suit went out in the open and shot more ducks than all the other gunners together were able to bring down.

The Dijon camouflage factory also turned out a large number of covers for the so-called Bessenaux hangars for airplanes. These hangars were large tents set up at aviation fields near the front. It was soon found to be impracticable to attempt to camouflage such tents by day, as they gave plenty of indication of their position in spite of the best efforts at concealment on the American and allied sides. However, the great danger at aviation fields came at night when the German bombing planes were abroad. Even on a dark night a white tent proved to be a good mark for the hostile airmen. Consequently the attempt was made to camouflage Bessenaux hangars at night only. It was found impracticable to paint the tents themselves, since the waterproof canvas would not take the paint readily. The solution was a large cover of burlap. This was painted in broken patches of color, much as artillery was painted in camouflage. At the factory such covers were spread out on the ground and painted with floor brushes dipped into water color.

All machinery at Dijon, with the exception of two lathes, two drill presses, and a shearing machine, was designed and built at the plant itself. The work of providing camouflage cover required enormous quantities of burlap to be cut up into strips. The English camouflage shops used stationary knives and a machine operated by a crank. American Engineers at Dijon designed a power-driven cutting machine with a large number of circular, whirling knife blades. The invention of this machine increases the production of burlap strips 900 per cent with the same force. The engineers at the plant also designed paint tanks and special machinery that would mix 4,000 gallons of color in a day.

There were about 1,000 French women employed in this plant. The executives paid great attention to their welfare. A special nursery, the "Creche," was built for their children. American Red Cross nurses cared for the babies during the time their mothers were at work. Many of the women employed were refugees driven from once comfortable homes. Their children fattened up with the good food provided by the army mess, and the mothers were correspondingly happy. Entertainments were frequently provided for the operators of the factories. The artists at the shop worked during their leisure moments and eventually produced the scenery and equipment for a genuine Yankee circus, animals and all, the menagerie, however, being made principally of papier-mÂchÉ with human operatives inside the beasts. The first performance of the circus was given on Thanksgiving Day, 1918, and the audience was so delighted that it demanded a repetition. After three encores of this sort it was suggested that performances be given in Dijon, a city of upward of 50,000 population, with admittance charged. This advice was followed, and the circus made such a hit that the Engineers were able to turn over to the French orphan fund a considerable sum of money.

CONCLUSION.

The foregoing account gives in a broad way an idea of the scope of activities and the achievements of the Engineers during the 19 months of actual warfare in France. To furnish the organization of technical troops and specialists which made all this possible, the original Engineer Arm of the United States Army was increased to 131.5 times its prewar strength, and the proportion of Engineer troops relative to the total forces was increased from 1.6 per cent to 10.8 per cent. To accomplish this, a heavy demand was made upon the technical professions and upon the industries of this country. In filling this demand most necessary assistance was given by the engineer societies and the engineering journals, whose patriotic work demands the highest praise.

In situations requiring special knowledge almost always there could be found some specialist capable of adapting himself and his work to the military needs. Engineer officers for the combatant regiments were younger members of the technical professions, who were sent to the training camps provided for the purpose and there given the essentials of strictly military knowledge. This training was later supplemented by courses in Engineer and line schools located in France. The training officers of the regiments were supplied from the Corps of Engineers, these men having both the military and technical knowledge fitting them for the command. The diversity of education and experience necessary in all branches of the Engineer service may be understood by a consideration of the duties of the different units sent to France during the war—specialist units, in addition to the strictly combatant divisional regiments, who also numbered among their commissioned and enlisted personnel many technical specialists of high attainment.

We had, for instance, seven railway construction regiments, two railway construction battalions, one regiment and five battalions for railway maintenance of way, two battalions for maintenance of railway equipment, four regiments and one battalion to operate our main military railways in France, three regiments to operate the light railways in France and their repair shops, two regiments for operating the regular railway shops, two regiments and six battalions for constructing buildings and other general construction work, two regiments for storing and transporting Engineer supplies, a forestry regiment, a light railway construction regiment, a regiment for building roads, a water supply regiment, a mining regiment, a quarrying regiment, a technical regiment for handling surveying, sound ranging, and location of enemy positions by means of special apparatus, three survey and printing battalions, two railway transportation battalions, an electrical and mechanical regiment, several companies to operate cranes, a camouflage service, five inland waterway companies, five ponton trains, a ponton park, a railway transportation and stores battalion, and a searchlight regiment.

Utilizing and applying the new knowledge and scientific achievements of recent years, drawing upon the fund of experience acquired by the Regular Army in its theoretical studies and past wars, making available the vast amount of technical skill which has assisted this Nation to its present commercial and industrial status, the Engineers of the United States Army worked and fought, planned, and accomplished in France a work which in magnitude exceeds any similar undertaking recorded in American history. From base port to first waves of an assault upon the enemy's position, Engineer troops have been constantly in action first to last and have "carried on" always with the high ideals of the profession and with the motto of the Corps of Engineers, "Essayons," before them.

In establishing contact between our great bases of supply on the French coast and interior points, as well as with the fighters in the various fields of operations, the Department of Military Railways of the Engineer Corps found it necessary to provide thousands of miles of railway track ranging from the standard gauge down to the narrow 60-centimeter type built right up to the border of No Man's Land, to construct and ship across seas thousands of almost every kind of freight cars, to build hundreds of locomotives and transport them to Europe, to provide in addition fabricated track that could be laid under heavy shell fire, and hospital trains that could care for our wounded.

It was on July 10, 1917, that Gen. Pershing cabled stating that the French had asked for 300 locomotives and 2,000 kilometers of track, in addition to numerous items of accessories that go with an order of this size. Delivery of the locomotives was requested by October 15, 1917, and of the track by December 31, 1917.

It was ascertained that the American Locomotive Works had built consolidation engines for France of an entirely satisfactory type, and that similar locomotives for the use of British forces on French soil had been turned out by the Baldwin Locomotive Works. After the decision to adopt the consolidation type of locomotive, which is generally used in freight service in the United States, arrangements were made at once with these two concerns to build 150 locomotives each.

The consolidation locomotive weighs 166,400 pounds, and is about the heaviest that can be used in France. It has one pair of engine truck wheels and four pairs of drivers. The engine is just as large as it is possible to use within the French tunnel and platform clearances. The type sent to France was, however, not nearly so large nor so heavy as the general run of freight engines used here.

