Allusion has already been made to one great characteristic of our age, namely, the replacement, in every department of industry, of manual labour by machines. A brief notice of even the main features of the various contrivances which have been made to take the place of men’s hands would more than occupy this volume. Accordingly, we must omit all reference to many branches of manufacture, although the products may be of very great utility, and the processes of very high interest; and in taking one example here and another there, we must be guided mainly by the extent and depth of the influence which the new invention appears destined to exert. This consideration has, with scarcely an exception, decided the selection of the topics already discussed, and it has also determined the introduction of the present article, which relates to machines of no less general importance than the rest, although at first sight it might seem to enter upon the details of merely a special branch of industry. But so general are the interests connected with the subject we are about to lay before our readers, that we are not sure it would not have been more logical to have placed the present article before all the rest. For whence comes the iron of which our steam engines, tools, rails, ships, cannon, bridges, and printing presses are made?—whence comes the fuel which supplies force to the engines?—whence come, in fine, the substances which form the matÉriel of every art? Plainly from the earth—the nurse and the mother of all, and in most cases from the bowels of the earth, for her treasures are hidden far below the surface—the coal, and the ores of iron and other metals, are not ready to our hand, exposed to the light of day. The railways also, and the canals, can be made only on condition that we cut roads through the solid rocks, and pierce with tunnels the towering mountains. Hence the tools which enable us to penetrate into the substance of the earth present the highest general interest from a practical point of view, and this interest is enhanced by the knowledge of the structure and past history of our planet acquired in such operations.

The operations by which solid rocks are penetrated in the sinking of shafts for mines, or in the driving of tunnels, drifts, headings, galleries, or cuttings for railways, mines, or other works, are easily understood. In the first place a number of holes—perhaps 3 ft. or 4 ft. deep and 2 in. or 3 in. in diameter—are formed in the rock. The holes are then charged with gunpowder or other explosive materials, a slow-burning match is adjusted, the miners retire to a safe distance, the explosion takes place—detaching, shattering, and loosening masses of the rock more or less considerable; and then gangs of workmen clear away the stones and dÉbris which have been detached by the explosion, and the same series of operations is renewed. The holes for the blasting charges are formed by giving repeated blows on the rock with a kind of chisel called a jumper—the end of which is formed of very hard steel, so that the rock is in reality chipped away. The dÉbris resulting from this operation is cleared away from time to time by a kind of auger or some similar contrivance. But for many purposes it is necessary to drill holes in rocks to great depths, hundreds of feet perhaps, as for example, in order to ascertain the nature of underlying strata, or to verify the presence of coal or other minerals before the expense of sinking a shaft is incurred. These bore-holes were commonly formed in exactly the same manner as the blast-holes already mentioned, by repeated blows of a chisel or jumper, which was attached to the end of a rod; and as the hole deepened, additional lengths of rod were joined on, and the rods were withdrawn from time to time to admit of the removal of the dÉbris by augers, or by cylinders having a valve at the bottom. The reciprocating movement is given to the chisels and rods either by hand or by steam or water power. When the length of the rods becomes considerable, of course the difficulty of giving the requisite blows in rapid succession is greatly increased, for the whole length of rods has to be lifted each time, and if allowed to fall with too much violence, the breaking of the chisel or the rods is the inevitable result. The time requisite for drawing out the rods, removing the fragments chipped out, and again attaching the rods and lowering, also increases very much as the bore gets deeper. Messrs. Mather and Platt, the Manchester engineers, have, in order to obviate these difficulties, constructed machines in which the chipping or cutting is done by the fall of a tool suspended from a rope, the great advantage resulting from the arrangement being the facility and rapidity with which the tools used for the cutting and for the removal of the dÉbris are lowered to their work and drawn up. It is necessary in using the jumper, whether in cutting blast-holes or bore-holes, to give the tool a slight turn after each blow, in order that the rock may be chipped off all round, and the action of the tool equalized. Many attempts have been made to drill rocks after the fashion in which iron is drilled—that is, by drilling properly so called, in which the tool has a rapid rotary motion. But even in comparatively soft rock, it is found that no steel can sufficiently withstand the abrading action of the rock, for the tool becomes quickly worn, and makes extremely slow progress. We shall have presently to return to the subject of bore-holes; but now let us turn our attention to an example which will illustrate the nature and advantages of the machinery which has in recent times been applied to work the jumpers by which the holes for blasting are formed.

THE MONT CENIS TUNNEL.

