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." Now, facts prove that man can, without danger, descend from an high elevation under a surface of much less than fifteen feet diameter; and the force to lift himself, as will be shown hereafter, is also comparatively small. He can walk up stairs, and likewise mount upon air, which, properly manipulated, becomes sufficiently solid. It has been demonstrated beyond a doubt, that the heaviest flying animals require the smallest amount of surface and power in proportion. The surface is less, because the resistance of the atmosphere is much greater toward one unbroken body than all the parts thereof if detached. Hence a stork, weighing eight times as much as a pigeon, needs only five square feet of surface, while the eight pigeons, with nearly one square foot each, possess together over seven square feet; and the common fly, if magnified to the size of the crane, would show a surface sixty times as large. The heaviest flyers require the least amount of power, because weight, as stated before, itself is power, which increases in a certain ratio. Hence we find the muscular force of the smaller beings, who possess little weight, to be enormous; this is particularly so with insects, who are the strongest in creation. A stag-beetle, of which two hundred weigh only one pound, can lift fourteen ounces; crickets leap eighty times their own length, and the "lively flea" can jump through space estimated at even two hundred times the length of its body--which accounts for the difficulty of catching it. If a mouse would simply reproduce the gait of a horse, its progress would be about twenty inches per minute only, and cats would soon find themselves out of employment.