The order for 150 engines was placed with the Baldwin concern on July 19, 1917, and the first locomotive of this order was ready for shipment on August 10, 1917, just 20 working days elapsing between the date of the placing of the order and the day when the first engine was completed and all set up ready for shipment.

This is believed to have established a new record for locomotive construction in the United States and probably in the world for an engine of this size. All the other locomotives in this order were delivered promptly—36 of the Baldwin engines being freighted from the factory in August, 71 in September, and the final 43 in October. Of the locomotives ordered from the American Locomotive Works, 133 were freighted in October and the remaining 17 in November.

On account of differences in the details of construction the original price fixed for the locomotives turned out by the American Locomotive Works was $51,000 each and for those of the Baldwin Works $46,000 apiece. Advance payments on these engines reduced the price by $1,000 each.

Changes in the painting and other small details resulted in a saving of $60 additional on each locomotive built by the Baldwin Works and $400 on each engine turned out by the American Works, so that the net cost of each Baldwin locomotive was $44,940, and of each American locomotive $49,600.

After much consideration, and after this initial order had been disposed of, it was determined that the Baldwin type of engine should be made the standard, and all subsequent orders for engines went to the Baldwin Works.

As orders were placed from time to time with the Baldwin people, reductions were made in price, so that the last engines of the total of 3,340 ordered from this concern were obtained for $37,000 each. Orders for 1,500 of these engines eventually were canceled without cost to the United States Government. The saving effected by the reduction in price on the engines ordered, using the original price as a basis of comparison, was $22,989,385.

There were shipped in all to the American Expeditionary Forces 1,303 locomotives, of which 908 had been put into service by November 11, 1918.

During the severe winter weather of 1917-18 and the simultaneous shortage of motive power on American railways, 142 of these consolidation engines built for the American Expeditionary Forces were turned over to the American railways to help out a critical situation in this country. It was possible to use these engines here by making changes in the couplers and some other slight additions to meet the requirements of our safety appliance laws.

At the time these engines were turned over to the Railway Administration we were producing locomotives for France much more rapidly than it was possible to provide tonnage to transport them overseas. These locomotives were in service helping out the transportation facilities in this country an average of 6 months and 28 days each before being recalled for shipment to France. They earned profits for the Government while in service for the Railroad Administration at the rate of 32.3 per cent a year.

It might also be noted that the Director General of Military Railways was appointed custodian of undelivered locomotives ordered by the Russian government from the Baldwin and American Works. In January, 1918, a total of 200 of these Russian locomotives was purchased, and the engines were converted to meet American requirements by a change in the gauge from 5 feet to 4 feet, 8½ inches, and a change in the coupling system to meet our standard. The price of these was $55,000 each. The Baldwin Works turned its 100 over to the Railroad Administration between February 3 and May 20, 1918, and the American Works made its deliveries between February 19 and May 30, 1918.

The combined cost of these locomotives was $11,000,000 and their total rental revenue from the railroads was $2,585,475 up to December 31, 1918, or 23.5 per cent of the cost price, or at the rental rate of 29.8 per cent per annum.

Orders for 90,103 freight cars to be used by the American Expeditionary Forces were also placed with American contractors. Of these the orders for nearly half—40,915 cars, in exact figures—had been placed just before the armistice, and these contracts were canceled at slight cost to the Government. Up to the end of the year 1918 a total of 18,313 freight cars had been shipped overseas, nearly all of these cars being of the 60,000-pound size. Close bargaining in the purchase of these cars resulted in a saving of $15,737,633 under the prices originally quoted.

For the first time in history American locomotives were shipped across the Atlantic stacked in ships on their own wheels. In our normal foreign trade, and even in the early locomotive shipments to the American Expeditionary Forces, both engines and cars had been disassembled at the seaboard and their parts put up in packages for convenient and economical loading on ships. Each of the first locomotives sent to France was crated in 19 packages, while the parts for an ordinary box car were put up in 26 packages.

On October 29, 1917, however, Gen. Atterbury called attention to the fact that the English were shipping locomotives across the Channel on their own wheels and stated that it would result in very great economies of time, money, and man power if such an arrangement could be made for shipments from the United States. Manufacturers of the locomotives, however, advised against this. So did our own embarkation people and the Shipping Control Committee. Efforts were unsuccessful to get car ferries from the Key West and Habana line and from Quebec for the transport of locomotives on their own wheels over the ocean.

Finally, however, after numerous efforts to get ships with large hatches the ore steamer Feltore was loaded with 33 locomotives on their own wheels, packed in baled hay. This steamer sailed May 18, 1918, and its arrival in France resulted in the following cable from Gen. Pershing:

Subsequently the steamers Cubore, Firmore, and Santore were assigned to the task of carrying these engines over on their own wheels. The total number of locomotives that went abroad in this manner was 533. After the signing of the armistice we sold the French Government 485 locomotives, of which 142 had been shipped up to January 1, 1919.

Efforts were likewise made to ship over freight cars already set up but this was also met with much objection. Finally, 1,000 cars were built to go over complete but the signing of the armistice stopped the shipment. The saving in the cost of shipping locomotives on their own wheels amounted to $775 for each one, and an average of $20 a car would have been saved by sending the cars over on their own wheels. But, in addition to this, the cost of erection on the other side, amounting to $800 for each locomotive, should also be added to the saving.

The number of cars actually shipped overseas for the American Expeditionary Forces, if made into one solid train, would be 140 miles long.

In August, 1918, there came a call from abroad to produce locomotives at the rate of 300 a month and freight cars at the rate of 8,200 monthly. Machinery for getting this production was started at once and was so effectual that during the months of September and October and up to the signing of the armistice engines were actually being produced and shipped from the Baldwin Locomotive Works at this rate. This company was turning out the greatest number of locomotives ever produced by any one company in the same length of time.

Arrangements for increasing production of freight cars to meet every possibility of tonnage facilities on the ocean were also made, and had the armistice not been signed we had planned to produce during the month of December 11,000 complete freight cars and to maintain this production rate until we had filled all orders from Gen. Pershing.

On our first purchase of rails, amounting to 102,000 tons, the price paid was $38 a ton for Bessemer steel and $40 a ton for open-hearth steel, as against a price of $59 a ton that the Russians were paying and prices between $54.36 and $61.87 that were being paid by the French. There was a saving in this item of approximately $2,040,000 as compared with the prices paid by the Russians and of $1,938,000 compared with the prices paid by the French.

