The Mastery of the Air Part 12

Even if one were not entirely ignorant of the number and disposition of the aerial fighting forces over the world-wide battle-ground, the Defence of the Realm Act would prevent us from making public the information. But when, more than a year ago, America entered the war, and talked of building 10,000 aeroplanes, no one gasped. For even in those days one thought of aeroplanes not in hundreds but in tens of thousands.

Before proceeding to give a few details of the most recent work of the Royal Flying Corps and Royal Naval Air Service, mention must be made of the armament of the aeroplane. In the first place, it should be stated that the war has gradually evolved three distinct types of flying machine: (1) the "general-purposes" aeroplane; (2) the giant bomb dropper; (3) the small single-seater "fighter".

As the description implies, the first machine fills a variety of roles, and the duties of its pilots grow more manifold as the war progresses.

"Spotting" for the artillery far behind the enemy's lines; "searching"

for ammunition dumps, for new dispositions by the enemy of men, material, and guns; attacking a convoy or bodies of troops on the march; sprinkling new trenches with machine-gun fire, or having a go at an aerodrome--any wild form of aerial adventure might be included in the diary of the pilot of a "general-purposes" machine.

It was in order to clear the air for these activities that the "fighter"

came into being, and received its baptism of fire at the Battle of the Somme. At first the idea of a machine for fighting only, was ridiculed.

Even the Germans, who, in a military sense, were awake and plotting when other nations were dozing in the sunshine of peace, did not think ahead and imagine the aerial duel between groups of aeroplanes armed with machine-guns. But soon the mastery of the air became of paramount importance, and so the fighter was evolved. n.o.bly, too, did the men of all nations rise to these heroic and dangerous opportunities. The Germans were the first to boast of the exploits of their fighting airmen, and to us in Britain the names of Immelmann and Bolcke were known long before those of any of our own fighters. The former claimed not far short of a hundred victims before he was at last brought low in June, 1916. His letters to his family were published soon after his death, and do not err on the side of modesty.

On 11th August, 1915, he writes: "There is not much doing here. Ten minutes after Bolcke and I go up, there is not an enemy airman to be seen. The English seem to have lost all pleasure in flying. They come over very, very seldom."

When allowance has been made for German brag, these statements throw some light upon the standard of British flying at a comparatively early date in the war. Certainly no German airman could have made any such complaint a year later. In 1917 the German airmen were given all the fighting they required and a bit over.

Certainly a very different picture is presented by the dismal letters which Fritz sent home during the great Ypres offensive of August, 1917.

In these letters he bewails the fact that one after another of his batteries is put out of action owing to the perfect "spotting" of the British airmen, and arrives at the sad conclusion that Germany has lost her superiority in the air.

An account has already been given of the skill and prowess of Captain Ball. On his own count--and he was not the type of man to exaggerate his prowess--he found he had destroyed fifty machines, although actually he got the credit for forty-one. This slight discrepancy may be explained by the scrupulous care which is taken to check the official returns.

The air fighter, though morally certain of the destruction of a certain enemy aeroplane, has to bring independent witnesses to substantiate his claim, and when out "on his own" this is no easy matter. Without this check, though occasionally it acts harshly towards the pilot, there might be a tendency to exaggerate enemy losses, owing to the difficulty of distinguishing between an aeroplane put out of action and one the pilot of which takes a sensational "nose dive" to get out of danger.

One of the most striking ill.u.s.trations of the growth of the aeroplane as a fighting force is afforded by the great increase in the heights at which they could scout, take photographs, and fight. In Sir John French's dispatches mention is made of bomb-dropping from 3000 feet. In these days the aerial battleground has been extended to anything up to 20,000 feet. Indeed, so brisk has been the duel between gun and aeroplane, that nowadays airmen have often to seek the other margin of safety, and can defy the anti-aircraft guns only by flying so low as just to escape the ground. The general armament of a "fighter" consists of a maxim firing through the propeller, and a Lewis gun at the rear on a revolving gun-ring.

