XIV.—SURFACE.
The next of the three properties necessary for flight, is the extension of the locomotive organs in winged beings—the planes. Although the wings in the different animals differ much in their form, texture, construction, number, and the matter which composes them, yet they resemble one another in the expansion and development of their surfaces, being stretched on each side of the body, and playing the part of a parachute. The animal, therefore, cannot fall like a stone, in obedience to the accelerated force of gravity, but it descends with a slow velocity; constant regular, and considerably abated.
This influence, then, exercised by the flat surface on the fall of masses, is seen in a sheet of paper of the same weight as a grain of lead, it will fall much more slowly. But if we make the paper a compact ball, and flatten the lead into a broad, thin sheet, the reverse result will be produced, and the paper reach the ground before the lead. Therefore, bodies in the air are light or heavy in proportion to their surfaces, and the heaviest may become light by an alteration of form. For successful flight, then, a just proportion of surface and weight is necessary; because, as stated, the air being elastic, its resistance is much more effectual with light bodies than heavy ones; and this proportion is such that the extent of surface is always in an inverse ratio to the weight of the winged animal.
The principle in the fall of flat surfaces is strictly applicable to the bird. Its weight, tending downwards, and being situated below the plain of suspension, keeps it well balanced, so that it cannot fall head over heels, nor rapidly. If the wings are inclined at an angle with the horizon, the bird will not descend vertically, but glide along an inclined plane with much greater swiftness, because the vertical distance remains unaltered in the same space of time. Hence their immense horizontal velocity, without comparatively any effort. This is in obedience to two forces—gravity, or weight, and resistance of surface.
XV.—POWER.
But for actual flight a third force is required—the propelling power, the necessary amount of which has greatly been overrated by many mathematicians.
Borelli estimated the power of a three pound bird to be over one hundred and thirty horses relatively. But, Navier, more reasonably, calculated a force of five horses sufficient for the flight of a pigeon. Coulomb, again, offset this "great liberality" by demonstrating that the surface to support a man must be two miles long and two hundred feet wide, with the power of a "Corliss engine" to propel such a "fifty acre ranch."