The successful construction, by the direction of Napoleon, of a broad and easy highway from Switzerland into Italy, crossing the lofty Alps amid the snows and glaciers of the Simplon, has justly been considered a feat of skill redounding to the glory of its designers. But we have recently witnessed a greater feat of engineering skill, for we have seen the Alps conquered by the stupendous work known as the Mont Cenis Tunnel. This tunnel is 7½ English miles in length; but it is not the mere length which has made the undertaking remarkable. The mountain which is pierced by the tunnel is formed entirely of hard rock, and what added still more to the apparently impracticable character of the proposal when first announced was the circumstance that it was quite impossible to sink vertical shafts, so that the work could not, as in the usual process, be carried on at several points simultaneously, but must necessarily be continued from the two extremities only, a restriction which would occasion a vast loss of time and much expense, to say nothing of the difficulties of ventilating galleries of more than three miles in length. The reader must bear in mind that the importance of this question of ventilation depends not simply on the renewing of the air contaminated by the respiration of the workmen, but on the quick removal of the noxious gases produced in the explosions of the blasting charges. A work surrounded by such difficulties would probably have never been attempted had not Messrs. Sommeiller and Co. invited the attention of engineers to an engine of their invention, worked by compressed air, and capable of automatically working “jumpers” which could penetrate the hardest rock. These rock-boring machines, having been examined by competent authorities in the year 1857, were pronounced so efficient that the execution of the long-spoken-of Alpine tunnel was at once resolved upon, and before the close of that year the work had actually been commenced, after a skilful and accurate survey of the proposed locality had been made, and the direction of the tunnel set out. The tunnel does not pass through Mont Cenis, although the post road from St. Michel to Susa passes over part of Mont Cenis, which gives its name to the pass. The mountain really pierced by the tunnel is known as the Grand Vallon, and the tunnel passes almost exactly below its summit, but at a depth the perpendicular distance of which is as nearly as possible one mile. The northern end of the tunnel is near a village named Fourneaux.

Pending the construction of the Sommeiller machines, and other machinery which was to supply the motive force, the work of excavation was commenced at both ends, in 1857, in the ordinary manner, that is, by hand labour, and in 1858 surveys of the greatest possible accuracy were meanwhile made, in order that the two tunnels might be directed so that they would meet each other in the heart of the mountain. The reader will at once perceive that the smallest error in fixing on the direction of the two straight lines which ought to meet each other would entail very serious consequences. The difficulties of doing this may be conceived when we remember that the stations were nearly 8 miles apart, separated by rugged mountains, in a region of snows, mists, clouds, and winds, over which the levels had to be taken, and a very precise triangulation effected. So successfully were these difficulties overcome, and so accurately were the measurements and calculations made, that the junction of the centre lines of the completed tunnel failed by only a few inches, a length utterly insignificant under the conditions.

The work was carried on by manual labour only, until the beginning of 1861, for it was found, on practically testing the machinery, that many important modifications had to be made before it could be successfully employed in the great work for which it was designed. After the machinery had been set to work, at the BardonnÊche end, breakages and imperfections of various parts of the apparatus, or the contrivances for driving it, caused delay and trouble, so that during the whole of 1861 the machines were in actual operation for only 209 days, and the progress made averaged only 18 in. per day, an advance much less than could have been effected by manual labour. The engineers, not disheartened or deterred by these difficulties and disappointments, encountered them by making improvement after improvement in the machinery as experience accumulated, so that a wonderful difference in the rate of progress showed itself in 1862, when the working days numbered 325, and the average rate of advance was three feet nine inches per day.

At the Fourneaux extremity more time was required for the preparation of the air-compressing machinery, and the machines had been at work in the other extremity, with more or less interruption, for nearly two years before the preparations at Fourneaux were completed.

The illustration at the head of this article, Fig. 178, represents the Sommeiller machines at work, the motive power being compressed air, conveyed by tubes from receivers, into which it is forced until the pressure becomes equal to that of six atmospheres, or 90 lbs. per square inch. The compression was effected by taking advantage of the natural heads of water, which were made to act directly in compressing the air; the pressure due to a column of water 160 ft. high being made to act upwards, to compress air, and force it through valves into the receivers; then the supply of water was cut off, and that which had risen up into the vessel previously containing air was allowed to flow out, drawing in after it through another valve a fresh supply of air; and then the operations were repeated by the water being again permitted to compress the air, and so on, the whole of the movements being performed by the machinery itself. The compressed air, after doing its work in the cylinders of the boring tools, escaped into the atmosphere, and in its outrush became greatly cooled, a circumstance of the greatest possible advantage to the workmen, for otherwise, from the internal warmth of the earth, and that produced by the burning of lights, explosions of gunpowder, and respiration, the heat would have been intolerable. At the same time, the escaping air afforded a perfect ventilation of the workings while the machines were in action. At other times, as after the explosion of the charges, it was found desirable to allow a jet of air to stream out, in order that the smoke and carbonic acid gas should be quickly cleared away. Even had the work been done by manual labour alone, a plentiful supply of compressed air would have been required merely for ventilation, so that there was manifest advantage in utilizing it as the motive power of the machines.

