The immobile anti-aircraft gun, as distinct from that attached to a travelling carriage such as a motor-car, may be subdivided into two classes. The one is the fixed arm which cannot be moved readily, mounted upon a permanent emplacement; the other is the field-piece which, while fired from a stationary position, may be moved from point to point upon a suitable carriage. The distinction has its parallel in ordinary artillery, the first-named weapon coinciding with the heavy siege gun, which is built into and forms part and parcel of the defensive or offensive scheme, while the second is analogous to the field artillery, which may be wheeled from position to position.

In this phase of artillery the Germans led the way, for the simple reason that they recognised the military value of aerial navigation years in advance of their contemporaries. Again, in this field the Krupp Organisation has played a prominent part. It embarked upon actual construction of weapons while its rivals in other countries were content to prepare their drawings, which were filed against "The Day." But it must not be thought that because the German manufacturers of armaments were ahead of their contemporaries they dominated the situation. Far from it. Their competitors in the market of destruction were every whit as keen, as ingenious, and as enterprising. Kruppism saw a commercial opportunity to profit from advertisement and seized it: its rivals were content to work in secret upon paper, to keep pace with the trend of thought, and to perfect their organisations so as to be ready for the crisis when it developed.

The first Krupp anti-aircraft field-piece was a 6.5 centimetre (2 9/16 inch) arm. It possessed many interesting features, the most salient of which was the design of the axle of the carriage. The rigid axle for the two wheels was replaced by an axle made in two sections, and joined together in the form of a universal coupling, so that each wheel virtually possessed its own axle, or rather half-axle. This was connected with the cradle of the gun in such a manner that the wheels were laterally pivoted thereon.

The result is that each axle can be turned forward together with its wheel, and thus the wheels have their rims brought into line to form an arc of a circle, of which the rear end of the spade of the gun carriage constitutes the centre. This acts as a pivot, about which the gun can be turned, the pair of wheels forming the runners for the achievement of this movement. The setting of the weapon in the firing position or its reversion to the travelling position can be easily and speedily effected merely by the rotation of a handwheel and gearing.

With this gun a maximum elevation of 60 degrees is possible, owing to the trunnions being carried well behind the breech in combination with the system of long steady recoil. The balancing spring which encloses the elevating screw is contained in a protected box. The recoil brake, together with the spring recuperator, follows the usual Krupp practice in connection with ordinary field pieces, as does also the automatic breech-closing and firing mechanism. In fact there is no pronounced deviation from the prevailing Krupp system, and only such modifications as are necessary to adapt the arm to its special duty. When the gun is elevated to high angles the shell, after insertion the breech is prevented from slipping out by means of a special device, so that the proper and automatic closing of the breech is not impaired in any way.

In such an arm as this, which is designed essentially for high-angle firing, the sighting and training facilities require to be carried out upon special lines, inasmuch as the objective is necessarily at a considerable altitude above the horizon of the gun. In other words, in firing at a high inclination, distance between the gun and the target cannot be utilised directly for the back sight. On the other hand, it is essential that in proportion as the angle from the horizontal increases, the back sight should be lowered progressively in a manner corresponding to the distance.

To assist the range-finder in his task of sighting it is necessary that he should be provided with firing tables set out in a convenient form, which, in conjunction with the telemeter, serve to facilitate training for each successive round. In this way it is possible to pick up the range quickly and to keep the objective in the line of fire until it either has been put hors de combat, or has succeeded in retiring beyond the range of the gun.

The sighting arrangements of these Krupp anti-aircraft guns are carried out upon these lines. Beneath the barrel of the back-sight is an observing glass with an eye-piece for the artillerist, while above and behind the observing glass is another eye-piece, to be used in conjunction with the manipulation of the back-sight. The eye-piece of the observation glass is so made that it can be turned through a vertical plane in proportion as the angle of fire increases in relation to the horizontal. The determination of the distance from the objective and from the corresponding back-sight as well as the observation of the altitude is carried out with the aid of the telemeter. This again carries an observation glass fitted with an eye-piece which can be turned in the vertical plane in the same manner as that of the fore-sight. By means of this ingenious sighting device it is possible to ascertain the range and angle of fire very easily and speedily.