It is pleasant to record that the Allies kept well ahead of the enemy in their use of aerial photography. Before a great offensive some thousands of photographs had to be taken of enemy dispositions by means of cameras built into the aeroplanes.

Plates were found to stand the rough usage better than films, and not for the first time in the history of mechanics the man beat the machine, a skilful operator being found superior to the ingenious automatic plate-fillers which had been devised.

The counter-measure to this ruthless exposure of plans was camouflage.

As if by magic-tents, huts, dumps, guns began, as it were, to sink into the scenery. The magicians were men skilled in the use of brush and paint-pot, and several leading figures in the world of art lent their services to the military authorities as directors of this campaign of concealment. In this connection it is interesting to note that both Admiralty and War Office took measures to record the pictorial side of the Great War. Special commissions were given to a notable band of artists working in their different "lines". An abiding record of the great struggle will be afforded by the black-and-white work of Muirhead Bone, James M'Bey, and Charles Pears; the portraits, landscapes, and seascapes of Sir John Lavery, Philip Connard, Norman Wilkinson, and Augustus John, who received his commission from the Canadian Government.

CHAPTER XL. The Atmosphere and the Barometer

For the discovery of how to find the atmospheric pressure we are indebted to an Italian named Torricelli, a pupil of Galileo, who carried out numerous experiments on the atmosphere toward the close of the sixteenth century.

Torricelli argued that, as air is a fluid, if it had weight it could be made to balance another fluid of known weight. In his experiments he found that if a gla.s.s tube about 3 feet in length, open at one end only, and filled with mercury, were placed vertically with the open end submerged in a cup of mercury, some of the mercury in the tube descended into the cup, leaving a column of mercury about 30 inches in height in the tube. From this it was deduced that the pressure of air on the surface of the mercury in the cup forced it up the tube to the height Of 30 inches, and this was so because the weight of a column of air from the cup to the top of the atmosphere was only equal to that of a column of mercury of the same base and 30 inches high.

Torricelli's experiment can be easily repeated. Take a gla.s.s tube about 3 feet long, closed at one end and open at the other; fill it as full as possible with mercury. Then close the open end with the thumb, and invert the tube in a basin of mercury so that the open end dips beneath the surface. The mercury in the tube will be found to fall a short distance, and if the height of the column from the surface of the mercury in the basin be measured you will find it will be about 30 inches. As the tube is closed at the top there is no downward pressure of air at that point, and the s.p.a.ce above the mercury in the tube is quite empty: it forms a VACUUM. This vacuum is generally known as the TORRICELLIAN VACUUM, after the name of its discoverer.

Suppose, now, a hole be bored through the top of the tube above the column of mercury, the mercury will immediately fall in the tube until it stands at the same level as the mercury in the basin, because the upward pressure of air through the liquid in the basin would be counterbalanced by the downward pressure of the air at the top, and the mercury would fall by its own weight.

A few years later Professor Boyle proposed to use the instrument to measure the height of mountains. He argued that, since the pressure of the atmosphere balanced a column of mercury 30 inches high, it followed that if one could find the weight of the mercury column one would also find the weight of a column of air standing on a base of the same size, and stretching away indefinitely into s.p.a.ce. It was found that a column of mercury in a tube having a sectional area of 1 square inch, and a height of 30 inches, weighed 15 pounds; therefore the weight of the atmosphere, or air pressure, at sea-level is about 15 pounds to the square inch. The ordinary mercury barometer is essentially a Torricellian tube graduated so that the varying heights of the mercury column can be used as a measure of the varying atmospheric pressure due to change of weather or due to alteration of alt.i.tude. If we take a mercury barometer up a hill we will observe that the mercury falls.

The weight of atmosphere being less as we ascend, the column of mercury supported becomes smaller.