The experience gained in the progress of the work suggested from time to time many improvements in the machinery and appliances, which finally proved so effectual that the progress was accelerated beyond expectation. At the end of 1864, when the machines had been in work about four years, it was calculated that the opening of the tunnel might be looked for in the course of the year 1875. But in point of fact it happened that on the 25th December, 1870, perforator No. 45 bored a hole from Italy into France, by piercing the wall of rock, about 4 yards thick, which then separated the workings from each other. The centre lines of the two workings, as set out from the different sides of the mountain, failed to coincide by only a foot, that set out on the Fourneaux side being this much higher than the other, but their horizontal directions exactly agreeing. The actual length of the tunnel was found to be some 15 yards longer than the calculated length, the calculation having given 7·5932 miles for the length, whereas by actual measurement it was found to be 7·6017 miles. The heights above the sea-level of the principal points are these:

	Feet.
Fourneaux, or northern entrance	3,801
BardonnÊche, or southern entrance	4,236
Summit of tunnel	4,246
Highest point of mountain vertically over the tunnel	9,527

The tunnel is lined with excellent brick and stone arching, and it is connected with the railways on either side by inclined lines, which are in part tunnelled out of the mountain, so that the extremities of the tunnel referred to above are not really entered by the trains at all; but these lateral tunnels join the other and increase the total distance traversed underground to very nearly 8 miles, or more accurately, 7·9806 miles. The time required by a train to pass from one side to the other is about 25 minutes. What a contrast is this to the old transit over the Mont Cenis pass by “diligence”! We have the scene depicted in Fig. 179, where we perceive, sliding down or toiling up the steep zigzag ascents, a series of curious vehicles drawn by horses with perpetually jingling bells.

The cost of the Mont Cenis Tunnel was about £3,000,000 sterling, or upwards of £200 per yard; but as a result of the experience gained in this gigantic work, engineers consider that a similar undertaking could now be carried out for half this cost. It is supposed that the profit to the contractors for the Mont Cenis Tunnel was not much less than £100 per yard. The greatest number of men directly employed on the tunnel at one time was 4,000, and the total horse-power of the machinery amounted to 860. From 1857 to 1860, by hand labour alone, 1,646 metres were excavated; from 1861 to 1870 the remaining 10,587 metres were completed by the machines. The most rapid progress made was in May, 1865, in which month the tunnel was driven forward at one end the length of 400 feet. When the workings were being carried through quartz, a very hard rock, the speed was greatly reduced—as, for example, during the month of April, 1866, when the machines could not accomplish more than 35 ft.

The perforators used in the Mont Cenis Tunnel were worked by compressed air, conveyed to a small cylinder, in which it works a piston, to the rod of which the jumper is directly attached. The air, being admitted behind the piston, impels the jumper against the rock, and the tool is then immediately brought back by the opening of a valve, which admits compressed air in front of the piston, at the same time that the air which has driven it forward is allowed to escape, communication with the reservoir of compressed air having previously been closed behind it. The whole of these movements are automatic, and they are effected in the most rapid manner, four or five blows being struck in every second, or between two and three hundred in one minute. Water was constantly forced into the holes, so as to remove the dÉbris as quickly as it was formed. A number of these machines were mounted on one frame, supported on wheels, running on the tramway which was laid along the gallery. The perforators had no connection with each other, for each one had its own tube for the conveyance of compressed air, and its own tube to carry the water used for clearing out the hole, and the cylinders were so fixed on the frames that the jumpers could be directed in any desired manner against any selected portion of the rock. They were driven to an average depth of about 2½ ft., and the process occupied from forty to fifty minutes. When a set of holes had thus been formed, the cylinders were shifted and another series commenced, until about eighty holes had been bored, the formation of the whole number occupying about six or seven hours, and the holes being so arranged that the next operation would detach the rock to the required extent. The flexible tubes, which conveyed the air and water to the machines from the entrances, were then removed from the machines and stowed away, the frame bearing the perforators was drawn back along the tramway, workmen advanced whose duty it was to wipe out the holes, charge them with powder, and fix the fuses ready for the explosion. When the slow-burning match was ignited, all retired behind strong wooden barricades, at a safe distance, until the explosion had taken place; and after the compressed air had been allowed to stream into the working, so as to clear away all the smoke and gas generated by the explosion, the workmen ran up on a special tramway the waggons which were to carry away all the detached stones; and when this had been done, the floor was levelled, the tramways were lengthened, and the frame bearing the drilling machines was brought up to begin a fresh series of operations, which were usually repeated about twice in the course of every twenty-four hours. A great part of the rock consists of very hard calcareous schist, interspersed with veins of quartz, one of the hardest of all rocks, which severely tries the temper of the steel tools, for a few blows on quartz will not unfrequently cause the point of a jumper to snap off.

ROCK-DRILLING MACHINES.

