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.
While the anti-aircraft gun represents the only force which has been brought to the practical stage for repelling aerial attack, and incidentally is the sole offensive weapon which has established its effectiveness, many other schemes have been devised and suggested to consummate these ends. While some of these schemes are wildly fantastic, others are feasible within certain limitations, as for instance when directed against dirigibles.
It has been argued that the atmosphere is akin to the salt seas; that an aerial vessel in its particular element is confronted with dangers identical with those prevailing among the waters of the earth. But such an analogy is fallacious: there is no more similarity between the air and the ocean than there is between an airship and a man-of-war. The waters of the earth conceal from sight innumerable obstructions, such as rocks, shoals, sandbanks, and other dangers which cannot by any means be readily detected.
But no such impediments are encountered in the ether. The craft of the air is virtually a free age in the three dimensions. It can go whither it will without let or hindrance so long as the mechanical agencies of man are able to cope with the influences of Nature. It can ascend to a height which is out of all proportion to the depth to which the submarine can descend in safety. It is a matter of current knowledge that a submarine cannot sink to a depth of more than 250 feet: an aerial vessel is able to ascend to 5,000, 8,000, or even 10,000 feet above the earth, and the higher the altitude it attains the greater is its degree of safety. The limit of ascension is governed merely by the physical capacities of those who are responsible for the aerial vessel's movement.
It is for this reason that the defensive measures which are practised in the waters of the earth are inapplicable to the atmosphere. Movement by, or in, water is governed by the depth of channels, and these may be rendered impassable or dangerous to negotiate by the planting of mines. A passing ship or submarine may circumvent these explosive obstructions, but such a successful manoeuvre is generally a matter of good luck. So far as submarines are concerned the fact must not be over looked that movements in the sea are carried out under blind conditions: the navigator is unable to see where he is going; the optic faculty is rendered nugatory. Contrast the disability of the submarine with the privileges of its consort in the air. The latter is able to profit from vision. The aerial navigator is able to see every inch of his way, at least during daylight. When darkness falls he is condemned to the same helplessness as his confrere in the waters below.
A well-known British authority upon aviation suggested that advantage should be taken of this disability, and that the air should be mined during periods of darkness and fog to secure protection against aerial invasion. At first sight the proposal appears to be absolutely grotesque, but a little reflection will suffice to demonstrate its possibilities when the area to be defended is comparatively limited. The suggestion merely proposes to profit from one defect of the dirigible. The latter, when bent upon a daring expedition, naturally prefers to make a bee-line towards its objective: fuel considerations as a matter of fact compel it to do so. Consequently it is possible, within certain limits, to anticipate the route which an invading craft will follow: the course is practically as obvious as if the vessel were condemned to a narrow lane marked out by sign-posts. Moreover, if approaching under cover of night or during thick weather, it will metaphorically "hug the ground." To attempt to complete its task at a great height is to court failure, as the range of vision is necessarily so limited.
Under these circumstances the mining of the air could be carried out upon the obvious approaches to a threatened area. The mines, comprising large charges of high-explosive and combustible material, would be attached to small captive balloons similar to the "sounding balloons" which are so much used by meteorologists in operations for sounding the upper strata of the atmosphere. These pilot balloons would be captive, their thin wires being wound upon winches planted at close intervals along the coast-line. The balloon-mines themselves would be sent to varying heights, ranging from 1,000 to 5,000 feet, and with several attached to each cable, the disposition of the mines in the air in such an irregular manner being in fact closely similar to the practice adopted in the mining of a channel for protection against submarines and hostile ships.
The suggestion is that these mines should be sent aloft at dusk or upon the approach of thick and foggy weather, and should be wound in at dawn or when the atmosphere cleared, inasmuch as in fine weather the floating aerial menace would be readily detected by the pilot of a dirigible, and would be carefully avoided. If the network were sufficiently intricate it would not be easy for an airship travelling at night or in foggy weather to steer clear of danger, for the wires holding the balloons captive would be difficult to distinguish.
The mines would depend upon detonators to complete their work, and here again they would bear a close resemblance to sea-mines. By looping the mines their deadliness could be increased. The unsuspicious airship, advancing under cover of darkness or thick weather, might foul one of the wires, and, driving forward, would tend to pull one or more mines against itself. Under the force of the impact, no matter how gentle, or slight, one or more of the detonating levers would be moved, causing the mine to explode, thus bursting the lifting bag of the vessel, and firing its gaseous contents. An alternative method, especially when a cable carried only a single mine, would be to wind in the captive balloon directly the wire was fouled by an invading aerial craft, the process being continued until the mine was brought against the vessel and thereby detonated.
