Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/321

 FLIGHT 309 water, the long axis of the body is inclined obliquely down wards and forwards, and the bird forces itself into and beneatli the water by the action of its feet, or wings, or both. In diving or subaquatic flight the feet strike up wards and backwards, the wings downwards and backwards (f&amp;gt; of fig. 4). In aerial flying everything is reversed. The long axis of the bird is inclined obliquely up wards and forwards, and the wings strike, not down wards and backwards, but downwards and forwards (c of fig. 4). These changes in the direction of the long axis of the bird in swim ming, diving, and flying, and in the direction of the stroke of the wings in sub- aquatic and aerial flight, are due to the fact that the bird is heavier than the air and lighter than the water. The physical properties of water and air explain in a great measure how the sailing ship differs from the balloon, and how the latter differs from the flying creature and flying machine constructed on the natural typo. The sailing ship is, as it were, immersed in two oceans, viz., an ocean of water and an ocean of air the former being greatly heavier and denser than the latter. The ocean of water buoys or floats the ship, and the ocean of air, or part of it in motion, swells the sails which propel the ship. The moving air which strikes the sails directly strikes the hull of the vessel indirectly and forces it through the water, which, as explained, is a comparatively dense fluid. When the ship is in motion it can be steered either by the sails alone, or by the rudder alone, or by both combined. A balloon differs from a sailing ship in being immersed in only one ocean, viz., the ocean of air. It resembles the ship in floating upon the air, as the ship floats upon the water ; in other words, the balloon is lighter than the air, as the ship is lighter than the water. But here all analogy ceases. The ship, in virtue of its being immersed in two fluids having different densities, can be steered and made to tack about in a horizontal plane in any given direction. This in the case of the balloon, immersed in one fluid, is impossible. The balloon in a calm can only rise and fall in a vertical line. Its horizontal movements, which ought to be the more important, are accidental movements due to air currents, and cannot be controlled ; the balloon, in short, cannot be guided. One might as well attempt, to steer a boat carried along by currents of water in the absence of oars, sails, and wind, as to steer a balloon carried along by currents of air. The balloon has no hold upon the air, and this consequently cannot be employed as a fulcrum for regulating its course. The balloon, because of its vast size and from its being lighter than the air, is completely at the mercy of the wind. It forms an integral part, so to speak, of the wind for the time being, and the direction of the wind in every instance determines the horizontal motion of the balloon. The force required to propel a balloon against even a moderate breeze would result in its destruction. The b.illoon cannot be transferred with any degree of certainty from ono point of the earth s surface to another, and hence the chief danger in its employment. It may, quite as likely as not, carry its occupants out to sea. The balloon is a mere lifting machine. It has no analogue in nature, and, as its history sufficiently shows, is incapable of improvement. The balloon, as is well known, was introduced by the Montgolfier brothers in 1782. It was first inflated by heated air obtained by burning trusses of straw under it, then by hydrogen gas, and lastly by coal gas. It is in no FIG. 4. The King renguin in the positions assumed by a bird in (a) swimming, (6) diving, and (c) flying. (Pcttigrew.) sense to be regarded a flying machine. It resembles the flying creature only in this that it is immersed in the ocean of air in which it sustains itself. The mode of suspension is wholly different. The balloon floats because it is lighter than the air ; the flying creature floats because it extracts from the air, by the vigorous downward action of its wings, a certain amount of upward recoil. The balloon is passive ; the flying creature is active. The balloon is controlled by the wind ; the flying creature con trols the wind. The balloon in the absence of wind can only rise and fall in a vertical line ; the flying creature can fly in a horizontal plane in any given direction. The balloon is inefficient because of its levity ; the flying creature is efficient because of its weight. Weight, however paradoxical it may appear, is necessary to flight. Everything which flies is vastly heavier than the air. The inertia of the mass of the flying creature enables it to control and direct its movements in the air. Many are of opinion that flight is a mere matter of levity and power. This is quite a mistake. No machine, however light and powerful, will ever fly whose travelling surfaces are not properly fashioned and properly applied to the air. It was supposed at one time that the air sacs of birds contributed in some mysterious way to flight, but this is now known to be erroneous. The bats and some of the best-flying birds have no air sacs. Similar remarks are to be made of the heated air imprisoned within the bones of certain birds. 1 Feathers even are not necessary to flight Insects and bats have no feathers, and yet fly well The only facts in natural history which appear even in- directly to countenance the flotation theory are the pres ence of a swimming bladder in some fishes, and the existence of membranous expansions or pseudo-wings in certain animals, such as the flying fish, flying dragon, and flying squirrel. As, however, the animals referred to do 1 According to Dr Crisp, the swallow, martin, snipe, and many birds of passage have no air in their Loiies. P rue. ZooL Soc. Land., part xxv., 1857, p. 13.