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 s.p.a.ce. 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 antic.i.p.ations 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 const.i.tutes the frame. Such a huge ma.s.s of metal const.i.tutes 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 ill.u.s.trates 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 const.i.tuted 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 circ.u.mstances 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 circ.u.mstances 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.

CHAPTER XVIII. AIRCRAFT AND NAVAL OPERATIONS

When once the flying machine had indicated its possibilities in connection with land operations it was only natural that endeavours should be made to adapt it to the more rigorous requirements of the naval service. But the conditions are so vastly dissimilar that only a meagre measure of success has been recorded. Bomb-throwing from aloft upon the decks of battleships appeals vividly to the popular imagination, and the widespread destruction which may be caused by dropping such an agent down the funnel of a vessel into the boiler-room is a favourite theme among writers of fiction and artists. But hitting such an objective while it is tearing at high speed through the water, from a height of several thousand feet is a vastly different task from throwing sticks and b.a.l.l.s at an Aunt Sally on terra firma: the target is so small and elusive.

Practically it is impossible to employ the flying machine, whether it be a dirigible or an aeroplane, in this field. Many factors militate against such an application. In the first place there is a very wide difference between dry land and a stretch of water as an area over which to manoeuvre. So far as the land is concerned descent is practicable at any time and almost anywhere. But an attempt to descend upon the open sea even when the latter is as calm as the proverbial mill-pond is fraught with considerable danger. The air-currents immediately above the water differ radically from those prevailing above the surface of the land. Solar radiation also plays a very vital part. In fact the dirigible dare not venture to make such a landing even if it be provided with floats. The chances are a thousand to one that the cars will become water-logged, rendering re-ascent a matter of extreme difficulty, if not absolutely impossible. On the other hand, the aeroplane when equipped with floats, is able to alight upon the water, and to rest thereon for a time. It may even take in a new supply of fuel if the elements be propitious, and may be able to re-ascend, but the occasions are rare when such operations can be carried out successfully.

In operations over water the airman is confronted with one serious danger--the risk of losing his bearings and his way. For instance, many attempts have been made to cross the North Sea by aeroplane, but only one has proved successful so far. The intrepid aviator did succeed in pa.s.sing from the sh.o.r.e of Britain to the coast of Scandinavia. Many people suppose that because an airman is equipped with a compa.s.s he must be able to find his way, but this is a fallacy. The aviator is in the same plight as a mariner who is compelled from circ.u.mstances to rely upon his compa.s.s alone, and who is debarred by inclement weather from deciding his precise position by taking the sun. A ship ploughing the waters has to contend against the action of cross currents, the speed of which varies considerably, as well as adverse winds. Unless absolute correction for these influences can be made the ship will wander considerably from its course. The airman is placed in a worse position.

He has no means of determining the direction and velocity of the currents prevailing in the atmosphere, and his compa.s.s cannot give him any help in this connection, because it merely indicates direction.

Unless the airman has some means of determining his position, such as landmarks, he fails to realise the fact that he is drifting, or, even if he becomes aware of this fact, it is by no means a simple straightforward matter for him to make adequate allowance for the factor. Side-drift is the aviator's greatest enemy. It cannot be determined with any degree of accuracy. If the compa.s.s were an infallible guide the airman would be able to complete a given journey in dense fog just as easily as in clear weather. It is the action of the cross currents and the unconscious drift which render movement in the air during fog as impracticable with safety as manoeuvring through the water under similar conditions. More than one bold and skilful aviator has essayed the crossing of the English Channel and, being overtaken by fog, has failed to make the opposite coast. His compa.s.s has given him the proper direction, but the side-drift has proved his undoing, with the result that he has missed his objective.

The fickle character of the winds over the water, especially over such expanses as the North Sea, const.i.tutes another and seriously adverse factor. Storms, squalls, gales, and, in winter, blizzards, spring up with magical suddenness, and are so severe that no aircraft could hope to live in them. But such visitations are more to be dreaded by the lighter-than-air than by the heavier-than-air machines. The former offers a considerable area of resistance to the tempest and is caught up by the whirlwind before the pilot fully grasps the significant chance of the natural phenomenon. Once a dirigible is swept out of the hands of its pilot its doom is sealed.