In connection with our first purchase of this steel rail, it should be stated that the Lackawanna Steel Co. and the United States Steel Products Co. agreed to sell us rail on this basis. Orders were placed with these companies, but not with two other companies—the Cambria Steel Co. and the Bethlehem Steel Co.—who declined to accept the price offered.

All subsequent orders for steel rail were on the basis of $55 and $57 a ton for Bessemer and open hearth, respectively, which figure was established by the War Industries Board pursuant to the Government policy to stabilize industry by establishing fixed prices alike for all purchasers—the Government itself, the allies, and the public.

A total of 937 miles of standard-gauge railway track was laid in France with material shipped from this country.

A big money saving was effected by changing the design of the freight cars asked for by our overseas forces. Their original call was for 17,000 four-wheel cars of the French type, these varying from 10 to 20 tons capacity per car. Our investigations here convinced us that the American type of car with 30-ton capacity could be used on the French railroads. Consequently we recommended that 6,000 of the 30-ton American-type cars be sent abroad instead of smaller-capacity French cars. Our recommendation was approved by officers abroad, and as a result there was a saving of $12,640,000 in the cost of this initial order of cars. From that time all cars shipped from the United States were of the American 8-wheel type, a fact which resulted in a saving of approximately $189,600,000 over what it would have cost to build and ship the lighter French cars.

Had the light French type of cars, as originally suggested, been adopted, 270,309 cars would have been required instead of 90,103 cars, and probably twice as much tonnage would have been necessary to transport these cars overseas.

Most of the steel rails were purchased from the Cambria Steel Co., the Lackawanna Steel Co., the Bethlehem Steel Co., the United States Steel Products Co., and the Sweets Steel Co. Raised pier, gantry, and locomotive cranes were turned out by the several crane builders in proportion to their ability to produce. The Standard Steel Car Co. made millions of dollars' worth of metallic parts for freight cars, and the Colorado Fuel & Iron Co. produced rails and bars. As previously mentioned, the Baldwin Locomotive Works got the bulk of the orders for locomotives, although the American Locomotive Co., the Vulcan Co., the H. E. Porter Co., and the Davenport Locomotive Works also made locomotives for our Expeditionary Forces.

HOSPITAL TRAINS.

Ambulance trains were called for by Gen. Pershing in his cablegram of July 15, 1917. It was stated in this message that plans for these ambulance trains would be furnished by the Surgeon General of the Army.

To build these ambulance trains, with their complicated designs and specialized equipment, in this country would have entailed lengthy delay and very heavy expense, as after they had been constructed it would have been necessary to knock them down for shipment. With this fact in mind our officers here took up the question with Sir Francis Dent, of the British railway commission, who was in this country at the time. He stated that ambulance trains built by the London & North Western Railway, which had proved wholly satisfactory in three years of service, could be turned out by that same concern there quickly if the English design were adopted for our Army.

After considerable discussion and consideration the English design was followed, and orders for our ambulance trains were placed abroad. Up to December 7, 1918, there had been completed for our Army 19 of these trains, with a total of 304 cars, and there were in the course of completion at that time or under order 29 additional ambulance trains.

Information from England shows that it was indeed the part of wisdom to order these ambulance trains abroad, as figures from England stated that the first 14 of these trains were produced at a cost to us of £3,845 per car, including repair parts. This means that at the present rate of exchange the cost of each coach was $18,302.20, while to have built these cars in this country, knocked them down, and shipped them overseas would have cost $40,000 each.

NARROW-GAUGE RAILWAY EQUIPMENT—FABRICATED STEEL TRACK.

The urgent necessity for narrow-gauge railway equipment for our armed forces in Europe was first brought home to us when Gen. Pershing cabled on July 15, 1917. In this message he asked for large quantities of 60-centimeter locomotives, cars, and track. The types requested were entirely new in this country.

Specifically, there were required 195 60-centimeter steam locomotives with a low center of gravity and with a maximum of 3½ tons axle load; 126 40-horsepower gasoline locomotives; 63 20-horsepower gasoline locomotives; and 3,332 freight cars of various types, including box cars and flat cars of 10-ton capacity, tank cars, and dump cars. To aid in the building of this new equipment many photographs and designs brought over from France were used. It was decided to build the 10-ton cars fitted with small 4-wheel trucks at each end, rather than to make them of the 4-wheeled type, as with this construction they would be better adapted for the rounding of short curves.

In turning out the different kinds of locomotives for the 60-centimeter railways new designs were made in order to produce locomotives that would run with equal facility in either direction. For the gasoline locomotives, designs of types similar to standard-gauge engines, a few of which had been in the service in this country, were made, and orders were placed with the Baldwin Locomotive Works for the first lot.

The first steam locomotives were delivered by the builders on October 3, 1917, and the first gas locomotives on November 7, 1917.

Orders for the freight cars for these narrow-gauge railways were placed with a number of the larger car-building companies of the country. The first of these cars were delivered November 24, 1917.

When the armistice was signed a total of 1,841 locomotives and 11,229 cars of the narrow-gauge type had been ordered and 427 locomotives and 6,134 cars completed. Up to the 11th of November 361 of the locomotives and 5,691 of the cars had been shipped overseas.

Of the 361 locomotives sent to France, 191 were steam engines, 108 had 50-horsepower gasoline engines, and 62 had 35-horsepower gasoline engines. Of the 5,691 cars that went to the Expeditionary Forces prior to the signing of the armistice, 600 were box cars, 166 were tank cars, 500 were flat cars, 1,555 were 8-wheeled gondola cars, 330 were dump cars, 100 were artillery truck cars, 970 were motor cars, 180 were inspection cars, 300 were hand cars, and 990 were push cars.

For the construction of the narrow-gauge railroad used in the combat areas behind the front line trenches a special type of fabricated track was designed. This consisted of short sections of rail bolted to steel crossties. The American narrow-gauge railway was so arranged that it could be packed in knockdown shape to save shipping space. Most of this track was in 5-meter lengths, although many shorter sections were used. All, however, were in multiples of 1¼ meters, accurately sawed so as to insure absolute fit of intermediate sections when shell fire made replacement necessary. Vast quantities of curved track, as well as innumerable switches and turnouts, also were built.

In all about 605 miles of fabricated, narrow-gauge steel track were purchased and 460 miles shipped to France. All but 192 miles of the fabricated track was built at the Lakewood Engineering Co., near Cleveland. The balance was obtained through the United States Steel Products Co. The cost of the straight track was about $7,400 a mile, while the cost of the curved sections was $8,000 a mile.

