Now that the thing has been done, it seems strange that it was not done earlier. At no time was it possible for man to forget his disabilities; the birds were always above him, in easy possession. If he attributed their special powers wholly to the lightness of their structure and the strength of their muscles, the variety of flying creatures might have taught him better. The fact is that there is no unique design for flight; given the power and its right use, almost anything can fly. If the sea-gull can fly, so can the duck, with a much heavier body and a much less proportion of wing. The moth can fly; but so can the beetle. The flying-fish can fly, or rather, can leap into the air and glide for a distance of many yards. With the requisite engine-power a portmanteau or a tea-tray could support itself in the air. The muscular power of man, it is now generally accepted, is not sufficient to support his weight in level flight on still air, but if the principles of flight had been understood, there was no need to wait for the invention of the powerful internal-combustion engine; a steam-engine in a well-designed aeroplane might have performed very useful flights. It was knowledge that lingered. Newton, when he saw an apple fall in his garden at Woolsthorpe, 'began to think of gravity extending to the orb of the moon'. If he had been in the habit of skimming flat stones on calm water, he might have bent his mind to the problem of flight, and might even have anticipated some of the discoveries in aerodynamics which were reserved for the last century--in particular, the relations of speed and angle of incidence to the reactions of air resistance on a moving plane. The fact which is the basis of all aeroplane flight is that a perfectly horizontal plane, free to fall through the air, has its time of falling much retarded if it is in rapid horizontal motion. This is what makes gliding possible. Now let the plane which is being propelled in a horizontal direction be slightly tilted up, so that its front, or leading edge, is higher than its back, or trailing edge. The reaction of the air can then be resolved into two components, technically called 'lift' and 'drag'; lift, which tends to raise the plane, and drag, which retards it in its forward motion. When the angle of incidence of the plane is small, that is, when it is only slightly tilted from its direction of motion, the greater part of the air reaction is converted into lift. This is what makes flying possible. A moderate speed through the air will enable the plane to lift much more than its own weight.