On the other hand, the speed attainable by the aeroplane const.i.tutes its safety. It can run before the wind, and meantime can climb steadily and rapidly to a higher alt.i.tude, until at last it enters a contrary wind or even a tolerably quiescent atmosphere. Even if it encounters the tempest head on there is no immediate danger if the aviator keep cool. This fact has been established times out of number and the airman has been sufficiently skilful and quick-witted to succeed in frustrating the destructive tactics of his natural enemy.

Only a short while ago in France, British airmen who went aloft in a gale found the latter too strong for them. Although the machine was driven full speed ahead it was forced backwards at the rate of 10 miles per hour because the independent speed of the aeroplane was less than the velocity of the wind. But a dirigible has never succeeded in weathering a gale; its bulk, area, and weight, combined with its relatively slow movement, are against it, with the result that it is hurled to destruction. All things considered, the dirigible is regarded as an impracticable acquisition to a fleet, except in the eyes of the Germans, who have been induced to place implicit reliance upon their monsters. The gullible Teuton public confidently believes that their Dreadnoughts of the air will complete the destruction of the British fleet, but responsible persons know full well that they will not play such a part, but must be reserved for scouting. Hitherto, in naval operations, mosquito water-craft, such as torpedo-boats, have been employed in this service. But these swift vessels suffer from one serious disability. The range of vision is necessarily limited, and a slight mist hanging over the water blinds them; the enemy may even pa.s.s within half-a-mile of them and escape detection.

The Zeppelin from its position 1,000 feet or more above the water, in clear weather, has a tremendous range of vision; the horizon is about 40 miles distant, as compared with approximately 8 miles in the case of the torpedo-boat. Of course an object, such as a battleship, may be detected at a far greater range. Consequently the German naval programme is to send the Zeppelin a certain distance ahead of the battleship squadron.

The dirigible from its coign of vantage would be able to sight a hostile squadron if it were within visual range and would communicate the fact to the commander of the fleet below. The latter would decide his course according to information received; thus he would be enabled to elude his enemy, or, if the tidings received from the aerial scout should be favourable, to dispose his vessels in the most favourable array for attack.

The German code of naval tactics does not foreshadow the use of dirigible aircraft as vessels of attack. Scouting is the primary and indeed the only useful duty of the dirigible, although it is quite possible that the aerial craft might partic.i.p.ate in a subsequent naval engagement, as, indeed, has been the case. Its partic.i.p.ation, however, would be governed entirely by climatic conditions. The fact that the dirigible is a weak unit of attack in naval operations is fully appreciated by all the belligerents.

The picture of a sky "black with Zeppelins" may appeal to the popular imagination, and may induce the uninitiated to cherish the belief that such an array would strike terror into the hearts of the foe, but the naval authorities are well aware that no material advantage would accrue from such a force. In the first place they would const.i.tute an ideal target for the enemy's vessels. They would be compelled to draw within range in order to render their own attack effective, and promiscuous shooting from below would probably achieve the desired end. One or more of the hostile aircraft would be hit within a short while. Such disasters would undoubtedly throw the aerial fleet into confusion, and possibly might interfere with the tactical developments of its own friends upon the water below.

The sh.e.l.ls hurled from the Zeppelins would probably inflict but little damage upon the warships beneath. Let it be conceded that they weigh about 500 pounds, which is two-thirds of the weight of the projectile hurled from the Krupp 128-centimetre howitzer. Such a missile would have but little destructive effect if dropped from a height of 1,000 feet.

To achieve a result commensurate with that of the 28-centimetre howitzer the airship would have to launch the missile from a height of about 7,000 feet. To take aim from such an alt.i.tude is impossible, especially at a rapidly moving target such as a battle-cruiser.

The fact must not be forgotten that Count Zeppelin himself has expressed the opinion, the result of careful and prolonged experiments, that his craft is practically useless at a height exceeding 5,000 feet. Another point must not be overlooked. In a spirited naval engagement the combatants would speedily be obliterated from the view of those aloft by the thick pall of smoke--the combination of gun-fire and emission from the furnaces and a blind attack would be just as likely to damage friend as foe.