Much of this narrow-gauge track that went to France was manufactured at the rate of between 5 and 6 miles of completed track a day.

Great quantities of the fabricated track produced by the Lakewood Engineering Co. were loaded upon camouflaged steamers in Cleveland in May, 1918, and sent direct to France, via Lake Erie, the Welland Canal, and the St. Lawrence River.

A vast quantity of motorized or portable equipment was required by the Engineer units of the American Expeditionary Forces and this had in the main to be furnished under the supervision of the Engineers in this country. The extent to which this material was produced is shown by such items as 6,923 trucks of all kinds, 2,082 portable buildings, 124 portable shop and material trucks, 51 portable pile drivers, 90 electric storage trucks, 6,006 boilers, and 3,504 dump cars. Two-thirds of this equipment was shipped overseas before the armistice was signed.

The development of mobile shops was one of the most interesting phases of this branch of engineering work. Quite early in the war, when we began the construction of the great base shops in France, we developed these portable machine shops, blacksmith shops, carpenter shops, and storeroom shops in demountable truck bodies, to be used for general service in the field. The shops were so constructed that they could be entirely closed up when the unit was in motion; but when the shop was ready for use the sides and ends of the inclosing structure were lowered, forming work tables when the shop was left on the truck chassis. If the shop were entirely demounted, these sides and ends, let down, formed extensions of the floor. With this arrangement a wide range of general repair and construction work could be handled on the spot on short notice. If it were necessary for the shop to stay in one place for several days or weeks, the body could be demounted, and the truck chassis was then used for transporting materials to and from the shop.

Each portable shop contained about 800 different items of tools and equipment. Each was mounted on a 5½ ton truck. The portable machine shop contained a workbench, a drill press, a portable electric drill, a grinder, and a 14-inch lathe, these being operated by an electric power plant carried on the truck; and it also had an equipment of necessary small tools and supplies, including an oxyacetylene welding outfit.

The portable blacksmith, plumbing, and tin shops each contained a workbench, forges, hoists, pipe-fitting machines, a shear and punch, vises, and a welding and cutting outfit, together with a power plant and switchboard and the necessary small tools and supplies. The portable carpenter shop contained boring machines, a drill press, a bench grinder, a workbench, a saw bench, a winch, power plant and switchboard, small tools and supplies.

A complete machine shop on wheels cost the Government about $8,500. The carpenter shop cost $7,600. As supply units for the portable shops, the Government built 30 material trucks, each containing about 600 items of tools and supplies. These material trucks cost $6,100 apiece.

Another successful development of this nature was the portable photolithographic press truck, already referred to in the account of the American Expeditionary Forces' lithographic equipment. These automobile presses, which were at our front soon after our troops went into the trenches, were able to print and distribute lithographic sketches and maps within 12 hours after the original sketches were submitted for reproduction. The French and British armies also had mobile photolithographic units which were much less portable than ours and much slower in operation. The best time made by the French and British outfits was four days for the same work.

We also supplied to the Engineering forces abroad special water sterilizers and water tanks, mounted on trucks. The Engineers put small job-printing shops on trucks and photographic dark rooms on trucks for use in the field.

They equipped trucks with derricks, capstans, and wrecking machinery. They furnished automotive road sprinklers and oilers, as well as trucks with special dump bodies for highway work.

They developed a light, portable pile driver unlike anything used theretofore in commercial work. This machine was constructed of structural steel and had a total weight of 4 tons. It was mounted on a truck drawn by horses or mules, and the pile driver itself was operated by a 25-horsepower gasoline engine. The pile driver could be used within 16 minutes after its arrival at any point.

One development of this sort, the mobile clam-shell derrick, is worth noting. This unique piece of machinery was built by the Winther Motor Truck Co., of Kenosha, Wis. When the American Expeditionary Forces issued a requisition for 120 clam-shell derricks mounted on motor trucks, no such piece of equipment was in existence anywhere on earth. The Winther Co. volunteered to attempt to produce the machine. By giving a wider tread of rear axle to the Winther motor truck, the company could provide a suitable vehicle, but, search as they might, they could not find a derrick of sufficient power to operate a half-yard clam shell and also light enough to mount on a 7-ton truck. No such derrick existed. The company, therefore, without knowing anything about the manufacture of derricks, put its engineering force to work to produce a design. This design was developed in two weeks, and the derrick built from it was less than half the weight of any derrick of equal capacity. After being perfected, the mobile derrick in tests showed that it could move 350 cubic yards of sand or gravel per day or 500 or 600 tons of coal. One man could operate it and the motive power was a 4-cylinder gasoline engine.

The Engineering Department approved this design and ordered 32 such clam-shell units. Nine of these were delivered before the armistice was signed. The company has continued production of these derricks with a view of selling them to road builders and excavators in civil life.

For use of the various Engineer units we manufactured 1,610 tool wagons and shipped most of them to France. Because of the rough nature of the shell-torn ground over which these wagons must be used, we designed each to be uncoupled and operated as two 2-wheeled carts.

The development of mobile industrial units mounted on motor trucks is likely to have a profound effect on American industry in the future. For instance, the special derrick or crane trucks which we built are almost certain to be adopted in commercial use. The locomotive crane has always been a useful machine, but its chief use has been in handling heavy materials which were being loaded on or off railway cars. A crane which can be moved rapidly to places where railway tracks are not located should be of almost equal importance. An accompanying illustration shows in operation one of the derrick trucks which we built for overseas use.

In the same way the mobile pile drivers designed by the Engineer Corps should be of great future service in road building in this country.

The various machine shops which were built for war purposes will, in their duplications and adaptations, undoubtedly serve a useful purpose in future commercial activities in this country. The increased use of motive power on farms has created a demand for machine repairs. The day may come when the traveling machine shop will be a familiar sight on our rural highways.

The Engineer troops required a great quantity of hoisting machinery. Our purchases in this respect amounted to 700 cranes, mostly of the locomotive type, and 886 hoisting engines, at a total cost of $4,996,000. About two-thirds of this equipment was sent to France and installed at the ports of debarkation and at depots. The rest was used at the shipping points in this country. This machinery was of great aid in the rapid handling of materials at tidewater.

A vast amount of small tools and construction material was required.

Some 21,000 tons of barbed wire, shipped abroad to be used principally in the construction of entanglements in front of American battle positions, were manufactured principally by the United States Steel Products Co., Jones & Laughlin, the Gulf States Steel Co., and the Colorado Fuel & Iron Co., although several other firms also supplied barbed wire.