Several forms of rock-drills, or perforators, have been constructed on the same principle as that used in the Mont Cenis Tunnel, and a description of one of them will give a good notion of the general principle of all. We select a form devised by Mr. C. Burleigh, and much used in America, where it has been very successfully employed in driving the Hoosac Tunnel, effecting a saving in the cost of the drilling amounting to one-third of the expense of that operation, and effecting also a still greater saving of time, for the tunnel, which is 5 miles in length, is to be completed in four years, instead of twelve, as the machines make an advance of 150 ft. per month, whereas the rate by hand labour was only 49 ft. per month. These machines are known as the “Burleigh Rock Drills,” and have been patented in England for certain improvements by Mr. T. Brown, who has kindly supplied us with the following particulars:

The Burleigh perforator acts by repeated blows, like Bartlett and Sommeiller’s, but its construction is more simple, and the machine is lighter and not half the size, while its action is even superior in rapidity and force. The Burleigh machines are composed of a single cylinder, the compressed air or steam acting directly on the piston, without the necessity of flywheel, gearing, or shafting. The regular rotation of the drills is obtained by means of a remarkably simple mechanical contrivance. This consists of two grooves, one rectilinear, the other in the form of a spiral cut into the piston-rod. In each of these channels, or grooves, is a pin, which works freely in their interior: these pins are respectively fixed to a concentric ring on the piston-rod. A ratchet wheel holds the ring, and the pin slides into the curve, causing it to turn always in the same direction, without being able to go back. By this eminently simple piece of mechanism, the regular rotation of the drill-holder is secured. The slide-valve is put into motion by the action of a projection, or ball-headed piston-rod, on a double curved momentum-piece, or trigger, which is attached to the slide-rod or spindle by a fork, thus opening and shutting the valve in the ascent and descent of the piston. Fig. 180 represents one of the machines attached in this instance by a clamp to the frame of a tripod. The principal parts of the machine are the cylinder, with its piston, and the cradle with guide-ways, in which the cylinder travels. The action of the piston is similar to that of the ordinary steam hammer, with this difference, that, in addition to the reciprocating, it has also a rotary, motion. The drill-point is held in a slip-socket, or clamp, at the end of the piston-rod, by means of bolts and nuts. The drill-point rotates regularly at each stroke of the piston, making a complete revolution in every eighteen strokes. For hard rocks it is generally made with four cutting edges, in the form of a St. Andrew’s cross, thus striking the rock in seventy-two places in one revolution, each cutting edge chipping off a little of the stone at each stroke in advance of the one preceding. The jumper makes, on an average, 300 blows per minute, and such is the construction of the machine, that the blows are of an elastic, and not of a rigid, nature, thus preventing the drill-point from being soon blunted. It has been found in practice, that a drill-point used in the Burleigh machine can bore on an average 20 ft. of Aberdeen granite without re-sharpening. As the drill pierces the rock, the machine is fed down the guide-ways of the cradle by means of the feed-screw (see Fig. 180), according to the nature of the rock and the progress made. When the cylinder has been fed down the entire length of the feed-screw, and if a greater depth of hole is required, the cylinder is run back, and a longer drill is inserted in the socket at the end of the piston-rod. The universal clamp may be attached to any form of tripod, carriage, or frame, according to the requirements of the work to be done; it enables the machines to work vertically, horizontally, or at any angle.

The following advantages are claimed for this machine: Any labourer can work it; it combines strength, lightness, and compactness in a remarkable degree, is easily handled, and is not liable to get out of order. No part of the mechanism is exposed; it is all enclosed within the cylinder, so there is no risk of its being broken. It is applicable to every form of rockwork, such as tunnelling, mining, quarrying, open cutting, shaft-sinking, or submarine drilling; and in hard rock, like granite, gneiss, ironstone, or quartz, the machine will, according to size, progress at the incredible rate of four inches to twelve inches per minute, and bore holes from ¾ in. up to 5 in. diameter. It will, on an average, go through 120 ft. of rock per day, making forty holes, each from 2 ft. to 3 ft. deep, and it can be used at any angle and in any direction, and will drill and clear itself to any depth up to 20 ft.

The following extract from the “Times,” September 24th, 1873, gives an account of some experiments with the machine, made at the meeting of the British Association in that year, before the members of the Section of Mechanical Science:

“Yesterday, considerable interest was taken in this section, as it had been announced that a ‘Burleigh Rock Drilling Machine’ would be working during the reading of a paper by Mr. John Plant. The machine was not, however, in the room, but was placed in the grounds outside, where it was closely examined by the members after the adjournment, and seen in full operation, boring into an enormous block of granite. The aspect of the machine cannot be called formidable in any respect, for it looks like a big garden syringe, supported upon a splendid tripod; but when at work, under about 80 lbs. pressure of compressed air, it would be deemed a very revolutionary agent indeed, against whose future power the advocates for manual labour in the open quarry, the tunnel, and even the deep mine, may well look aghast. Placed upon a block of granite a yard deep, the machine was handled and its parts moved by the fair hands of many of the lady associates of scientific proclivities; but once the source of power was turned on, the drill began its poundings, eating holes 2 in. in diameter in the block of granite, and making a honeycomb of it as easily as a schoolboy would demolish a sponge cake. It pounds away at the rate of 300 strokes, and progresses forward about 12 in., in the minute, making a complete revolution of the drill in eighteen strokes, and keeping the hole free of the pounded rock. The machine was fixed to work at any angle, almost as readily as a fireman can work his hose; and its adaptation to a wide range of stone-getting, by drilling for blasting, and cutting large blocks for building and engineering, with a saving of capital and labour, was admitted by many members of the section. The tool is called the ‘Burleigh Rock Drill,’ invented by Mr. Charles Burleigh, a gentleman hailing from Massachusetts, United States. The patent is the property of Messrs. T. Brown and Co., of London. The principal feature of this new machine is, that it imitates in every way the action of the quarryman in boring a hole in the rock.”