The weight of the special Krupp anti-aircraft field-piece, exclusive of the protecting shield, is approximately identical with that of the ordinary light artillery field-piece. It throws a shell weighing 8.8 pounds with an initial velocity of about 2,066 feet per second.

Although the German armament manufacturers were among the first to enter the field with an anti-aircraft gun of this character they were speedily followed by the French, who devised a superior weapon. In fact, the latter represented such a decisive advance that the German artillerists did not hesitate to appropriate their improvements in sundry essential details, and to incorporate them with their own weapons. This applies especially to the differential recoil system which is utilised in the small anti-aircraft guns now mounted upon the roofs of high buildings of cities throughout Germany for the express purpose of repelling aerial attack.

The French system is admitted by the leading artillery technicians of the world to be the finest which has ever been designed, its remarkable success being due to the fact that it takes advantage of the laws of Nature. In this system the gun is drawn back upon its cradle preparatory to firing. In some instances the barrel is compressed against a spring, but in the more modern guns it is forced to rest against a cushion of compressed air contained within a cylinder. When first bringing the gun into action, the barrel is brought into the preliminary position by manually compressing the air or spring by means of a lever. Thereafter the gun works automatically. When the gun is fired the barrel is released and it flies forward. At a critical point in its forward travel the charge is fired and the projectile speeds on its way. The kick or recoil serves to arrest the forward movement of the barrel and finally drives it back again against the strong spring or cushion of compressed air within the cylinder to its normal position, when it is ready for the introduction of the next shell.

The outstanding feature of this system is that the projectile is given a higher initial velocity than is possible with the barrel held rigid at the moment of discharge, because the shell is already travelling at the moment of firing.

The fixed anti-aircraft guns such as are stationed upon eminences and buildings are of the quick firing type, the object being to hurl a steady, continuous stream of missiles upon the swiftly moving aeroplane. Some of the weapons throw a one-pound shell and are closely similar to the pom-pom which proved so effective during the South African war. Machine guns also have been extensively adopted for this duty by all the combatants, their range of approximately 2,000 yards and rapidity of fire being distinctly valuable when hostile aircraft descend to an altitude which brings them within the range of the weapon.

The greatest difficulty in connection with this phase of artillery, however, is not so much the evolution of a serviceable and efficient type of gun, as the determination of the type of projectile which is likely to be most effective. While shrapnel is employed somewhat extensively it has not proved completely satisfactory. It is difficult to set the timing fuse even after the range has been found approximately, which in itself is no easy matter when the aircraft is moving rapidly and irregularly, but reliance is placed thereon in the hope that the machine may happen to be within the cone of dispersion when the shell bursts, and that one or more of the pieces of projectile and bullets may chance to penetrate either the body of the airman or a vital part of the mechanism.

It is this uncertainty which has led to a preference for a direct missile such as the bullet discharged from a machine gun. A stream of missiles, even of rifle calibre, maintained at the rate of some 400 shots per minute is certain to be more effective, provided range and aim are correct, than shrapnel. But the ordinary rifle-bullet, unless the objective is within very close range, is not likely to cause much harm, at least not to the mechanism of the aerial vessel.

It is for this reason that greater attention is being devoted, especially by the French artillerists, to the Chevalier anti-aircraft gun, a weapon perfected by a Swiss technician resident in Great Britain. It projects a formidable missile which in fact is an armour-piercing bullet 1/2- to 3/4-inch in diameter. It is designed for use with an automatic machinegun, which the inventor has devised more or less upon the well-known French system. The bullet has a high velocity—about 2,500 feet per second—and a maximum range of 6,000 to 8,000 feet at the maximum elevation. Should such a missile strike the motor or other mechanism of the vessel it would wreak widespread havoc, and probably cause the machine to come to earth. This arm has been designed for the express purpose of disabling the aeroplane, and not for the subjugation of the airman, which is a minor consideration, inasmuch as he is condemned to a descent when his craft receives a mortal wound.