The Engineering Department ordered in the United States, during the fighting, equipment and supplies which cost approximately $754,201,407.

We furnished in all 85,120 steel shelters of various sizes, of which 38,320 were of the individual type which could be carried by one man. The steel used in these individual shelters was about one-eighth of an inch thick.

There may be expected to be great incidental benefit to future American industry from improvements and inventions brought out by American military engineering in 1917 and 1918.

One important work, for instance, which the Engineer Department undertook was that of standardizing the requirements for paints and varnishes. At the outset our Army needs ran into 29 shades of color in 315 different paint and varnish mixtures. Without affecting any of our camouflage projects or other important undertakings, we reduced the number of shades required from 29 to 16 and brought the total number of commodities down from 315 to 99. This reduction in the range of commodities will be of great use to the paint and varnish industry in the future.

At the beginning of the war the mechanical rubber industry had but few standard specifications. The Engineers, after considerable research, developed 30 standard specifications for mechanical rubber goods, which class included such materials as hose, packing, and sleeves. The representatives of the rubber industry verbally stated that the Engineer Department in this short time did more good to the trade than it had been able to accomplish for itself in the previous three or four years of effort. Immediately after hostilities stopped rubber concerns began asking the Engineer Department for its standard specifications.

In the manufacture of hardware and kitchen utensils there was also considerable standardization done, and changes in manufacturing methods were recommended which were put into effect by the producers. All spun goods were eliminated, and the industry confined itself to straight stamping, which meant a reduction in labor. A standard cobalt coating for enamelware was developed by which the industry conserved about 30 tons of nitre per month and made a more durable and satisfactory enamel coating, with the result that to-day the Army is purchasing its vast quantities of enamelware subject to certain tests, whereas, in the past, practically all this material was bought purely upon the manufacturers' statements. The shortage of tin was of considerable importance. Upon the recommendation of an Engineer officer enormous quantities of cafeteria trays were coated with zinc and large amounts of tin thereby conserved. The finished tray was entirely satisfactory and gave essentially the same service as that plated with tin. Horseshoe nails, formerly a variable product, were standardized and tested, and methods were devised by which the Army was enabled to control their quality.

Before the war there was no standard rating for internal-combustion engines, each manufacturer rating his motors according to his own ideas. Our studies of small engines of the type used for driving pumps or operating woodworking and metal-working machines resulted in many improvements, which have been adopted by the manufacturers of internal-combustion engines. Out of these studies came the so-called army rating, a standard which is bound to result in the more careful rating of commercial engines.

The Engineer Department brought out a modification of the design of the existing line of gasoline-driven shovels by applying caterpillar traction to the larger sizes, thus doing away with the labor required to plank up and block shovels that move on wheels.

When we entered the war, the explosive trinitrotoluol was standard for our Army for mining and demolition purposes. The Bureau of Mines, in cooperation with the Engineer Department, developed an explosive which is cheaper than T. N. T. and promises to replace it for engineering operations.

We also improved the devices commercially used in electrical detonation of distributed charges, our improved detonators being more certain and reliable than anything in use.

Commercial machines for detonating as many as 250 standard No. 8 caps were developed for the Panama Canal, but the machines in common use had seen little improvement for 25 years. As a result of the development by the Engineer Corps, a machine capable of detonating 120 caps was obtained, weighing no more than the 30-cap commercial blasting machine and costing slightly less.

A second machine was developed, capable of exploding 500 caps, at a price not greatly above the price of a 30-cap commercial machine. Mining engineers who saw this development stated that it would have a high commercial value, as these improved machines would make electric blasting more positive and dependable than any other form of detonation, as well as making it possible to set off a large series of charges simultaneously. The Panama Canal machine weighed 35 pounds and cost $126. Our 500-cap machine weighed 30 pounds and cost $35. The du Pont 30-cap machine weighed 25 pounds and cost $25. Our small machine weighed 20 pounds, cost $22.50, and would fire 120 caps.

In addition to this there might be mentioned other projects developed primarily for war purposes but which will be available for the industrial uses of peace. These included portable well-drilling outfits of a new type, alcohol stills of a small size for the utilization of waste products in small units, sound reducers on the exhaust pipes of gasoline engines, air strainers to minimize the chances of dust and grit entering gasoline engines. When the war ended we were working on the problem of hastening the setting of concrete and were also studying the production in this country of photographic colors and tone chemicals formerly secured only from Germany.

In general, mention should be made of the exhaustive tests in many industries conducted by the Engineer depot and by special detachments of Engineers. Tests were made of hundreds of pieces of apparatus, and these tests led to many improvements in American manufacture. Illustrating how these tests were regarded by individual concerns, the Cleveland Tractor Co., after a test of its equipment conducted by Army Engineers, stated: "Our people consider this test to be the most valuable ever undertaken by this company." This is indicative of benefits scattered throughout American industry by the engineering war tests.

While practically all of the research work which resulted in the developments and improvements noted was conducted by Engineer officers while on duty at the General Engineer Depot in Washington, since the transfer of the functions of the General Engineer Depot to the Division of Purchase, Storage and Traffic, November 1, 1918, much of this research work has been and still is being carried on by the latter division.

For handling Engineer materials there were established the General Engineer Depot at Washington, D. C, embarkation depots at South Kearney, N. J., and Norfolk, Va., and shipping depots at Baltimore, Md., Philadelphia, Pa., Jacksonville, Fla., New Orleans, La., and Mobile, Ala. In addition, subdepots were organized at all of the divisional camps and cantonments.

The war demanded the production in America of quantities of precision instruments. These were required not only by the Ordnance Department for the equipment of artillery with sights and indirect fire-control apparatus but also by the Engineer Corps, the Signal Corps, the Bureau of Aircraft Production, and the Medical Department. These instruments were such things as aneroid barometers, pocket compasses, measuring tapes, surveyors' equipment generally, map-drawing outfits, draftsmen's supplies, and so on. For a large period of the war the procurement of precision instruments was in the hands of the General Engineer Depot. Later, when the War Department's supply activities were being consolidated, the purchasing of precision instruments, except the highly technical sound-ranging devices, was taken over by the Director of Purchase and Storage, the organization of the General Engineer Depot going along in the transfer. The development and the production of searchlights and sound-ranging apparatus remained in the hands of the Engineer Corps.

