When the airship and the aeroplane became accepted units of warfare it was only natural that efforts should be concentrated upon the evolution of ways and means to compass their destruction or, at least, to restrict their field of activity. But aircraft appeared to have an immense advantage in combat. They possess virtually unlimited space in which to manoeuvre, and are able to select the elevation from which to hurl their missiles of destruction.

There is another and even more important factor in their favour. A projectile fired, or even dropped, from a height, say of 5,000 feet, is favourably affected by the force of gravity, with the result that it travels towards the earth with accumulating energy and strikes the ground with decisive force.

On the other hand, a missile discharged into space from a weapon on the earth has to combat this action of gravity, which exercises a powerful nullifying influence upon its flight and velocity, far in excess of the mere resistance offered by the air. In other words, whereas the projectile launched from aloft has the downward pull of the earth or gravitational force in its favour, the shell fired from the ground in the reverse direction has to contend against this downward pull and its decelerating effect.

At the time when aircraft entered the realms of warfare very little was known concerning the altitudes to which projectiles could be hurled deliberately. Certain conclusive information upon this point was available in connection with heavy howitzer fire, based on calculations of the respective angles at which the projectile rose into the air and fell to the ground, and of the time the missile took to complete its flight from the gun to the objective. But howitzer fire against aircraft was a sheer impossibility: it was like using a six-inch gun to kill a fly on a window pane at a thousand yards' range. Some years ago certain experiments in aerial firing with a rifle were undertaken in Switzerland. The weapon was set vertically muzzle upwards and discharged. From the time which elapsed between the issue of the bullet from the muzzle until it struck the earth it was possible to make certain deductions, from which it was estimated that the bullet reached an altitude of 600 feet or so. But this was merely conjecture.

Consequently when artillerists entered upon the study of fighting air-craft with small arms and light guns, they were compelled to struggle in the dark to a very pronounced extent, and this darkness was never satisfactorily dispelled until the present war, for the simple reason that there were no means of getting conclusive information. The German armament manufacturers endeavoured to solve the problem by using smoking shells or missiles fitted with what are known as tracers. By following the ascensional path of the projectiles as revealed by the smoke it was possible to draw certain conclusions. But these were by no means convincing or illuminating, as so many factors affected the issue.

Despite the peculiar and complex difficulties associated with the problem it was attacked some what boldly. In this trying field of artillery research the prominent German armament manufacturers, Krupp of Essen and Ehrhardt of Dusseldorf, played a leading part, the result being that before the airship or the aeroplane was received within the military fold, the anti-aircraft gun had been brought into the field of applied science. The sudden levelling-up serves to illustrate the enterprise of the Germans in this respect as well as their perspicacity in connection with the military value of aircraft.

Any gun we can hope to employ against aircraft with some degree of success must fulfil special conditions, for it has to deal with a difficult and elusive foe. Both the lighter-than-air and the heavier than-air craft possess distinctive features and varying degrees of mobility. Taking the first-named, the facility with which it can vary its altitude is a disconcerting factor, and is perplexing to the most skilful gunner, inasmuch as he is called upon to judge and change the range suddenly.

On the other hand, the artilleryman is favoured in certain directions. The range of utility of the airship is severely limited. If its avowed mission is reconnaissance and conclusive information concerning the disposition of forces, artillery and so forth is required, experience has proved that such work cannot be carried out satisfactorily or with any degree of accuracy at a height exceeding 5,000 feet, and a distance beyond six miles. But even under these circumstances the climatic conditions must be extremely favourable. If the elements are unpropitious the airship must venture nearer to its objective. These data were not difficult to collect, inasmuch as they were more or less available from the results of military observations with captive balloons, the conditions being somewhat similar. With the ordinary captive balloon it has been found that, in clear weather, a radius of about 3 3/4 miles at the maximum elevation constitutes its range of reliable utility.

With the aeroplane, however, the conditions are very dissimilar. In the first place the machine owing to its diminutive size as compared with the airship, offers a small and inconspicuous target. Then there is its high independent speed, which is far beyond that of the airship. Furthermore its mobility is greater. It can wheel, turn sharply to the right or to the left, and pursue an irregular undulating flight in the horizontal plane, which renders it well nigh impossible for a gunner to pick it up. The machine moves at a higher relative speed than that at which the gun can be trained. It is the rapid and devious variation which so baffles the gunner, who unless he be highly skilled and patient, is apt to commence to fire wildly after striving for a few moments, and in vain, to pick up the range; he trusts to luck or depends upon blind-shooting, which invariably results in a waste of ammunition.

A gun, to be of tangible destructive efficiency when directed against aircraft, especially those depending upon the gas-bag for equilibrium, must be of special design. It must be capable of firing at an angle only a few degrees less than the absolute vertical, and in order to follow the rapid and involved movements of its objective, must be so mobile that it can be trained through a complete circle at any angle of inclination less than its maximum. At the same time, if the weapon is being used in field operations it must be mounted upon a carriage of adequate mobility to enable it to follow the airship, and thereby keep pace with the latter, so that the aerial craft may be sorely harassed if not actually hit. The automobile is the obvious vehicle for this duty, and it has accordingly been extensively used in this service.