Many forms of carriages and supports have, from time to time, been made to suit the work for which the ‘Burleigh’ machines have been required. The machine is attached to these carriages, or supports, by means of the universal clamp, by which it can be worked in any direction and at any angle. Of these carriages we select for notice only two forms, one of which is shown in Fig. 181. This carriage can be used to great advantage in adits and drifts. It consists of an upright column, with a screw clamp-nut for holding and raising or lowering the machine, the whole being mounted on a platform which can slide right across the carriage, and thus the machine can be brought to work on any point of a heading. It is secured in position by means of a jack-screw in the top of the column; and as the carriage is mounted on wheels, it is easily moved to permit of blasting. Fig. 182 represents a carriage which is the result of many years’ experience with mining machinery, and it is considered a very perfect appliance. It is constructed of wood and iron, and it runs on wheels. The supports for the machines, four of which may be mounted at once, are two horizontal bars, the lower of which can be raised or lowered, as may be necessary. The two parallel sides of the carriage are joined only at the upper side, and there is nothing to prevent it from being run into the heading, though the way between the rails may be heaped up with broken rock, if only the rails are clear. Drilling, and the removal of the broken rock, may then proceed simultaneously; for, by means of a narrow gauge inside the carriage rails, small cars may be taken right up to the dÉbris. It is made in different sizes, to suit the dimensions of the tunnel required. To give the carriage steadiness in working, it is raised from the wheels by jack-screws, and held in position by screws in a similar manner to the carriage represented in Fig. 181.

An extremely interesting system of drilling rocks—totally different from that on which the machines we have just described are constructed—has, within the last few years, been introduced by Messrs. Beaumont and Appleby. What does the reader think of boring holes in rocks with diamonds? It has long been a matter of common knowledge that the diamond is the hardest of all substances, and that it will scratch and wear down any other substances, while it cannot itself be scratched or worn by anything but diamond. In respect to wearing down or abrading hard stones, the diamond, according to experiments recently made by Major Beaumont, occupies a position over all other gems and minerals to a degree far beyond that which has been generally attributed to it; for in these experiments it was found that on applying a diamond, or rather a piece of the “carbonate” about to be described, fixed in a suitable holder, to a grindstone in rapid rotation, the grindstone was quickly worn down; but on repeating a similar experiment with sapphires and with corundum, it was these which were worn down by the grindstone. Without, on the present occasion, entering into the natural history of the diamond, we may say that there are, besides the pure colourless transparent crystals so highly prized as gems, several varieties of diamond, and that those which are tinged with pink, blue, or yellow, are far from having the same value for the jeweller. Then there is another impure variety called boort, which appears to be employed only to furnish a powder by which the brilliants are ground and polished. In the diamond gravels of Brazil, from which we derive our regular supply of these gems, there was discovered in 1842 a curious variety of dark-coloured diamond, in which the crystalline cleavage, or tendency to split in certain directions (which belongs to the ordinary stones), appears to be almost absent; and the substance might be regarded as a transition form between the diamond and graphite but for its hardness. This substance was until lately used for the same purposes as boort, which is a nearer relative of the pure crystal, and like it, splits along certain planes. It received from the miners the name of “carbonado,” and with regard to the application we are considering, it has turned out to be a sort of Cinderella among diamonds; for its unostentatious appearance is more than compensated for by its surpassing all its more brilliant sisters in the useful property to which reference has been made. This Brazilian term is doubtless the origin of the English name by which the substance in question is known among the English diamond merchants, who call it “carbonate”—an unfortunate word, for it is used in chemistry with an entirely different signification. “Carbonate” it is, however, which supplies the requirements of the rock-drill, and the selected stones are set in a crown, or short tube, of steel, represented by c in Fig. 183. In this they are secured as follows: holes are drilled in the rim of the tube, and each hole is then cut so that a piece of the diamond exactly fits it, and when this piece has been inserted, the metal is drawn round by punches, so as almost to cover the stone, leaving only a point projecting, b b. The portions of the crown between the stones are somewhat hollowed out, as at a, for a purpose which will presently be mentioned. The crown thus set with the boring gems is attached to the end of a steel tube, by which it is made to rotate with a speed of about 250 revolutions per minute while pressed against the rock to be bored. Water is forced through the steel tube, and passing out between the rock and the crown, especially under the hollows, c c, makes its escape between the outside of the boring-tube and the rock, thus washing away all the dÉbris and keeping the drill cool. The pressure with which the crown is forced forward depends, of course, on the nature of the rock to be cut, and varies from 400 lbs. to 800 lbs. In this way the hardest rocks are quickly penetrated—sometimes, for example, at the rate of 4 in. per minute, compact limestone at 3 in., emery at 2 in., and quartz at the rate of 1 in. per minute. It is found that, even after boring through hundreds of feet of such materials, the diamonds are not in the least worn, but as fit for work as before: they are damaged only when by accident one of the stones gets knocked out of its setting; and this machine surpasses all in the rapidity with which it eats its way through the firmest rocks. This, it must be observed, is the special privilege of the diamond drill—that, since the begemmed steel crown and the boring-rods are alike tubular, the rock is worn away in an annular space only, and a solid cylinder of stone is detached from the mass, which cylinder passes up with the hollow rods, where, by means of certain sliding wedges, it is held fast, and is drawn away with the rods.