In April, 1917, there were probably not more than a dozen recognized American manufacturers of high-grade precision instruments. As an indication of the expansion of manufacturing capacity required by the war, one concern, the Taylor Instrument Cos., of Rochester, N. Y., which had manufactured in peace times watch-pocket compasses at the rate of 15,000 a year, were called upon to turn them out at the rate of 10,000 weekly to fill an order for 200,000 of them. In order to handle this contract the Taylor Instrument Cos. built a new factory building in 20 days. A certain type of aneroid barometer required by the exigencies had never before been produced in America. The Taylor Instrument Cos. succeeded in producing 1,240 of these barometers.

The Lufkin Rule Co., of Saginaw, Mich., was called upon to manufacture 700 band chain measuring tapes for surveying, graduated throughout according to the metric system, and also 1,240 special outfits for repairing such tape. These band tapes when broken are fastened together by tiny rivets, which are produced by special machinery. Because of the inability of the machine-tool industry, swamped as it was with war demands, to produce the special rivet-making machines, it was necessary to reduce in the specifications for repair outfits the quantity of metal rivets for each kit from 4 ounces of rivets to 2 ounces.

Field artillery required a precision instrument known as the miniature telescopic alidade of the Gale type. It is unlikely that 150 of these instruments had been made in the United States during 10 years, yet the Artillery demands called for the production of 1,110 of them. The W. & L. E. Gurley Co., of Troy, N. Y., not only manufactured half of this order, but, in order that the Government might obtain a sufficient supply of these instruments, it turned over to a competing firm, the Eugene Dietzgen Co., of Chicago, the lenses, prisms, hermetically-sealed bubbles, and other parts for 555 instruments.

The Army required large numbers of hand tally registers to be used by checkers and observers. The Benton Manufacturing Co., of New York, had been making less than 15,000 registers of this sort in a year, yet it increased its facilities and turned out 62,000 of them for the Army within two months.

The Army required 35,000 complete sketching outfits for the use of military observers. The contents of these outfits were manufactured by a dozen different concerns.

Drawing instrument sets were produced by the Eugene Dietzgen Co. Each set included a pair of proportional dividers. Our draftsmen had always obtained their dividers from Europe. The divider, which nearly everyone has seen, appears to be a simple device, yet it must be made with the utmost precision, or else it is valueless. In manufacture it goes through more than 100 distinct factory operations.

Marching compasses for troops were made by the Sperry Gyroscope Co., of Brooklyn, N. Y., the quantity in manufacture being over 200,000 instruments.

Many other delicate instruments of most difficult manufacture, whose description is too technical to be set down here, were produced successfully in America during the war period.

In childhood we were enthralled by the tales of those magic persons whose keen hearing could detect even the whisper of the growing grass. As camouflage developed, modern warfare yearned for such supernatural gifts of sense that troops might detect the unseen presence of the enemy. Accordingly Science, the fairy godmother of today's soldiers, raised her wand, and lo, the Army was equipped with the wonderful ears of the fairy tale, uncanny no longer, but a concrete manufacturing proposition.

Artillery practice nowadays abhors the wasted shot. The time when cannon fired in the general direction of the enemy, and hoped to hit something, passed when the long-range rifles and howitzers, with their marvelously accurate sighting instruments, came into existence. Whole books have been written on the subject of pointing a modern cannon in the modern way. A great proportion of our industrial effort in the recent conflict was devoted to the sole end that we might aim our artillery accurately.

For instance, to this end almost exclusively was devoted the enormous production of aircraft material. The observer in the airplane or balloon trusted not to his eyes but to the finer sight of the photographic camera; and this again occasioned a large war industry—the production of cameras and their operation in the field, which included the production of finished photography in the field dark rooms. But, as the airplane and aerial camera were perfected, camouflage was undertaken as a protection from discovery from aloft; and so might be brought in another chapter—the production of camouflage material and the work of camouflage experts in the field. Presently camouflage succeeded in baffling the camera to a great extent, and this made necessary the development of instruments that could detect the location of the enemy by sound. Since the unaided ear was not keen enough to supply the desired information, applied science came to the rescue with the various devices embraced in the general classification of sound-ranging equipment. The production of this equipment was under the direction of the Engineer Department of the Army.

In three classes of military work we needed hearing refined to the razor-edge. With keen enough ears we could detect those subterranean operations of the enemy known as mining; with ears of that sort we could detect and locate the positions of hostile cannon; and still again we could employ such sensitiveness of hearing to find in the darkest sky at night the hostile raiding airplane.

One of these long-distance ear drums which man invented for himself as an aid to his military operations was known as the geophone. The first geophone used by the western powers in the war was invented by the French. It was a simple mechanism. The device or drum which received the sound waves and magnified them consisted of a small closed box with a confined air space. This box was weighted with a leaden disk to give it the required inertia. The geophone was placed upon the ground and the vibrations of the earth were communicated through the medium of the confined air space. The sounds then reached the listener's ears via a rubber tube and an ordinary stethoscope horn. By means of this device the slightest vibrations of the ground were rendered audible.

The geophone was used to detect enemy mining operations. The listener placed the weighted box on the floor of an underground gallery or on solid earth or rock. If the enemy were burrowing in the ground anywhere within a distance of 75 yards the geophone would tell about it. In order to enable the listener to know in what direction the sounds came, two geophone boxes were provided, one connected with each ear. By placing the boxes a small distance apart from each other and them moving them until the vibrations in both ear horns were equalized, the listener could tell approximately in what direction the enemy mining operation was located.

Geophones were used by both sides, and so effective did they prove to be that it is reported that they were largely instrumental in stopping mining operations altogether. If an enemy mine were located by one of these devices, a counter mine could be started at once and carried through, usually with disastrous results to the hostile miners.

As our first step in the production of geophones, we adopted the French device; but later on we developed an instrument with nearly one-third greater range than the French geophone had. This improvement was developed by the Engineers and bureau specialists at the Bureau of Standards in Washington with money provided by the Engineering Department. We produced the improved model in sufficient quantities to meet the requirements of the American Expeditionary Forces.

We also developed an electromechanical geophone that could be connected up by wire to a central listening station some distance back from an exposed location. The sound-receiving boxes or microphones were placed out in No Man's Land and hidden under trash or earth. They were so sensitive that they would not only record any subterranean activities of the enemy within their range, but at night would betray enemy raiding parties attempting to cross to our positions, the sensitive boxes picking up the vibrations of their speech or footsteps. The central listener could locate approximately the position of hostile operations by observing which boxes were receiving the sounds in greatest intensity. The boxes could also pick up and send to the central listening stations conversations carried on by the enemy parties even in low tones, the apparatus thus acting as the dictatorship of the war.