Another proposed mining method differs materially in its application. In this instance it is suggested that the mines should be sent aloft, but should not be of the contact type, and should not be fired by impact detonators, but that dependence should be placed rather upon the disturbing forces of a severe concussion in the air. The mines would be floating aloft, and the advance of the airship would be detected. The elevation of the mines in the vicinity of the invading craft would be known, while the altitude of the airship in relation thereto could be calculated. Then, it is proposed that a mine within d certain radius of the approaching craft, and, of course, below it, should be fired electrically from the ground. It is maintained that if the charge were sufficiently heavy and an adequate sheet of flame were produced as a result of the ignition, an airship within a hundred yards thereof would be imperilled seriously, while the other mines would also be fired, communicating ignition from one to the other. The equilibrium of the airship is so delicate that it can be readily upset, and taking into account the facts that gas is always exuding from the bag, and that hydrogen has a tendency to spread somewhat in the manner of oil upon water, it is argued that the gas would be ignited, and would bring about the explosion of the airship.
Another method has even been advocated. It is averred in authoritative circles that when the aerial invasion in force of Great Britain is attempted, the Zeppelins will advance under the cover of clouds. Also that the craft will make for one objective—London. Doubtless advantage will be taken of clouds, inasmuch as they will extend a measure of protection to the craft, and will probably enable the invading fleet to elude the vigilance of the aeroplane scouts and patrols. Under these circumstances it is suggested that balloon-mines should be sent aloft and be concealed in the clouds. It would be impossible to detect the wires holding them captive, so that the precise location of the lurking danger would not be divined by the invader. Of course, the chances are that the invading airship would unconsciously miss the mines; on the other hand the possibilities are equally great that it would blunder into one of these traps and be blown to atoms.
An English airman has recently suggested a means of mining invading Zeppelins which differs completely from the foregoing proposals. His idea is that aeroplanes should be equipped with small mines of the contact type, charged with high explosives, and that the latter should be lowered from the aeroplane and be trawled through the atmosphere. As an illustration I will suppose that a hostile aircraft is sighted by a patrolling aeroplane. The pilot's companion in the latter immediately prepares his aerial mine, fixing the detonator, and attaching the mine to the wire. The latter is then dropped overboard, the wire being paid out from a winch until it has descended to the level of the hostile craft. The airman now manoeuvres in the air circling about the airship, dragging his mine behind him, and endeavouring to throw it across or to bring it into contact with the airship below. Naturally the latter, directly it observed the airman's object, would endeavour to elude the pursuing trawling mine, either by crowding on speed or by rising to a greater altitude. The aeroplane, however, would have the advantage both in point of speed and powers of climbing, while there is no doubt that the sight of the mine swinging in the air would exert a decisive moral effect upon those in the airship.
Attempts to render the mine harmless by discharging it prematurely with the aid of rifle and machine-gun fire would, of course, be made by the crew of the airship, but the trawling mine would prove a very difficult target to strike. If such a missile were used against an airship of the proportions of a Zeppelin the mine would inevitably be trawled across the vessel sooner or later. Once the airship had been fouled, the aviator would merely have to drive ahead, dragging the wire and its charge across the gas-bag until at last one of the contact levers of the mine was moved by being dragged against some part of the vessel, when the mine would be exploded. In such operations the aviator would run a certain risk, as he would be more or less above the airship, and to a certain degree within the zone of the ultimate explosion. But there is no doubt that he would succeed in his "fishing" exploit within a very short time.
This ingenious scheme has already been tested upon a small scale and has been found effective, the trawling bomb being drawn across its target and fired by contact within a few minutes. The experiment seems to prove that it would be simpler and more effectual to attack a hostile aircraft such as a Zeppelin in this manner than to drop free bombs at random. Moreover, we cannot doubt that the sight of a mine containing even ten or twelve pounds of high explosive dangling at the end of a wire would precipitate a retreat on the part of an airship more speedily than any other combative expedient.
The advocate of this mine-trawling method, who is a well-known aviator, anticipates no difficulty in manoeuvring a mine weighing 30 pounds at the end of 300 feet of fine wire. Success depends in a great measure on the skill of the aviator in maintaining a constant tension upon the line until it falls across its objective.