Attempts have been and still are being made to adapt an explosive projectile to this gun, but so far the measure of success achieved has not proved very promising. There are immense difficulties connected with the design of an explosive shell of this class, charged with a high explosive, especially in connection with the timing. So far as dependence upon percussive detonation is concerned there is practically no difficulty. Should such a missile strike, say, the motor of an aeroplane, or even the hull of the craft itself, the latter would be practically destroyed. But all things considered, it is concluded that more successful results are likely to be achieved by the armour-piercing bullet striking the mechanism than by an explosive projectile.

The Krupp company fully realised the difficulties pertaining to the projectile problem in attacks upon aerial craft. So far as dirigibles are concerned shrapnel is practically useless, inasmuch as even should the bag be riddled by the flying fragments, little effective damage would be wrought—the craft would be able to regain its haven. Accordingly efforts were concentrated upon the perfection of two new types of projectiles, both of which were directed more particularly against the dirigible. The one is the incendiary shell—obus fumigene—while the other is a shell, the contents of which, upon coming into contact with the gas contained within the gas-bag, set up certain chemical reactions which precipitate an explosion and fire.

The incendiary shells are charged with a certain compound which is ignited by means of a fuse during its flight. This fuse arrangement coincides very closely with that attached to ordinary shrapnel, inasmuch as the timing may be set to induce ignition at different periods, such as either at the moment it leaves the gun, before, or when it strikes the envelope of the dirigible. The shell is fitted with a "tracer," that is to say, upon becoming ignited it leaves a trail of smoke, corresponding with the trail of a rocket, so that its passage through the air may be followed with facility. This shell, however, was designed to fulfil a dual. Not only will it fire the gaseous contents out of the dirigible, but it has an explosive effect upon striking an incombustible portion of the aircraft, such as the machinery, propellers or car, when it will cause sufficient damage to throw the craft out of action.

The elaborate trials which were carried out with the obus fumigene certainly were spectacular so as they went. Two small spherical balloons, 10 feet in diameter, and attached to 1,000 feet of cable, were sent aloft. The anti-aircraft guns themselves were placed about 5,100 feet distant. Owing to the inclement weather the balloons were unable to attain a height of more than 200 feet in a direct vertical line above the ground. The guns were trained and fired, but the one balloon was not hit until the second round, while the third escaped injury until the fifth round. When struck they collapsed instantly. Though the test was not particularly conclusive, and afforded no reliable data, one point was ascertained—the trail of smoke emitted by the shell enabled its trajectory to be followed with ease. Upon the conclusion of these trials, which were the most successful recorded, quick-firing tests in the horizontal plane were carried out. The best performance in this instance was the discharge of five rounds in eight seconds. In this instance the paths of the projectiles were simple and easy to follow, the flight of the shell being observed until it fell some 18,670 feet away. But the Krupp firm have found that trials upon the testing ground with a captive balloon differ very materially from stern tests in the field of actual warfare. Practically nothing has been heard of the two projectiles during this war, as they have proved an absolute failure.

Some months ago the world was startled by the announcement that the leading German armament firm had acquired the whole of the interest in an aerial torpedo which had been evolved by the Swedish artillerist, Gustave Unge, and it was predicted that in the next war widespread havoc would be wrought therewith. Remarkable claims were advanced for this projectile, the foremost being that it would travel for a considerable distance through the air and alight upon the objective with infallible accuracy. The torpedo in question was subjected to exacting tests in Great Britain, which failed to substantiate all the claims which were advanced, and it is significant to observe that little has been heard of it during the present conflict. It is urged in certain technical quarters, however, that the aerial torpedo will prove to be the most successful projectile that can be used against aircraft. I shall deal with this question in a later chapter.

During the early days of the war anti-aircraft artillery appeared to be a much overrated arm. The successes placed to its credit were insignificant. This was due to the artillerymen being unfamiliar with the new arm, and the conditions which prevail when firing into space. Since actual practice became possible great advances in marksmanship have been recorded, and the accuracy of such fire to-day is striking. Fortunately the airman possesses the advantage. He can manoeuvre beyond the range of the hostile weapons. At the moment 10,000 feet represents the extreme altitude to which projectiles can be hurled from the arms of this character which are now in use, and they lack destructiveness at that range, for their velocity is virtually expended.

