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.