The process calls for a certain manifestation of skill in manoeuvring the aeroplane in relation to the airship, judgment of distance, and ability to operate the aeroplane speedily. The rapid ascensional capability of the airship, as compared with that of the aeroplane, is a disadvantage, but on the other hand, the superior mobility and speed of the aeroplane would tell decisively for success.
Among the many wonders which the Krupp organisation is stated to have perfected, and which it is claimed will create considerable surprise, is the aerial torpedo. Many of the Krupp claims are wildly chimerical, as events have already proved, but there is no doubt that considerable effort has been expended upon this latest missile, for which the firm is said to have paid the inventor upwards of L25,000—$125,000. Curiously enough the projectile was perfected within gunshot of the British aerodrome of Hendon and is stated to have been offered to the British Government at the time, and to have met with a chilling reception. One fact, however, is well established. The inventor went to Germany, and submitted his idea to Krupp, by whom it was tested without delay. Upon the completion of the purchase, the great armament manufacturers did not fail to publish broadcast the fact that they had acquired a mysterious new terror of the skies. That was some three years ago, and in the interval the cleverest brains of the German firm have been steadily devoting their time and energies to the improvement of the missile, the first appearance of which was recorded, in a somewhat hazy manner, in the closing days of December.
While the exact mechanism of this missile is a secret, the governing principles of its design and operation are known to a select few technicians in this country. Strange to say, the projectile was designed in the first instance in the interests of peace and humanity, but while engaged upon his experiments the inventor suddenly concluded that it would be a more profitable asset if devoted to the grim game of war. At the time the military significance of the airship and the aeroplane were becoming apparent; hence the sudden diversion of the idea into a destructive channel.
This aerial torpedo is a small missile carrying a charge of high explosive, such as trinitrotoluene, and depends for its detonation upon impact or a time fuse. It is launched into the air from a cradle in the manner of the ordinary torpedo, but the initial velocity is low. The torpedo is fitted with its own motive power, which comes automatically into action as the missile climbs into the air. This self-contained energy is so devised that the maximum power is attained before the missile has lost the velocity imparted in the first instance, the result being that it is able to continue its flight in a horizontal direction from the moment it attains the highest point in its trajectory, which is naturally varied according to requirements. But there is no secret about the means of propulsion. The body is charged with a slow-burning combustible, in the manner of the ordinary rocket, whereby it is given a rapid rotary motion.
Furthermore it is stated to be fitted with a small gyroscope in the manner of the torpedo used in the seas, for the purpose of maintaining direction during flight, but upon this point there is considerable divergence of opinion among technicians, the general idea being that the torpedo depends upon an application of the principle of the ordinary rocket rather than upon a small engine such as is fitted to the ordinary torpedo. The employment of a slow combustible ensures the maintenance of the missile in the air for a period exceeding that of the ordinary shell. It is claimed by the Germans that this projectile will keep aloft for half-an-hour or more, but this is a phantasy. Its maintenance of flight is merely a matter of minutes.
The belated appearance of this much-lauded projectile and its restricted use suggest that it is unreliable, and perhaps no more effective than the aerial torpedo which appeared in the United States during the Spanish-American War, and proved a complete failure. An effective and reliable means of combating or frustrating a dirigible attack, other than by gun-fire or resort to the drastic remedy of ramming the enemy, has yet to be devised.
In a previous chapter the various methods of signalling between the ground and the airman aloft have been described. Seeing that wireless telegraphy has made such enormous strides and has advanced to such a degree of perfection, one naturally would conclude that it constitutes an ideal system of communication under such conditions in military operations.
But this is not the case. Wireless is utilised only to a very limited extent. This is due to two causes. The one is of a technical, the other of a strategical character.
The uninitiated, bearing in mind the comparative ease with which wireless installations may be established at a relatively small expense, would not unreasonably think that no serious difficulties of a technical character could arise: at least none which would defy solution. But these difficulties exist in two or three different fields, each of which is peculiarly complex and demands individual treatment.
In the first place, there is the weight of the necessary installation. In the case of the dirigible this may be a secondary consideration, but with the aeroplane it is a matter of primary and vital importance. Again, under present conditions, the noise of the motor is apt to render the intelligent deciphering of messages while aloft a matter of extreme difficulty, especially as these are communicated in code. The engine noise might be effectively overcome by the use of a muffler such as, is used with automobiles, but then there is the further difficulty of vibration.