Picking up the range is still as difficult as ever. The practice followed by the Germans serves to indicate the Teuton thoroughness of method in attacking such problems even if success does not ensue. The favourite German principle of disposing anti-aircraft artillery is to divide the territory to be protected into equilateral triangles, the sides of which have a length of about six miles or less, according to the maximum effective range of the pieces at an elevation of 23 1/2 degrees.

The guns are disposed at the corners of the triangles as indicated in Figs. 13-14. Taking the one triangle as an example, the method of picking up the range may be explained as follows. The several guns at the comers of the triangle, each of which can be trained through the 360 degrees in the horizontal plane, are in telephonic touch with an observer O stationed some distance away. The airman A enters the area of the triangle. The observer takes the range and communicates with the gunner B, who fires his weapon. The shell bursts at 1 emitting a red flame and smoke. The observer notes the altitude and relative position of the explosion in regard to the aircraft, while gunner B himself observes whether the shell has burst to the right or to the left of the objective and corrects accordingly. The observer commands C to fire, and another shell is launched which emits a yellow flame and smoke. It bursts at 2 according to the observer, while gunner C also notes whether it is to the right or to the left of the target and corrects accordingly. Now gunner D receives the command to fire and the shell which explodes at 3 throws off a white flame and smoke. Gunner D likewise observes whether there is any deviation to right or left of the target and corrects in a similar manner. From the sum of the three rounds the observer corrects the altitude, completes his calculations, and communicates his instructions for correction to the three gunners, who now merely train their weapons for altitude. The objective is to induce the shells hurled from the three corners of the triangle to burst at a common point 4, which is considered to be the most critical spot for the aviator. The fire is then practically concentrated from the three weapons upon the apex of a triangular cone which is held to bring the machine within the danger zone.

This method of finding the range is carried out quickly—two or three seconds being occupied in the task. In the early days of the war the German anti-aircraft artillerymen proved sadly deficient in this work, but practice improved their fire to a marvellous degree, with the result that at the moment it is dangerous for an aviator to essay his task within an altitude of 6,000 feet, which is the range of the average anti-aircraft gun.

The country occupied by a belligerent is divided up in this manner into a series of triangles. For instance, a machine entering hostile territory from the east, enters the triangle A-B-C, and consequently comes within the range of the guns posted at the comers of the triangle. Directly he crosses the line B-C and enters the adjacent triangle he passes beyond the range of gun A but comes within the range of the gun posted at D, and while within the triangular area is under fire from the guns B-C-D. He turns and crosses the line A-C, but in so doing enters another triangle A-C-E, and comes range of the gun posted at E.

The accompanying diagram represents an area of country divided up into such triangle and the position of the guns, while the circle round the latter indicate the training arc of the weapons, each of which is a complete circle, in the horizontal plane. The dotted line represents the aviator's line of flight, and it will be seen that no matter how he twists and turns he is always within the danger zone while flying over hostile territory. The moment he outdistances one gun he comes within range of another.

The safety of the aviator under these circumstances depends upon his maintaining an altitude exceeding the range of the guns below, the most powerful of which have a range of 8,000 to 10,000 feet, or on speed combined with rapid twisting and turning, or erratic undulating flight, rendering it extremely difficult for the gun-layer to follow his path with sufficient celerity to ensure accurate firing.

At altitudes ranging between 4,000 and 6,000 feet the aeroplane comes within the range of rifle and machine-gun firing. The former, however, unless discharged in volleys with the shots covering a wide area, is not particularly dangerous, inasmuch as the odds are overwhelmingly against the rifleman. He is not accustomed to following and firing upon a rapidly moving objective, the result being that ninety-nine times out of a hundred he fails to register a hit. On the other hand the advantage accruing from machine-gun fire is, that owing to the continuous stream of bullets projected, there is a greater possibility of the gun being trained upon the objective and putting it hors de combat.

But, taking all things into consideration, and notwithstanding the achievements of the artillerist, the advantages are overwhelmingly on the side of the aviator. When one reflects upon the total sum of aircraft which have been brought to earth during the present campaign, it will be realised that the number of prizes is insignificant in comparison with the quantity of ammunition expended.