But by far the most important work done by listening instruments was in locating the positions of enemy gun batteries. This was one scientific instrument, at any rate, which the Germans were never able to produce successfully for themselves. During the final months of the war more enemy guns were located by listening instruments than by any other means. An American instrument with the Army spotted 117 German gun positions in a single day by surface sound ranging. This was the high American record set in the war, but at all times our sound-detecting equipment had an uncanny accuracy. Up to the end of the fighting, no way had been discovered to conceal the location of a gun from sound-ranging instruments suitably placed and properly operated.

The instruments used for locating gun positions were of such a highly complicated and technical nature that no one but designers and mechanics skilled in the production of complex electrical equipment could build them at all. The recording instruments, or microphones, were of a sort so delicate that their use theretofore had never been considered outside of laboratories. Yet they were required to operate successfully amid the din and concussion of heavy bombardments. All useless sounds and jars were filtered out so that only the sought-for vibrations could come to the central recording mechanism.

Studies of gunfire showed that when a cannon fires an explosive shell of high velocity there are three distinct concussions. One of these is the sharp crack produced in the air when the shell, dragging a short vacuum trail behind it, passes over the head of the observer. As the air rushes into this vacuum and collides with itself, it produces a crack that is similar in origin to ordinary thunder. The second concussion to be heard is that produced at the muzzle of the gun by the expanding gases that propel the shell. There is still a third, the break, or explosion. In order to locate a battery or gun exactly only one of these concussions—the explosion at the muzzle of the gun—must be picked up by the microphone. The first and third shocks, and all other sounds not useful to the work should be damped out and excluded.

A number of these microphones would be placed in scattered positions, usually in a trench, and then connected with the central recording mechanism. When a microphone picked up a hostile gun explosion the disturbance was instantly transmitted through several miles of wire. An ingenious and complicated mechanism actuated an electromagnetic needle, which instantly recorded this disturbance on a tape of photographic paper, calibrated to show fifths of seconds in time. Each microphone on outpost duty was represented on this tape by a parallel line; and, as six microphones were usually used, the tape was striped with six parallel lines. As the other microphones at the front picked up the concussion of the gun, their records were made on their respective lines; and the observers at the central station, by noting the difference in time between the reports of the various microphones, and by making calculations based on the rate at which sound travels, could locate the gun that set up the disturbances by means of ordinary surveyor's triangulations. So accurately would this mechanism do the work that a gun position could be determined within 50 or 60 feet.

Incidentally, it is interesting to note that the practice of our Army was to secure in advance, by means of surface sound ranging and other methods, the positions of all the enemy's guns that could be learned. Then, often after intervals of hours or even days, the fire began simultaneously upon all these gun positions just as our attack started.

In this country we had two experimental stations for the development of sound-ranging apparatus. We began experiments in this work in June, 1917. Before we had perfected any satisfactory instruments, the British had met with great success with the Bull-Tucker system; and we adopted that type for the use of the American Expeditionary Forces. From plans and models sent to this country we produced an American Bull-Tucker machine, utilizing standard American electrical equipment wherever we could. At the close of the war we had in operation along the American front 12 complete American outfits. The six microphones of each recording machine in action were set about 5,000 feet apart along the front, so that each sound-ranging section covered a frontage of approximately 5 miles. The 12 outfits in use were sufficient to locate the guns of the enemy on a 60-mile front.

About a month before the fighting stopped we sent to France a new model sound-ranging set which had been developed with the cooperation of the Bureau of Standards. The reports from the American Expeditionary Forces indicated that this American development was superior in several important particulars to anything else in use when the war came to an end. The American instruments were lighter, easier to carry about, easier to install, and much cheaper than those of the British type, and would operate under more adverse weather conditions. The impulses received by the microphone in this equipment were recorded on a running tape smoked by an acetylene flame.

Sound ranging for the detection of airplanes at night requires an equipment which consists fundamentally of a sound-gathering device and a listening mechanism, the combination enabling the observer to tell the direction from which the sound is coming. When a bombing plane approaches at night the hum of the motor can be heard at a distance from 1 to 3 miles, or even more, depending upon conditions. But the direction of this sound is elusive to the unaided ear, as anyone can testify who has heard an airplane in broad daylight but could not locate it with his eyes. Before the invention of aerial sound ranging the searchlights hunting for the hostile airplane were obliged to sweep the sky aimlessly in an endeavor to locate it; and the pilot of the plane could often maneuver to keep out of the light. But by use of the sound detectors not only can the approach of the airplane be detected at a distance beyond the hearing range of the unaided ear, but, what is more important, its direction can be determined within an angle of 3°. The use of these sound detectors greatly increased the chances of locating airplanes at night by searchlight.

The Engineer Department conducted extensive experiments in the development of aerial sound detectors. One form developed consisted of a set of long horns with listening tubes attached to the small ends and leading to receivers on the observer's head set. These horns were mounted on a turntable which the observer could revolve, so that the horns could be turned in the general direction of the sound. Four horns were used in this mechanism—two to indicate the direction of the airplane on a horizontal circle (in azimuth), and the other pair to indicate the direction on the vertical arc (in elevation). Under favorable conditions the sensitiveness of this device was three times that of the unaided ear, and the airplane could be located within an angle of 1°. The horn detector, however, was large and cumbersome and not satisfactory for a mobile unit.

For field sound ranging, when the listener may wish to move from place to place, the parabloid sound reflector was developed. This hemispherical object, like a huge fountain basin in shape, was made of material similar to building board and shaped in parabolic lines. Such a sound collector echoed or reflected the sound from every point of its surface to a focal point where the listening instrument was located. The observer turned the parabloid on its universal mount until the sound was equalized in his ears, and then the exact direction of the airplane would be indicated by the azimuth and elevation pointers on the machine. The paraboloids developed by our Engineering Department had a sensitiveness three times that of the unaided ear and could locate sound within 3° of arc.

We were not pioneers in developing the parabloid, however, the French having built them ahead of us; but our apparatus possessed marked advantages over that of the French. In the first place, the French collecting device weighed 3½ tons and was so heavy and cumbersome that it could scarcely be moved at all. The total weight of the American collecting device was only 1,300 pounds. The American instrument was thus much lighter and more portable. It was so simple that it could be set up in about one-sixth the time that it took to erect the French device. The cost of our machine was only about two-fifths that of the French mechanism.