The automobile and the gun mounted thereon follow widely different lines. Some vehicles are designed especially for this duty, while others are improvisations, and be it noted, in passing, that many of the latter have proved more serviceable than the former. Still, the first-named is to be preferred, inasmuch as necessarily it is designed to meet the all-round requirements imposed, and consequently is better able to stand up to the intended work, whereas the extemporised vehicle is only serviceable under favourable conditions.

The Krupp Company has evolved many designs of anti-aircraft motor-driven guns—"Archibalds" the British airmen term them with emphatic levity. They are sturdily-built vehicles fitted with heavy motors, developing from 40 to 50 horse-power, with the chassis not widely dissimilar from that adopted for motor-omnibus traffic. Consequently, they are not necessarily condemned to the high-roads, but within certain limits are able to travel across country, i.e., upon fields or other level expanses, where the soil is not unduly soft.

But the very character of the problem rendered the evolution of the vehicle a somewhat perplexing matter. There were many factors which had to be taken into consideration, and it was possible to meet the imposed requirements only within certain limits. In the first place, the weight of the gun itself had to be kept down. It was obviously useless to overload the chassis. Again, the weight of the projectile and its velocity had to be borne in mind. A high velocity was imperative. Accordingly, an initial velocity varying from 2,200 to 2,700 feet per second, according to the calibre of the gun, was determined.

Moreover, as mobility was an indispensable condition, the gun had to be so mounted that it could be fired from the motor-car even if the latter were travelling at high speed. This requirement entailed another difficulty. The gun had to be mounted in such a manner as to enable the gunner to train it easily and readily through the complete circle and through its complete range of vertical inclination. As the result of prolonged experiments it was ascertained that the most suitable arrangement was a pedestal mounting, either within a turret or upon an open deck. To meet the weight of the gun, as well as the strains and stresses incidental to firing, the chassis was strengthened, especially over the rear axle near which the mounting is placed.

The heaviest gun of this type is the 10.5 centimetre (4 1/4-inch) quick-firer, throwing a shell weighing nearly forty pounds, with an initial velocity of 2,333 feet per second. This "Archibald" is totally unprotected. The gun is mounted centrally upon the carriage over the rear axle, and occupies the centre of the deck between the driver's seat and that of the gun crew behind. The whole of the deck is clear, thereby offering no obstruction to the gunner in training the weapon, while the space may be widened by dropping down the wings of the vehicle. At the rear is a seat to accommodate the gun crew, beneath which the ammunition is stowed. When travelling and out of action, the gun lies horizontally, the muzzle pointing from the rear of the car.

To reduce the strains arising from firing, the arm is fitted with what is known as the "differential recoil." Above the breach is an air recuperator and a piston, while there is no hydraulic brake such as is generally used. The compressor is kept under compression while the car is travelling with the gun out of action, so that the arm is available for instant firing. This is a departure from the general practice in connection with such weapons. When the gun is loaded the bolt which holds the compressor back is withdrawn, either by the hand for manual firing, or by the action of the automatic closing of the breech when the arm is being used as a quick-firer. In firing the gun is thrown forward under the pressure of the released air which occurs at the moment of discharge. The energy of the recoil brings the gun back and at the same time recharges the compressed air reservoir.

The gun is so mounted upon its pedestal as to enable a maximum vertical inclination of 75 degrees to be obtained. The mounting system also enables the weapon to be trained in any desired direction up to the foregoing maximum elevation throughout a complete circle, and it can be handled with ease and celerity. A smaller "Archibald" is the 7.5 centimetre (3-inch gun) throwing a 14.3 pound shell at an initial velocity of about 2,170 feet per second.

The turret anti-aircraft gun carried upon a motor-car differs from the foregoing very considerably. This is a protected arm. The gun of 7.1 centimetres—approximately 2.75 inches—is mounted in the same manner upon the car-deck and over the driving axle, but is enclosed within a sheet steel turret, which is proof against rifle and machine-gun fire. This turret resembles the conning-tower of a battleship, and is sufficiently spacious to house the whole of the gun crew, the internal diameter being about seven feet. Access to the turret is obtained through a rear door. This gun has a maximum elevation of about 75 degrees, while its operation and mechanism are similar to those of the unprotected weapon.

The vehicle itself is practically identical with the armoured motor-car, which has played such an important part during the present campaign, the driver being protected by a bullet-proof steel screen similar in design to the ordinary glass wind-screen fitted to touring automobiles. This is carried sufficiently high to offer complete protection to his head when seated at the wheel, while through a small orifice in this shield he is able to obtain a clear view of the road. The engine and its vital parts are also adequately protected. The ammunition is carried in a cupboard-like recess forming part of the driver's seat, encased in bullet-proof steel sheeting with flap-doors. This device enables the shells to be withdrawn readily from the side of the car and passed to the crew within the turret. The caisson is of sufficient dimensions to receive 69 shells.