When the diamond drill is used merely for driving the holes for blasting, this cylinder of rock is not an important matter; but there is an application of the drill where this cylinder is of the greatest value, furnishing as it does a perfect, complete, and easily preserved section of the whole series of strata through which the drill may pass when a bore-hole is sunk in the operation of searching for minerals (which is so significantly called in the United States “prospecting,” a phrase which seems to be making its way in England in mining connections); for the core is uniformly cylindrical, the surface is quite smooth, and any fossils which may be present come up uninjured, so far as they are contained in the solid core, and thus the strata are readily recognized. Contrast this with the old method, where the bore-hole in prospecting is made by the reciprocating action imparted to a steel tool, and merely the pounded material is obtained, usually in very small fragments, by augers or sludge-pumps: the fossils, which might afford the most valuable indications, crushed and perhaps incapable of being recognized; and instead of the beautifully definite and continuous cylinder, a mere mass of dÉbris is brought up. In the prospecting-bores the diameter of the hole is from 2 in. to 7 in. The size adopted depends on the nature of the strata to be penetrated, and on the depth to which it is proposed to carry the boring. When the strata are soft, the operation is commenced with a bore of 7 in., and when this has been carried to an expedient depth, the danger of the sides of the hole falling in is avoided by putting down tubes, and then the diamond drill, fixed to tubes of a somewhat smaller diameter, will be again inserted, and the boring recommenced; or the hole can be widened, so as to receive the lining-tubes. Of course, in boring through hard rocks, such as compact limestones, sandstone, &c., no lining-tubes are necessary.

In a very interesting paper, read before the members of the Midland Institute of Mining Engineers, by Mr. J. K. Gulland, the engineer of the Diamond Rock-Boring Company, who have the exclusive right of working the patents for this remarkable invention, that gentleman concludes by remarking that “the leading feature of the diamond drill is that it works without percussion, thus enabling the holing of rocks to be effected by a far simpler class of machinery than any which has to strike blows. Every mechanical engineer knows, often enough to his cost, that he enters upon a new class of difficulties when he has to recognize it as a normal state of things with any machinery he is designing that portions of it are brought violently to rest. These difficulties increase very much when the power, as in the case of deep bore-holes, has to be conveyed for a considerable distance. Where steel is used a percussive action is necessitated, as, if a scraping action is used, the drill wears quicker than the rock. The extraordinary hardness of the diamond places a new tool in our hands, as its hardness, compared with ordinary rock, say granite, is practically beyond comparison. Putting breakages on one side, a piece of “carbonate” would wear away thousands of times its own bulk of granite. Irrespective of the private and commercial success which this invention has attained, it is a boon to a country such as ours, where minerals constitute in a great measure our national wealth and greatness.”

