An aeroplane is any flat or slightly curved surface propelled through the air. Since it is considerably heavier than air, an inquiring mind may well ask: Why does it stay aloft? Why does it not fall?

It is the air pressure beneath the plane and the motion of the plane that keep it up. A balloon can remain stationary over a given spot in a calm, but an aeroplane must constantly move if it is to remain in the air. The monoplanes and biplanes of Blériot, Curtiss, and the Wrights are somewhat in the position of a skater on thin ice. The skater must move fast enough to reach a new section of ice before he falls; the aeroplane must move fast enough to reach a new section of air before it falls. Hence, the aeroplane is constantly struggling with gravitation.

The simplest and most familiar example of an aeroplane is the kite of our boyhood days. We all remember how we kept it aloft by holding it against the wind or by running with it if there happened to be only a gentle breeze. When the wind stopped altogether or the string broke, the kite fell. Above all things it was necessary to hold the kite's surface toward the wind, — an end which we accomplished with a string.

The eagle is an animated kite without a string; it keeps its outspread wings to the wind by muscular power. If we can find a substitute for the string, some device in other words which will enable us to hold the kite in the proper direction, we have invented a flying-machine. The pull or the thrust of an engine-driven propeller is the accepted substitute for the string of a kite and the muscles of an eagle.

If only these simple principles were involved in a solution of the age-old problem of artificial flight, aeroplanes would have skimmed the air decades ago. Many other things must be considered besides mere propelling machinery. Chief among these is the very difficult art of balancing the plane so that it will glide on an even keel. Even birds find it hard to maintain their balance. In the constant effort to steady himself a hawk sways from side to side as he soars, like an acrobat on a tight rope. Occasionally a bird will catch the wind on the top of his wing, with the result that he will capsize and fall some distance before he can recover himself. If the living aeroplanes of nature find the feat of balancing so difficult, is it any wonder that men have been killed in endeavouring to discover their secret?

If you have ever watched a sailing yacht in a stiff breeze you will readily understand what this task of balancing an aeroplane really means, although the two cases are mechanically not quite parallel. As the pressure of the wind on the sail heels the boat over, the ballast and the crew must be shifted so that their weight will counterbalance the wind pressure. Otherwise the yacht will capsize. In a yacht maintenance of equilibrium is comparatively easy; in an aeroplane it demands incessant vigilance, because the sudden slight variations of the wind must be immediately met. The aeroplane has weight; that is, it is always falling. It is kept aloft because the upward air pressure is greater than the falling force. The weight or falling tendency is theoretically concentrated in a point known as the centre of gravity. Opposed to this gravitative tendency is the upward pressure of the air against the under surface of the plane, which effect is theoretically concentrated in a point known as the centre of air pressure. Gravitation (weight) is constant; the air pressure, because of the many puffs and gusts of which even a zephyr is composed, is decidedly inconstant. Hence, while the centre of gravity remains in approximately the same place, the centre of air pressure is as restless as a drop of quicksilver on an unsteady glass plate.

The whole art of maintaining the side-to-side balance of an aeroplane consists in keeping the centre of gravity and the centre of air pressure on the same vertical line. If the centre of air pressure should wander too far away from that line of coincidence, the aeroplane is capsized. The upward air pressure being greater than the falling tendency and having been all thrown to one side, the aeroplane is naturally upset.

Obviously there are two ways of maintaining side-to-side balance, — the one by constantly shifting the centre of gravity into coincidence with the errant centre of air pressure; the other by constantly shifting the centre of air pressure into coincidence with the centre of gravity.

The first method (that of bringing the centre of gravity into alignment with the centre of air pressure) involves ceaseless, flash-like movements on the part of the aviator; for by shifting his body he shifts the centre of gravity. It happened that one of the first modern experimenters with the aeroplane met a tragic death after he had succeeded in making over two thousand short flights in a gliding-machine of his own invention, simply because he was not quick enough in so throwing his weight that the centres of air pressure and gravity coincided. He was an engineer named Otto Lilien-thal, and he was killed in 1896. Birds were to him the possessors of a secret which he felt that scientific study could reveal. Accordingly, he spent many of his days in the obscure little hamlet of Rhinow, Prussia. The cottage roofs of that hamlet were the nesting places of a colony of storks. He studied the birds as if they were living machines. After some practical tests, he invented a bat-like apparatus composed of a pair of fixed, arched wings and a tail-like rudder. Clutching the horizontal bar to which the wings were fastened, he would run down a hill against the wind and launch himself by leaping a few feet into the air. In this manner he could finally soar for about six hundred feet, upheld merely by the pressure of the air beneath the outstretched wings. In order to balance himself he was compelled to shift his weight incessantly so that the centre of gravity coincided with the centre of air pressure. Since they rarely remain coincident for more than a second, Lilienthal had to exercise considerable agility to keep his centre of gravity pursuing the centre of air pressure, which accounts for the apparently crazy antics he used to perform in flights. One day he was not quick enough. His machine was capsized, and his neck was broken. Pilcher, an Englishman, slightly improved on Lili-enthal's apparatus, and after several hundred flights came to a similar violent end. Crude as Lilienthal's machine undoubtedly was, it startled the world when its first flights were made. It taught the scientific investigator of the problem much that he had never even suspected, and laid the foundation for later researches.