The Ehrhardt airship fighting ordnance is similarly adapted to motor-car operations, one type being especially powerful. The whole of the vehicle is encased in armour-plating impervious to rifle and machine-gun fire. The driver is provided with a small orifice through which he is able to obtain a clear uninterrupted view of the road ahead, while the armouring over the tonneau is carried to a sufficient height to allow head-room to the gun crew when standing at the gun. All four wheels are of the disk type and fashioned from heavy sheet steel. The motor develops 40-50 horse-power and, in one type, in order to mitigate the risk of breakdown or disablement, all four wheels are driven. The gun, a small quick-firer, is mounted on a pedestal in a projecting conning-tower. The mounting is placed behind the driver's seat, and is trained and operated from the tonneau. The maximum elevation is 75 degrees, and like the gun carriage bearing the tube guide it can be moved through a complete circle, being free to rotate in the fixed pivot jack to enable this end to be attained.

The foregoing may be said to represent the most powerful types of mobile anti-aircraft weapons used by the Austro-German forces to-day. Arms of similar design, roughly speaking, have also been introduced into the French and Russian services. In addition many semi-armoured weapons of this character are in operation, some specially built for the work, while others have been improvised. In the semi-armoured motor-car the carriage follows the usual lines; it has an open top, the armouring comprising the body of the tonneau and the diskwheels, which are made of light bullet-proof steel. Here again the prevailing practice is to mount the gun as nearly above the rear axle as possible, and to work it from the tonneau. The maximum elevation is also 75 degrees, with training throughout the entire circle.

Another type comprises a very light machine gun of rifle calibre, and this is intended for attachment to an ordinary motor car. There is a pedestal mounting which can be set within the tonneau, while the weapon is pivoted in an outrigger, the latter being free to rotate in its pivot jack. This arrangement enables the arm to cover a wide range, while it also admits of training through an extensive angle of elevation.

The Allied forces improvised travelling anti-aircraft offences by mounting the latest types of Vickers, Hotchkiss, and other machine guns in armoured motor cars. Some of these have the domed turret form, with the gun projecting through the roof, while others are protected against hostile attack from the side only, the carriage being panelled with bullet-proof steel sheeting. While such weapons are useful, inasmuch as they can maintain a hot fire ranging up to 750 shots per minute, they are not to be compared with the "Archibalds," which are able to throw heavy shrapnel and incendiary shells, and have a vertical range of about 6,000 to 8,000 feet.

The improvised motor-gun has not proved a complete success, except in those instances when the hostile aircraft has ventured to approach somewhat closely to the ground. The more formidable weapons cannot be mounted upon ordinary vehicles, inasmuch as the increase in weight, which is appreciable, impairs the efficiency of the vehicle, and at the same time enhances the possibility of breakdown at a critical moment. For such arms a special and substantial chassis is imperative, while the motive power and gearing must be adapted to the circumstances.

Motor-mounted anti-aircraft weapons, however, have not proved an unqualified success. The fact that the vehicles are condemned to the high roads, or at least to comparatively smooth and level ground, constitutes a severe handicap. Again, when travelling at high speed, and this is essential when pursuing a fast aeroplane, the accurate laying of the weapon is extremely difficult, owing to the oscillation of the vehicle itself, especially if the road surface is in a bad condition. The sighting arrangements are of a wonderfully complete character, as described elsewhere, but the irregular rolling movement arising from high speed is a nullifying quantity. It is tolerably easy for the aircraft, especially an aeroplane, to evade successful pursuit, either by rising to an elevation beyond the range of the gun, or by carrying out baffling evolutions such as irregular undulating flight, wheeling, and climbing. According to the reports of the British and French airmen the "Archibald" has failed to establish the glowing reputation which was anticipated, for the simple reason that, unless it has a clear straight road and can maintain its high speed, it can easily be out-distanced by the fleet human bird.

The motor-car suffers from another serious disability. It cannot manoeuvre with sufficient celerity. For instance, if it is necessary to turn round in a narrow lane, valuable time is lost in the process, and this the airman turns to account. In hilly country it is at a still greater disadvantage, the inclines, gradients, and sinuosities of the roads restricting its effectiveness very pronouncedly. It must also be remembered that, relatively speaking, the "Archibald" offers a better target to the airman than the aeroplane offers to the man behind the anti-aircraft gun on the motor below. A few well-placed bombs are sufficient to induce the pursuers to cease their activities. Even if the missiles fail to strike the motor-car itself they can wreak disaster in directly by rendering the road impassable or dangerous to negotiate at high speed. On the whole therefore, the "Archibald" is a greatly exaggerated weapon of offence against aircraft, and, so far as is known, has failed to fulfil expectations. In fact, the Germans have practically abandoned the idea of using it in the manner of a pursuing arm; they work the weapon as a fixture, depending upon the car merely as a means of moving it from point to point. Thus, in reality, it has been converted into a light field-piece, and may almost be included in the category of fixed weapons for combating aerial operations.