Even if the aircraft ventured to descend as low as 5,000 feet it would be faced with another adverse influence. The discharge of the heavy battleship guns would bring about such an agitation of the air above as to imperil the delicate equilibrium of an airship. Nor must one overlook the circ.u.mstance that in such an engagement the Zeppelins would become the prey of hostile aeroplanes. The latter, being swifter and nimbler, would harry the c.u.mbersome and slow-moving dirigible in the manner of a dog baiting a bear to such a degree that the dirigible would be compelled to sheer off to secure its own safety. Desperate bravery and grim determination may be magnificent physical attributes, ut they would have to be superhuman to face the stinging recurrent attacks of mosquito-aeroplanes.

The limitations of the Zeppelin, and in fact of all dirigible aircraft, were emphasised upon the occasion of the British aerial raid upon Cuxhaven. Two Zeppelins bravely put out to overwhelm the cruisers and torpedo boats which accompanied and supported the British sea-planes, but when confronted with well-placed firing from the guns of the vessels below they quickly decided that discretion was the better part of valour and drew off. In naval operations the aeroplane is a far more formidable foe, although here again there are many limitations. The first and most serious is the severely limited radius of action. The aeroplane motor is a hungry engine, while the fuel capacity of the tank is restricted. The German military authorities speedily realised the significance of this factor and its bearing upon useful operations, and forth with carried out elaborate endurance tests. In numerable flights were made with the express purpose of determining how long a machine could remain in the air upon a single fuel supply.

The results of these flights were collated and the achievements of each machine in this direction carefully a.n.a.lysed, a mean average drawn up, and then pigeon-holed. The results were kept secret, only the more sensational records being published to the world. As the policy of standardisation in the construction of aeroplanes was adopted the radius of action of each type became established. It is true that variations of this factor even among vessels exactly similar in every respect are inevitable, but it was possible to establish a reliable mean average for general guidance.

The archives of the Berlin military department are crowded with facts and figures relating to this particular essential, so that the radius of action, that is the mileage upon a single fuel charge, of any cla.s.s and type of machine may be ascertained in a moment. The consequence is that the military authorities are able to decide the type of aeroplane which is best suited to a certain projected task. According to the dossier in the pigeon-hole, wherein the results of the type are filed, the aeroplane will be able to go so far, and upon arriving at that point will be able to accomplish so much work, and then be able to return home. Consequently it is dispatched upon the especial duty without any feeling of uncertainty.

Unfortunately, these experimental processes were too methodical to prove reliable. The endurance data were prepared from tests carried out in the aerodrome and from cross-country trials accomplished under ideal or fair-weather conditions. The result is that calculations have been often upset somewhat rudely by weather conditions of a totally unexpected character, which bring home vividly the striking difference between theory and practice.

The British and French aviation authorities have not adopted such methodical standardisation or rule of thumb inferences, but rather have fostered individual enterprise and initiative. This stimulation of research has been responsible for the creation of a type of aeroplane specially adapted to naval service, and generically known as the water plane, the outstanding point of difference from the aeroplane being the subst.i.tution of canoes or floats for the wheeled cha.s.sis peculiar to the land machine. The flier is st.u.r.dily built, while the floats are sufficiently substantial to support the craft upon the water in calm weather. Perhaps it was the insular situation of the British nation which was responsible for this trend of development, because so far as Britain is concerned the sea-going aeroplane is in dispensable. But the salient fact remains that to-day the waterplane service of Great Britain is the most efficient in the world, the craft being speedy, designed and built to meet the rough weather conditions which are experienced around these islands, and ideal vessels for patrol and raiding duties.

So far as the British practice is concerned the waterplane is designed to operate in conjunction with, and not apart from, the Navy. It has been made the eyes of the Navy in the strictest interpretation of the term. In any such combination the great difficulty is the establishment of what may be termed a mobile base, inasmuch as the waterplane must move with the fleet. This end has been achieved by the evolution of a means of carrying a waterplane upon, and launching it from, a battleship, if necessary.

For this purpose a docking cradle or way has been provided aft where the aeroplane may be housed until the moment arrives for its employment.