This problem is being attacked in an ingenious manner. It is proposed to substitute for audible signals visual interpretations, by the aid of an electric lamp, the fluctuations in which would correspond to the dots and dashes of the Morse code. Thus the airman would read his messages by sight instead of by sound.
This method, however, is quite in its infancy, and although attractive in theory and fascinating as a laboratory experiment or when conducted under experimental conditions, it has not proved reliable or effective in aeronautical operations. But at the same time it indicates a promising line of research and development.
Then there are the problems of weight and the aerial. So far as present knowledge goes, the most satisfactory form of aerial yet exploited is that known as the trailing wire. From 300 to 700 feet of wire are coiled upon a reel, and when aloft this wire is paid out so that it hangs below the aeroplane. As a matter of fact, when the machine is travelling at high speed it trails horizontally astern, but this is immaterial. One investigator, who strongly disapproves of the trailing aerial, has carried out experiments with a network of wires laid upon and attached to the surface of the aeroplane's wings. But the trailing wire is generally preferred, and certainly up to the present has proved more satisfactory.
The greatest obstacle, however, is the necessary apparatus. The average aeroplane designed for military duty is already loaded to the maximum. As a rule it carries the pilot and an observer, and invariably includes a light arm for defence against an aerial enemy, together with an adequate supply of ammunition, while unless short sharp flights are to be made, the fuel supply represents an appreciable load. Under these circumstances the item of weight is a vital consideration. It must be kept within a limit of 100 pounds, and the less the equipment weighs the more satisfactory it is likely to prove, other things being equal.
The two most successful systems yet exploited are the Dubilier and the Rouget. The former is an American invention, the latter is of French origin. Both have been tested by the British Military Aeronautical Department, and the French authorities have subjected the French system to rigorous trials. Both systems, within their limitations, have proved satisfactory.
The outstanding feature of the Dubilier system is the production of sine waves of musical frequency from continuous current, thus dispensing with the rotary converter. The operating principle is the obtaining of a series of unidirectional impulses by a condenser discharge, the pulsating currents following one another at regular intervals at a frequency of 500 impulses per second, which may be augmented up to 1,000 impulses per second. The complete weight of such an apparatus is 40 pounds; the electric generator, which is no larger than the motor used for driving the ordinary table ventilating fan, accounts for 16 pounds of this total. Under test at sea, upon the deck of a ship, a range of 250 miles has been obtained. The British Government carried out a series of experiments with this system, using a small plant weighing about 30 pounds, with which communication was maintained up to about 20 miles.
In the French system the Reuget transmitter is employed. The apparatus, including the dynamo, which is extremely small, weighs in all 70 pounds. A small alternator of 200 watts and 100 volts is coupled direct to the aeroplane motor, a new clutch coupler being employed for this purpose. By means of a small transformer the voltage is raised to 30,000 volts, at which the condenser is charged. In this instance the musical spark method is employed.
The whole of the high tension wiring is placed within a small space so as not to endanger the pilot, while the transformer is hermetically sealed in a box with paraffin. The aerial comprises a trailing wire 100 feet in length, which, however, can be wound in upon its reel within 15 seconds. This reeled antenna, moreover, is fitted with a safety device whereby the wire can be cut adrift in the event of an accident befalling the aeroplane and necessitating an abrupt descent. With this apparatus the French authorities have been able to maintain communication over a distance of 30 miles.
In maintaining ethereal communication with aeroplanes, however, a portable or mobile station upon the ground is requisite, and this station must be within the radius of the aerial transmitter, if messages are to be received from aloft with any degree of accuracy and reliability. Thus it will be recognised that the land station is as important as the aeroplane equipment, and demands similar consideration.
A wide variety of systems have been employed to meet these conditions. There is the travelling automobile station, in which the installation is mounted upon a motor-car. In this instance the whole equipment is carried upon a single vehicle, while the antenna is stowed upon the roof and can be raised or lowered within a few seconds. If motor traction is unavailable, then animal haulage may be employed, but in this instance the installation is divided between two vehicles, one carrying the transmitting and receiving apparatus and the generating plant, the other the fuel supplies and the aerial, together with spare parts.
The motive power is supplied by a small air cooled petrol or gasoline motor developing eight horse-power, and coupled direct to a 2-kilo watt alternator. At one end of the shaft of the latter the disk discharger is mounted, its function being to break up the train of waves into groups of waves, so as to impart a musical sound to the note produced in the receiver. A flexible cable transmits the electric current from the generator to the wagon containing the instruments. The aerial is built up of masts carried in sections.