Although valuable work in detecting gun positions was done by sound ranging, yet both sides located guns by watching their flashes. We improved flash-ranging sets of the allies. These were simple in principle. A number of observers at posts commanding good views were equipped with observation telescopes mounted on tripods to watch for the flashes of enemy guns. Whenever two or more of them observed the same flash and reported its direction, the position of the gun could be determined by ordinary triangulation.

However, in operation the system was not so simple, because of the fact that the observers reporting might not have turned their instruments upon the same flash. This difficulty was met by furnishing each observer with an outpost switch set. As soon as he observed the flash through his telescope he closed the switch, and that action turned on a small electric light at the headquarters station, which might be miles away. Then, as soon as he could, he telephoned in the direction of the flash observed. If the operator at the switchboard saw two or three of the lights flash simultaneously, he knew the observers at the front had probably caught the same flash. Lights that came on a little ahead or a little behind the simultaneous lights were disregarded when the observers telephoned reports.

In developing the telescope for this system considerable difficulty was experienced on account of the shortage of the proper optical glass in this country. We were, therefore, obliged to buy our telescopes in France until our supply would be available. These telescopes were expensive mechanisms, and in some of the work of the flash-ranging sections two of them were originally required at each observing station—one to determine the position of a shell burst in elevation and the other its position on the horizontal circle in azimuth. Since the declaration of the armistice an American Engineer officer has designed a telescope eyepiece which enables this work to be done by observing through a single instrument, thus effecting a marked saving in the number of telescopes which might be required in the future.

When the fighting stopped our military scientists and others cooperating with them were developing a type of ground sound-ranging apparatus which it was hoped could be utilized to give troops warning of the firing of heavy artillery shells in their general direction. Preliminary experiments show that at a distance of 4.1 miles this mechanism could record the firing of a gun some 19 seconds before the arrival of the shell. Under proper circumstances this elapsed time would enable troops properly warned to seek cover from the explosion of the projectile. This development of sound-ranging apparatus and its application to the protection of personnel was made possible by the far greater speed with which shock vibrations travel through a dense medium like the earth than through the usual sound-conveying medium, the atmosphere.

SEARCHLIGHTS.

The searchlight equipment of the United States Army prior to 1914 consisted chiefly of lights located at our coast defenses. In 1916 we began the development of mobile searchlight-and-power units for field-army work, four horse-drawn equipments, with 36-inch lights, being ordered first, and later eight other sets, with extensible towers and gasoline electric generators. When the war was approaching we ordered 85 sets of the limber-and-caisson type. The caissons of these sets carried 24-inch lights on extensible towers. In January, 1917, we ordered 50 high-intensity lights to replace as many low-intensity lamps at our seacoast fortifications. The first war order was placed in April, 1917, and consisted of 20 additional searchlights of the 60-inch dimension, the largest light ordered by the War Department.

After the entrance of America in the war the Engineer Department began studying the requirements abroad for searchlights used in defense against hostile aircraft; and in September, 1917, this investigation resulted in orders for 360 high-intensity searchlights, 693 high-intensity arc mechanisms, and 1,000 glass mirrors of standard design.

About this time we began looking to the improvement of existing searchlight equipment. The cooperation of leading scientists, manufacturers, and Government bureaus was obtained, and the product of exhaustive experiments was 18 different new kinds of searchlights either partially or wholly developed.

The first of these were produced, shipped, and were in operation with the Second Field Army in France on October 1, 1918. This was a new form of searchlight more powerful than any that had been produced before that time. It weighed one-eighth as much as lights of former design, cost only one-third as much, was about one-fourth as large in bulk, and threw a light 10 per cent stronger than any other portable projector in existence.

Without going into the details of this mechanism its most striking innovation, from the standpoint of the nontechnical observer, was the absence of the front glass through which the beams of the older type lamps are sent. The absence of the glass, while reducing the weight and cost of a light, also increased the intensity of the beam of the searchlight, since any glass, no matter how conducive to rays, absorbs light to a considerable extent.

In the first part of the war we took the 36-inch lights which the Government had on hand and mounted them on motor trucks. For generating power for the lights, motor trucks were equipped with electric generators operated by the crank-shaft of the truck engine. In moving about each truck carried not only the light and power unit and accessories, but provided space for the crew and their equipment.

When we went into the war there was only one firm in the United States that could make the large searchlight mirrors, but two other concerns developed the art and the faculties during the hostilities. These mirrors were of glass and cost about $1,000 at prewar prices. The maximum output in the United States before the war was three 60-inch mirrors per week. As the result of governmental encouragement the production of the 60-inch mirrors increased until it reached the stage of 15 a week in November, 1918; and the price was reduced to about $900 per mirror, even under war-time conditions with respect to labor and material. This was equivalent to a price of about $700 per mirror under normal conditions, or a saving of 30 per cent.

A remarkable contribution of the United States to searchlight science was the production of a satisfactory metal mirror for projecting the beam. The metal mirror not only weighed a little less than the glass mirror, but it cost only one-third as much as the glass one, could be produced in one-fifth the time, was much less fragile, and extended the possibility of manufacture to a wide number of industries. The metal mirror possessed 97 per cent of the reflectivity of the glass mirror. This slight dullness is inappreciable in searchlight work and more than compensated for by the other qualities of the metal reflector. This type of mirror, however, had not yet been put in production when the war ended.

Our inventors during the 19 months of hostilities succeeded in reducing the size of carbons used in 200-ampere lamps from 2 inches in diameter to 1? inches. This cut the cost of carbons in two, but the improvements tripled the amount of light developed.

In November, 1918, we were working with assurance of success to develop a simple system whereby field searchlights could be pointed and controlled from a distance. Such controls had been used in experimental work prior to 1917, but the mechanisms were complicated and not suitable for field service.

The searchlight section of the Corps of Engineers also developed optical finding devices, which doubled the range of all searchlights without requiring any modification of the lights themselves. Neither the ordinary telescope nor night glass is suitable for target finding by searchlight. The result of our investigation was the development of a combined observer's chair, eye protector, and searchlight target finder, the new equipment adding only 10 per cent to the cost of the searchlight unit.

The range of our modern high-power searchlight, whose target is a ship at sea, is about 15,000 yards; the range of this searchlight when its target is an airplane is about 15,000 feet.

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