THERE are certain desolate and terrible tracts of the earth's surface where there are no birds and no flying insects; no bees or butterflies hover over the flowerless ground, and no feathers beat the empty air. Supposing this condition had obtained over the whole world, supposing man had never seen or heard of flight of bird or insect, would he have conceived of it as being even possible ?
Probably not. Let us imagine our world exactly as it is, but bereft of every form of flying creature— that is to say, of nearly all the birds, a great proportion of the insects, and just a few of the fish and little animals. Into this singularly unattractive world that we have pictured the thought of flight could hardly have entered. Its inhabitants would know all about the action of gravity; they would know that if they fell from a height they would be drawn relentlessly downwards through the yielding air; how could they conceive that any body heavier than air could possibly be kept afloat in the atmosphere except, perhaps, by such means as a balloon ?
And yet man is marvellously ingenious, and has worked out for himself more difficult problems even than this. Perhaps a philosopher of this birdless world watching the dead leaves whirled aloft by the autumn breeze, a yachtsman with carefully set sheet close-hauled, a miller contemplating his revolving sails, a rueful motorist in a swiftly moving car whose hat had just been carried high aloft from his head, all or any of these might have guessed the secret. Or if not, then surely it must have occurred to some Isaac Newton watching his little son fly a kite—an object heavier than air, and yet capable of rising and sustaining itself above the earth.
Yielding and attenuated as it is, there is a power of resistance in the air with which we are all familiar. We feel it on a windy day when we stand still and the wind blows upon us. We feel it on a still day when we rush rapidly through the air in a motor-car, and the wind we feel is the wind of our own motion. On these and other countless familiar occasions this air resistance is exerted horizontally; but with a suitable contrivance it can be employed to exert an upward force, and here lies the secret that birds and insects, and millions of years later man himself, has discovered, the secret of flight.
The condition under which the resistance of the air can be made to afford its greatest upward lift is when a light, flat (or nearly flat) surface set at a small angle is driven rapidly forward. Newton himself first formulated the law that the pressure exerted by a fluid (and air is as much a fluid as water) is ' normal' —that is, at right angles—to the surface. Make a little picture in your mind, or on paper if it suits you better, of an upright plane with the pressure of a fluid (in this case the force of the wind) acting upon it. Obviously, as Newton said, it will tend to move forward in a direction at right angles to itself—that is, horizontally. But supposing the plane is not upright, but at an angle to the wind that blows upon it, then according to our law it will still tend to move forward in a direction at right angles to itself, which means that it will rise, or try to rise, at an angle into the air. This is the great principle of the inclined plane, the fundamental principle that raises every bird and insect and kite and aeroplane from the ground.
By way of parable let us picture a fine afternoon with a fresh breeze blowing steadily, and we ourselves upon some open common not far remote from the haunts of man. It is Saturday afternoon, and on the grass are two or three groups of children busy with their kites. One schoolboy has a large square box-kite, another the ' fin-bat' variety, with a little subsidiary plane standing out at right angles to the main surface; a child of smaller size, but equally enthusiastic, toddles about with the old-fashioned familiar toy with a tail of knotted paper attached. Sympathetically, for who does not love a kite ? we watch their efforts, and see the big box-kite, deftly thrown into the air, catch the wind and rise higher and higher into the sky until it strains our eyes to distinguish it against the blue. The ' fin-bat' gives some trouble at first, for the string is attached in the wrong place and the thing swoops and curves and plunges to the ground until the balance is adjusted, when it rises triumphantly to its limit, which is not, however, as high as that of the box-kite, because, as its owner is anxious to explain, not being so large, it has not the strength to lift so great a weight of string.
But presently the rival meets with disaster ; for the kite-string suddenly breaks, and the kite, released from the pull that kept it at an angle to the wind, is promptly blown right over, and falls headlong in a distant field with disastrous results. Meantime the toddler, after much exertion and fruitless endeavour, becomes loudly tearful at his lack of success, until a compassionate elder, by knotting some more paper on to the tail of his kite and thus altering the weight and balance, induces it to rise to modest but satisfying altitudes. As the afternoon wears on, the wind, which is-falling, becomes fitful in its gusts, the kites swoop earthward, and the boys run over the ground, pulling their kites by the strings behind them, the motion thus imparted to the kites themselves making up temporarily for the lack of motion of the wind.
But the breeze dies with the day, the children gather their toys and depart homewards, and next we see come out on to the common a young man with a model aeroplane, clearly of his own design and construction. Carefully he winds the stretched elastic which revolves the tiny propeller, carefully he launches his craft into the air, and his face beams with satisfaction when after a few abortive attempts and a few minor adjustments—a wing straightened out, an elevator tip turned upwards—the dainty little toy springs forward and skims gracefully through the air, some 300 yards and more, until, the force of its untwisting elastic expended, it sinks gently on to the grass.