The advantages of the diamond drill may be illustrated by the case of what is termed the Sub-Wealden Exploration. From certain geological considerations, which need not be entered upon here, several eminent British and continental geologists have arrived at the conclusion that it is probable that coal underlies the Wealden strata of Kent and Sussex, and that it may be perhaps met with at a workable depth. If such should really prove to be the case, the industrial advantages to the south of England would be very great, for the existence of coal so comparatively near to the metropolis would prove not only highly lucrative to the owners of the coal, but confer a direct benefit upon thousands by cheapening the cost of fuel. A number of property owners and scientific men, having resolved that the matter should be tested by a bore, raised funds for the purpose, and a 9 in. bore had been carried down to a depth of 313 ft. in the ordinary manner, when a contract was entered into with the Diamond Rock-Boring Company for a 3 in. bore extracting a cylinder of rock 2 in. in diameter. The company, as a precautionary measure, lined the old hole with a 5 in. steel tube; and in spite of some delay caused by accidents, they increased the depth of the hole to 1,000 ft. in the interval from 2nd February, 1874, to 18th June, 1874–-the progress of the work being regarded with the greatest interest by the scientific world. Unfortunately, the further progress of the work has been prevented by an untoward event, namely, the breaking of the boring-rod, or rather tube; and, although the company is prepared with suitable tackle for extracting the tubes in case of accidents of this kind, and generally succeeds in lifting them by a taper tap, which, entering the hollow of the tube, lays hold of it by a few turns—yet, in this instance, where there have been special difficulties, the extraction of so great a length of tubes is, as the reader may imagine, by no means an easy task. Six attempts have been made to remove the boring-rods which have dropped down; but so difficult has this operation proved, that, all these efforts having failed, it has been decided to abandon the old work and commence a new boring on an adjacent spot. A contract has been entered into with the Diamond Boring Company, who have undertaken to complete the first 1,000 ft. for £600, which is only £200 more than it would have cost to completely line the old bore-holes with iron tubes—an operation which was contemplated by the committee in charge of the exploration. The terms agreed to by the company are very favourable to the promoters of the Sub-Wealden Exploration, although the cost of the second 1,000 ft. will be £3,000 more; and the committee are relying upon the public for contributions to enable them to carry on their enterprise. It is most probable that funds will be forthcoming, and should the boring result in the finding of coal measures beneath the Wealden strata, all the nation will be the richer and participate in the advantages resulting from an undertaking carried on by private persons. Already a totally unexpected source of wealth has been met with by the old bore showing the existence of considerable beds of gypsum in these strata, and the deposits of gypsum are about to be worked. Whether coal be found or not found, there is no doubt that a bore-hole going down 2,000 ft. will greatly increase our geological knowledge, and may reveal facts of which we have at present no conception.

The boring-tubes, it maybe remarked, are made in 6 ft. lengths, and are so contrived that the joints are nearly flush—that is, there is no projection at the junctions of the tubes. Fig. 184 is engraved from a photograph of the machinery used for working the diamond drill when boring a hole for “prospecting.” This looks at first sight a very complicated machine, but in reality each part is quite simple in its action, and is easily understood when its special purpose has been pointed out. We cannot, however, do more than indicate briefly the general nature of the mechanism. The reader will on reflection perceive that, although the idea of causing a rod to rotate in a vertical hole may be simple, yet in practically carrying it out a number of different movements and actions have to be provided for in the machinery. The weight of the rods cannot be thrown on the cutters, nor borne by the moving parts of the machine—hence the movable disc-shaped weights attached to the chains are to balance the weight of the boring-rods as the length of the latter is increased. There must also be a certain amount of feed given to the cutters, regulated and adjusting itself to avoid injurious excess: hence a nut which feeds the drill is encircled by a friction-strap in which it merely slips round without advancing the cutter when the proper pressure is exceeded. There must be means of throwing this into or out of gear, or advancing the tool in the work and of withdrawing it—hence the handles seen attached to the brake-straps. Water must be drawn from some convenient source, and caused to pass down the drill-tube—hence the force-pump seen in the lowest part of the figure. The rods must be raised by steam power and lowered by mechanism under perfect control—hence suitable gearing is provided for that purpose.

The reader may be interested in learning what is the cost of “prospecting” with this unique machinery. The company usually undertake to bore the first 100 ft. for £40, but the next 100 ft. cost £80–-that is, for 200 ft. £120 would be charged; the third 100 ft. would cost £120–-that is to say, the first 300 ft. would cost £240, and so on—each lower 100 ft. costing £40 more than the 100 ft. above it. Some of the holes bored have been of very great depth, and have been executed in a marvellously short space of time. Thus, in 54 days, a depth of 902 ft. was reached at Girrick in a boring for ironstone; another for coal at Beeston reached 1,008 ft.; and at Walluff in Sweden 304½ ft. were put down in one week!

These machines are peculiarly suitable for submarine boring, for they work as well under water as in the air; and they will no doubt be put into requisition in the preliminary experiments about to be made for that great project which bids fair to become a sober fact—the Channel Tunnel between England and France; and as, by the time these pages will be before the public, the work of the greatest and boldest rock-boring yet attempted will have commenced, and the scheme itself will be the theme of every tongue, the Author feels that the present article would be incomplete without some particulars of the great enterprise. [1875.]

THE CHANNEL TUNNEL.

The notion of connecting England and France by a submarine line of railways is not of the latest novelty, but has been from time to time mooted by the engineers of both countries. The most carefully prepared scheme, however, is embodied in the joint propositions of Sir J. Hawkshaw and Messrs. Brunlees and Low among English engineers; and those of M. Gamond on the French side, which these gentlemen have prepared at the invitation of the promoters of the scheme, give the clearest and most authentic account of the considerations on which this gigantic enterprise will be based, and from this document we draw the following passages:

The undersigned engineers, some of whom have been engaged for a series of years in investigating the subject of a tunnel between France and England, having attentively considered those investigations and the facts which they have developed, beg to report thereon jointly for the information of the committee.