The Germans employ a mobile apparatus which is very similar, but in this instance the mast is telescopic. When closed it occupies but little space. By turning the winch handle the mast is extended, and can be carried to any height up to a maximum of about 100 feet. The capacity of these mobile stations varies within wide limits, the range of the largest and most powerful installations being about 200 miles. The disadvantage of these systems, however, is that they are condemned to territories where the ground at the utmost is gently undulating, and where there are roads on which four-wheeled vehicles can travel.
For operation in hilly districts, where only trails are to be found, the Marconi Company, has perfected what may be described as "pack" and "knapsack" installations respectively. In the first named the whole of the installation is mounted upon the backs of four horses. The first carries the generator set, the second the transmitting instruments, the third the receiving equipment, and the fourth the detachable mast and stays.
The generator is carried upon the horse's saddle, and is fitted with a pair of legs on each side. On one side of the saddle is mounted a small highspeed explosion motor, while on the opposite side, in axial alignment with the motor, is a small dynamo. When it is desired to erect the installation the saddle carrying this set is removed from the horse's back and placed upon the ground, the legs acting as the support. A length of shaft is then slipped into sockets at the inner ends of the motor and dynamo shafts respectively, thus coupling them directly, while the current is transmitted through a short length of flexible cable to the instruments. The mast itself is made in lengths of about four feet, which are slipped together in the manner of the sections of a fishing rod, and erected, being supported by means of wire guys. In this manner an antenna from 40 to 50 feet in height may be obtained.
The feature of this set is its compactness, the equal division of the sections of the installation, and the celerity with which the station may be set up and dismantled in extremely mountainous country such as the Vosges, where it is even difficult for a pack-horse to climb to commanding or suitable positions, there is still another set which has been perfected by the Marconi Company. This is the "knapsack" set, in which the whole of the installation, necessarily light, small, and compact, is divided among four men, and carried in the manner of knapsacks upon their backs. Although necessarily of limited radius, such an installation is adequate for communication within the restricted range of air-craft.
Greater difficulties have to be overcome in the mounting of a wireless installation upon a dirigible. When the Zeppelin was finally accepted by the German Government, the military authorities emphasised the great part which wireless telegraphy was destined to play in connection with such craft. But have these anticipations been fulfilled? By no means, as a little reflection will suffice to prove.
In the first place, a wireless outfit is about the most dangerous piece of equipment which could be carried by such a craft as the Zeppelin unless it is exceptionally well protected. As is well known the rigidity of this type of airship is dependent upon a large and complicated network of aluminium, which constitutes the frame. Such a huge mass of metal constitutes an excellent collector of electricity from the atmosphere; it becomes charged to the maximum with electricity.
In this manner a formidable contributory source of danger to the airship is formed. In fact, this was the reason why "Z-IV" vanished suddenly in smoke and flame upon falling foul of the branches of trees during its descent. At the time the Zeppelin was a highly charged electrical machine or battery as it were, insulated by the surrounding air. Directly the airship touched the trees a short circuit was established, and the resultant spark sufficed to fire the gas, which is continuously exuding from the gas bags.
After this accident minute calculations were made and it was ascertained that a potential difference of no less than 100,00 volts existed between the framework of the dirigible and the trees. This tension sufficed to produce a spark 4 inches in length. It is not surprising that the establishment of the electric equilibrium by contact with the trees, which produced such a spark should fire the hydrogen inflation charge. In fact the heat generated was so intense that the aluminium metallic framework was fused. The measurements which were made proved that the gas was consumed within 15 seconds and the envelope destroyed within 20 seconds.
As a result of this disaster endeavours were made to persuade Count Zeppelin to abandon the use of aluminium for the framework of his balloon but they were fruitless, a result no doubt due to the fact that the inventor of the airship of this name has but a superficial knowledge of the various sciences which bear upon aeronautics, and fully illustrates the truth of the old adage that "a little learning is a dangerous thing." Count Zeppelin continues to work upon his original lines, but the danger of his system of construction was not lost upon another German investigator, Professor Schiitte, who forthwith embarked upon the construction of another rigid system, similar to that of Zeppelin, at Lanz. In this vessel aluminium was completely abandoned in favour of a framework of ash and poplar.