Although the atmosphere has been proved to be over 200 miles high, it has by no means the same density throughout. Like all gases, air is subject to the law that the density increases directly as the pressure, and thus the densest and heaviest layers are those nearest the sea-level, because the air near the earth's surface has to support the pressure of all the air above it. As airmen rise into the highest portions of the atmosphere the height of the column of air above them decreases, and it follows that, having a shorter column of air to support, those portions are less dense than those lower down. So rare does the atmosphere become, when great alt.i.tudes are reached, that at a height of seven miles breathing is well-nigh impossible, and at far lower alt.i.tudes than this airmen have to be supported by inhalations of oxygen.

One of the greatest alt.i.tudes was reached by two famous balloonists, Messrs. c.o.xwell and Glaisher. They were over seven miles in the air when the latter fell unconscious, and the plucky aeronauts were only saved by Mr. c.o.xwell pulling the valve line with his teeth, as all his limbs were disabled.

CHAPTER XLI. How an Airman Knows what Height he Reaches

One of the first questions the visitor to an aerodrome, when watching the alt.i.tude tests, asks is: "How is it known that the airman has risen to a height of so many feet?" Does he guess at the distance he is above the earth?

If this were so, then it is very evident that there would be great difficulty in awarding a prize to a number of compet.i.tors each trying to ascend higher than his rivals.

No; the pilot does not guess at his flying height, but he finds it by a height-recording instrument called the BAROGRAPH.

In the last chapter we saw how the ordinary mercurial barometer can be used to ascertain fairly accurately the height of mountains. But the airman does not take a mercurial barometer up with him. There is for his use another form of barometer much more suited to his purpose, namely, the barograph, which is really a development of the aneroid barometer.

The aneroid barometer (Gr. a, not; neros, moist) is so called because it requires neither mercury, glycerine, water, nor any other liquid in its construction. It consists essentially of a small, flat, metallic box made of elastic metal, and from which the air has been partially exhausted. In the interior there is an ingenious arrangement of springs and levers, which respond to atmospheric pressure, and the depression or elevation of the surface is registered by an index on the dial. As the pressure of the atmosphere increases, the sides of the box are squeezed in by the weight of the air, while with a decrease of pressure they are pressed out again by the springs. By means of a suitable adjustment the pointer on the dial responds to these movements. It is moved in one direction for increase of air pressure, and in the opposite for decreased pressure. The positions of the figures on the dial are originally obtained by numerous comparisons with a standard mercurial barometer, and the scale is graduated to correspond with the mercurial barometer.

From the ill.u.s.tration here given you will notice the pointer and scale of the "A. G" aero-barograph, which is used by many of our leading airmen, and which, as we have said, is a development of the aneroid barometer. The need of a self-registering scale to a pilot who is competing in an alt.i.tude test, or who is trying to establish a height record, is self-evident. He need not interfere with the instrument in the slightest; it records and tells its own story. There is in use a pocket barograph which weighs only 1 pound, and registers up to 4000 feet.

It is claimed for the "A. G." barograph that it is the most precise instrument of its kind. Its advantages are that it is quite portable--it measures only 6 1/4 inches in length, 3 1/2 inches in width, and 2 1/2 inches in depth, with a total weight of only 14 pounds--and that it is exceptionally accurate and strong. Some idea of the labour involved in its construction may be gathered from the fact that this small and insignificant-looking instrument, fitted in its aluminium case, costs over L8.

CHAPTER XLII. How an Airman finds his Way

In the early days of aviation we frequently heard of an aviator losing his way, and being compelled to descend some miles from his required destination. There are on record various instances where airmen have lost their way when flying over the sea, and have drifted so far from land that they have been drowned. One of the most notable of such disasters was that which occurred to Mr. Hamel in 1914, when he was trying to cross the English Channel. It is presumed that this unfortunate pilot lost his bearings in a fog, and that an accident to his machine, or a shortage of petrol, caused him to fall in the sea.