Several vessels have been devoted to this nursing duty and are known as parent ships to the waterplane service. All that is requisite when the time arrives for the use of the seaplane is to lift it bodily by derrick or crane from its cradle and to lower it upon the water. It will be remembered that the American naval authorities made an experiment with a scheme for directly launching the warplane from the deck of a battleship in the orthodox, as well as offering it a spot upon which to alight upon returning from a flight, while Wing-Commander Samson, R.N., D.S.O., the famous British airman, repeated the experiment by flying from a similar launching way installed upon H.M.S. Hibernia. But this practice has many shortcomings. So far as the British and French navies are concerned, the former process is preferred. Again, when the waterplane returns from a flight it is admitted that it is simpler, quicker, and safer for it to settle upon the water near the parent ship and to be lifted on board.

As a sea-scout the waterplane is overwhelmingly superior to the dirigible as events have conclusively proved. Its greater mobility and speed stand it in excellent stead because it is able to cover a larger area within a shorter s.p.a.ce of time than its huge and unwieldy contemporary. Furthermore, it is a difficult target to hit and accordingly is not so likely to be brought down by hostile fire. There is another point in its favour. The experience of the war has proved that the numerically inferior enemy prefers to carry out his naval operations under the cover of the mist and haze which settle upon the water, and yet are of sufficient depth to conceal his ident.i.ty and composition. Such mists as a rule comprise a relatively thin bank of low-lying vapour, which while enveloping the surface of the water in an impenetrable pall, yet permits the mast-heads of the vessels to stand out clearly, although they cannot be detected from the water-level or even from the control and fighting tops of a warship. A scouting waterplane, however, is able to observe them and note their movement, and accordingly can collect useful information concerning the apparent composition of the hidden force, the course it is following, its travelling speed, and so forth, which it can convey immediately to its friends.

The aeroplane has established its value in another manner. Coal-burning vessels when moving at any p.r.o.nounced speed invariably throw off large quant.i.ties of smoke, which may be detected easily from above, even when the vessels themselves are completely hidden in the mist. It was this circ.u.mstance which revealed the presence of the British squadron in the affair of the Bight of Heligoland.

The German airman on patrol duty from the adjacent base on the island of Heligoland detected the presence of this smoke, above the low-lying bank of fog, although there were no other visible signs of any vessels. Fully cognisant of the fact that the German Fleet was at anchor in a safe place he naturally divined that the smoke proceeded from a hostile squadron, evidently bent upon a raid. He returned to his headquarters, conveyed the intelligence he had collected to his superior officers, upon receipt of which a German cruiser squadron was sent out and engaged the British vessels to its own discomfiture. But for the airman's vigilance and smartness there is no doubt that the British squadron would have accomplished a great coup.

This incident, however, served to reveal that the aerial scout is p.r.o.ne to suffer from over-keenness and to collect only a partial amount of information. Upon this occasion the German watchman detected the presence of the British torpedo-boat and light cruiser force. Had he continued his investigations and made a wider sweep he would have discovered the proximity of the British battle-cruiser squadron which routed the German force, the latter having acted on incomplete information.

While the low-lying sea-fog is the navigator's worst enemy, it is the airman's greatest friend and protection. It not only preserves him against visual discovery from below, but is an excellent insulator of sound, so that his whereabouts is not betrayed by the noise of his motor. It is of in calculable value in another way. When a fog prevails the sea is generally as smooth as the pro verbial mirror, enabling the waterplanes to be brought up under cover to a suitable point from which they may be dispatched. Upon their release by climbing to a height of a few hundred feet the airmen are able to reach a clear atmosphere, where by means of the compa.s.s it is possible to advance in approximately the desired direction, safe from discovery from below owing to the fog.

If they are "spotted" they can dive into its friendly depths, complete their work, and make for the parent ship.

Low-lying sea-fogs are favourable to aerial raids provided the scout is able to catch sight of the upper parts of landmarks to enable him to be sure of the correctness of his line of flight-in cases where the distance is very short compa.s.s direction is sufficiently reliable-because the bank of vapour not only const.i.tutes a perfect screen, but serves as a blanket to the motor exhaust, if not completely, at least sufficiently to mislead those below. Fogs, as every mariner will testify, play strange tricks with the transmission of sound. Hence, although those on the vessels below might detect a slight hum, it might possibly be so faint as to convey the impression that the aviator was miles away, when, as a matter of fact, he was directly overhead. This confusion arising from sound aberration is a useful protection in itself, as it tends to lure a naval force lying in or moving through the fog into a false sense of security.