We see that the gnat, one of the lightest of insects, has an expanse of wing of no less than 48·9 square feet for each pound of weight, while the heavy cockchafer has only 5·1 square feet for each pound. With birds, the sparrow has 2·7 square feet of wing-surface for each pound of weight, while the great Australian crane has only 0·41 of a square foot, and yet this bird undertakes remote journeys, and, the eagle excepted, Hies higher, and keeps on the wing longest, of all the travelers.
It would appear, then, that our flying-machine, while heavy, need not necessarily have a very broad expanse of flying surface. Indeed, Paradoxical as it may seem, weight is really an essential feature. Set in motion by muscular effort, the weight of a bird acts somewhat like the fly-wheel of an engine: the power is stored up during the downward stroke of the wing, to be given out again on its upward stroke, and probably it is weight also that enables the bird to successfully combat and take advantage of the force of the wind. It is noteworthy that all sailing-birds, like the hawk or vulture, have comparatively heavy bodies. The magnificent albatross, in rising from the water, is said to beat the air with great energy, but, when fairly launched, in a brisk gale, will sweep around in broad circles for hours together, hardly ever deigning to flap a wing. Darwin, in his "Voyage of the Beagle," speaks of watching the condor sailing in a similar way at a great height, without, so far as he could notice, any flapping action whatever.
At the same time, it is hard to understand how such a condition of affairs could exist. The condor's wings, inclined to the wind, have been compared to a kite, and if there were a string stretching from the bird to some fixed point, the whole thing would be clear; but every boy knows to his cost that, if the string slips or breaks, the kite quickly seeks some other point of support—probably a telegraph-wire. But Professor Pettigrew has suggested that the string is the invisible one representing the attraction of gravitation, and that "the string and the hand are to the kite what the weight of the flying creature is to the inclined planes formed by its wings." This, however, does not make the matter much clearer, for the force of gravity acts in vertical lines, and a vertical kite-string, with the kite flying directly overhead, is a thing, it is safe to say, no boy ever saw. Why should not our bird drift with the wind unless he uses some muscular effort to overcome its force or to keep himself from falling?
Once elevated, he can utilize his weight in a number of ways. A body will naturally fall along a line of least resistance, and if the front edge of the wings be tipped slightly downward the bird will glide forward while falling, gaining velocity and momentum; and then, by reversing the inclination of the wings, he can again glide up an aerial incline until this stored-up energy has been expended. But the resistance of the air must be overcome, and there must be continual loss from the imperfect sustaining power of the wings.