There are several reasons why air pilots go out of their course, even though they are supplied with most efficient compa.s.ses. One cause of misdirection is the prevalence of a strong side wind. Suppose, for example, an airman intended to fly from Harwich to Amsterdam. A glance at the map will show that the latter place is almost due east of Harwich. We will a.s.sume that when the pilot leaves Earth at Harwich the wind is blowing to the east; that is, behind his back.

Now, however strong a wind may be, and in whatever direction it blows, it always appears to be blowing full in a pilot's face. Of course this is due to the fact that the rush of the machine through the air "makes a wind", as we say. Much the same sort of thing is experienced on a bicycle; when out cycling we very generally seem to have a "head" wind.

Suppose during his journey a very strong side wind sprang up over the North Sea. The pilot would still keep steering his craft due east, and it must be remembered that when well out at sea there would be no familiar landmarks to guide him, so that he would have to rely solely on his compa.s.s. It is highly probable that he would not feel the change of wind at all, but it is even more probable that when land was ultimately reached he would be dozens of miles from his required landing-place.

Quite recently Mr. Alexander Gross, the well-known maker of aviation instruments, who is even more famous for his excellent aviation maps, claims to have produced an anti-drift aero-compa.s.s, which has been specially designed for use on aeroplanes. The chief advantages of this compa.s.s are that the dial is absolutely steady; the needle is extremely sensitive and shows accurately the most minute change of course; the anti-drift arrangement checks the slightest deviation from the straight course; and it is fitted with a revolving sighting arrangement which is of great importance in the adjustment of the instrument.

Before the airman leaves Earth he sets his compa.s.s to the course to be steered, and during the flight he has only to see that the two boldly-marked north points--on the dial and on the outer ring--coincide to know that he is keeping his course. The north points are luminous, so that they are clearly visible at night.

It is quite possible that if some of our early aviators had carried such a highly-efficient compa.s.s as this, their lives might have been saved, for they would not have gone so far astray in their course. The anti-drift compa.s.s has been adopted by various Governments, and it now forms part of the equipment of the Austrian military aeroplane.

When undertaking cross-country flights over strange land an airman finds his way by a specially-prepared map which is spread out before him in an aluminium map case. From the ill.u.s.tration here given of an aviator's map, you will see that it differs in many respects from the ordinary map. Most British aviation maps are made and supplied by Mr Alexander Gross, of the firm of "Geographia", London.

Many airmen seem to find their way instinctively, so to speak, and some are much better in picking out landmarks, and recognizing the country generally, than others. This is the case even with pedestrians, who have the guidance of sign-posts, street names, and so on to a.s.sist them.

However accurately some people are directed, they appear to have the greatest difficulty in finding their way, while others, more fortunate, remember prominent features on the route, and pick out their course as accurately as does a homing pigeon.

Large sheets of water form admirable "sign-posts" for an airman; thus at Kempton Park, one of the turning-points in the course followed in the "Aerial Derby", there are large reservoirs, which enable the airmen to follow the course at this point with the greatest ease. Railway lines, forests, rivers and ca.n.a.ls, large towns, prominent structures, such as gasholders, chimney-stalks, and so on, all a.s.sist an airman to find his way.

CHAPTER XLIII. The First Airman to Fly Upside Down

Visitors to Brooklands aerodrome on 25th September, 1913, saw one of the greatest sensations in this or any other century, for on that date a daring French aviator, M. Pegoud, performed the hazardous feat of flying upside down.

Before we describe the marvellous somersaults which Pegoud made, two or three thousand feet above the earth, it would be well to see what was the practical use of it all. If this amazing airman had been performing some circus trick in the air simply for the sake of attracting large crowds of people to witness it, and therefore being the means of bringing great monetary gain both to him and his patrons, then this chapter would never have been written. Indeed, such a risk to one's life, if there had been nothing to learn from it, would have been foolish.