The fact that the aluminium constituted a dangerous collector of electricity rendered the installation of wireless upon the Zeppelin not only perilous but difficult. Very serious disturbances of an electrical nature were set up, with the result that wireless communication between the travelling dirigible and the ground below was rendered extremely uncertain. In fact, it has never yet been possible to communicate over distances exceeding about 150 miles. Apart from this defect, the danger of operating the wireless is obvious, and it is generally believed in technical circles that the majority of the Zeppelin disasters from fire have been directly attributable to this, especially those disasters which have occurred when the vessel has suddenly exploded before coming into contact with terrestrial obstructions.
In the later vessels of this type the wireless installation is housed in a well insulated compartment. This insulation has been carried, to an extreme degree, which indicates that at last the authorities have recognised the serious menace that wireless offers to the safety of the craft, with the result that every protective device to avoid disaster from this cause has been freely adopted.
The fact that it is not possible to maintain communication over a distance exceeding some 20 miles is a severe handicap to the progressive development of wireless telegraphy in this field. It is a totally inadequate radius when the operations of the present war are borne in mind. A round journey of 200, or even more miles is considered a mere jaunt; it is the long distance flight which counts, and which contributes to the value of an airman's observations. The general impression is that the fighting line or zone comprises merely two or three successive stretches of trenches and other defences, representing a belt five miles or so in width, but this is a fallacy. The fighting zone is at least 20 miles in width; that is to say, the occupied territory in which vital movements take place represents a distance of 20 miles from the foremost line of trenches to the extreme rear, and then comes the secondary zone, which may be a further 10 miles or more in depth. Consequently the airman must fly at least 30 miles in a bee-line to cover the transverse belt of the enemy's field of operations. Upon the German and Russian sides this zone is of far greater depth, ranging up to 50 miles or so in width. In these circumstances the difficulties of ethereal communication 'twixt air and earth may be realised under the present limitations of radius from which it is possible to transmit.
But there are reasons still more cogent to explain why wireless telegraphy has not been used upon a more extensive scale during the present campaign. Wireless communication is not secretive. In other words, its messages may be picked up by friend and foe alike with equal facility. True, the messages are sent in code, which may be unintelligible to the enemy. In this event the opponent endeavours to render the communications undecipherable to one and all by what is known as "jambing." That is to say, he sends out an aimless string of signals for the purpose of confusing senders and receivers, and this is continued without cessation and at a rapid rate. The result is that messages become blurred and undecipherable.
But there is another danger attending the use of wireless upon the battlefield. The fact that the stations are of limited range is well known to the opposing forces, and they are equally well aware of the fact that aerial craft cannot communicate over long distances. For instance, A sends his airmen aloft and conversation begins between the clouds and the ground. Presently the receivers of B begin to record faint signals. They fluctuate in intensity, but within a few seconds B gathers that an aeroplane is aloft and communicating with its base. By the aid of the field telephone B gets into touch with his whole string of wireless stations and orders a keen look-out and a listening ear to ascertain whether they have heard the same signals. Some report that the signals are quite distinct and growing louder, while others declare that the signals are growing fainter and intermittent. In this manner B is able to deduce in which direction the aeroplane is flying. Thus if those to the east report that signals are growing stronger, while the stations on the west state that they are diminishing, it is obvious that the aeroplane is flying west to east, and vice versa when the west hears more plainly at the expense of the east. If, however, both should report that signals are growing stronger, then it is obvious that the aircraft is advancing directly towards them.
It was this ability to deduce direction from the sound of the signals which led to the location of the Zeppelin which came down at Luneville some months previous to the war, and which threatened to develop into a diplomatic incident of serious importance. The French wireless stations running south-east to north-west were vigilant, and the outer station on the north-west side picked up the Zeppelin's conversation. It maintained a discreet silence, but communicated by telephone to its colleagues behind.
Presently No. 2 station came within range, followed by Nos. 3, 4, 5, 6, and so on in turn. Thus the track of the Zeppelin was dogged silently through the air by its wireless conversation as easily and as positively as if its flight had been followed by the naked eye. The Zeppelin travellers were quite ignorant of this action upon the part of the French and were surprised when they were rounded-up to learn that they had been tracked so ruthlessly. Every message which the wireless of the Zeppelin had transmitted had been received and filed by the French.
Under these circumstances it is doubtful whether wireless telegraphy between aircraft and the forces beneath will be adopted extensively during the present campaign. Of course, should some radical improvement be perfected, whereby communication may be rendered absolutely secretive, while no intimation is conveyed to the enemy that ethereal conversation is in progress, then the whole situation will be changed, and there may be remarkable developments.