These investigations supported the theory that the Straits of Dover were not opened by a sudden disruption of the earth at that point, but had been produced naturally and slowly by the gradual washing away of the upper chalk; that the geological formations beneath the Straits remained in the original order of their deposit, and were identical with the formations of the two shores, and were, in fact, the continuation of those formations.

Mr. Low proposed to dispense entirely with shafts in the sea, and to commence the work by sinking pits on each shore, driving thence, in the first place, two small parallel driftways or galleries from each country, connected at intervals by transverse driftways. By this means the air could be made to circulate as in ordinary coal-mines, and the ventilation be kept perfect at the face of the workings.

Mr. Low laid his plans before the Emperor of the French in April, 1867, and in accordance with the desire of his Majesty, a committee of French and English gentlemen was formed in furtherance of the project.

For some years past Mr. Hawkshaw’s attention has been directed to this subject, and ultimately he was led to test the question, and to ascertain by elaborate investigations whether a submarine tunnel to unite the railways of Great Britain with those of France and the Continent of Europe was practicable.

Accordingly, at the beginning of the year 1866, a boring was commenced at St. Margaret’s Bay, near the South Foreland; and in March, 1866, another boring was commenced on the French coast, at a point about three miles westward of Calais; and simultaneously with these borings an examination was carried on of that portion of the bottom of the Channel lying between the chalk cliffs on each shore.

The principal practical and useful results that the borings have determined are that on the proposed line of the tunnel the depth of the chalk on the English coast is 470 ft. below high water, consisting of 175 ft. of upper or white chalk and 295 ft. of lower or grey chalk; and that on the French coast the depth of the chalk is 750 ft. below high water, consisting of 270 ft. of upper or white chalk and 480 ft. of lower or grey chalk; and that the position of the chalk on the bed of the Channel, ascertained from the examination, nearly corresponds with that which the geological inquiry elicited.

In respect to the execution of the work itself, we consider it proper to drive preliminary driftways or headings under the Channel, the ventilation of which would be accomplished by some of the usual modes adopted in the best coal-mines.

As respects the work itself, the tunnel might be of the ordinary form, and sufficiently large for two lines of railway, and to admit of being worked by locomotive engines, and artificial ventilation could be applied; or it might be deemed advisable, on subsequent consideration, to adopt two single lines of tunnel. The desirability of adopting other modes of traction may be left for future consideration.

Such are the essential passages of the report which, in 1868, was submitted to the Government of the Emperor Louis Napoleon, and was made the subject of a special commission appointed by the Emperor to inquire into the subject in all its bearings. The commission presented its report in 1869, and these are the chief conclusions contained in it:

I. The commission, after having considered the documents relative to the geology of the Straits, which agree in establishing the continuity, homogeneity, and regularity of level of the grey chalk between the two shores of the Channel,

Are of opinion that driving a submarine tunnel in the lower part of this chalk is an undertaking which presents reasonable chances of success.

Nevertheless they would not hide from themselves the fact that its execution is subject to contingencies which may render success impossible.

II. These contingencies maybe included under two heads: either in meeting with ground particularly treacherous—a circumstance which the known character of the grey chalk renders improbable; or in an influx of water in a quantity too great to be mastered, and which might find its way in either by infiltration along the plane of the beds, or through cracks crossing the body of the chalk.

Apart from these contingencies, the work of excavation in a soft rock like grey chalk appears to be relatively easy and rapid; and the execution of a tunnel, under the conditions of the project, is but a matter of time and money.

III. In the actual state of things, and the preparatory investigations being too incomplete to serve as a basis of calculation, the commission will not fix on any figure of expense or the probable time which the execution of the permanent works would require.

The chart, Fig. 185, and the section, Fig. 186, will give an idea of the course of the proposed tunnel, which will connect the two countries almost at the nearest points. The depth of the water in the Channel along the proposed line nowhere exceeds 180 ft.—little more than half the height of St. Paul’s Cathedral, which building would, therefore, if sunk in the midst of the Channel, still form a conspicuous object rising far above the waves. But the tunnel will pass through strata at least 200 ft. below the bottom of the Channel, rising towards each end with a moderate gradient; and from the lower points of these inclines the tunnel will rise slightly with a slope of 1 in 2,640 to the centre, or just sufficient for the purposes of drainage. On the completion of the tunnel a double line of rails will be laid down in it, and trains will run direct from Dover to Calais. Companies have already been formed in England under the presidency of Lord Richard Grosvenor, and in France under that of M. Michel Chevalier, and the legislation of each country has sanctioned the enterprise. Verily the real magician of our times is the engineer, who, by virtually abolishing space, time, and tide, is able to transport us hither and thither, not merely one or two—almost like the magicians we read of in the “Arabian Nights,” with their enchanted horses or wonderful carpets—but by hundreds and by tens of hundreds.

Fig. 185.—Chart of the Channel Tunnel.