Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/327

 FLIGHT 315 When the wings descend they elevate the body, the wings being active and the body passive ; when the body descends it contributes to the elevation of the wings, 1 the body being active and -the wings passive. It is in this way that weight forms a factor in ilight, the wings and the weight of the body reciprocating and mutually assisting and relieving each other. This is an argu ment for employing four wings in artificial flight, the wings being so arranged that the two which are up shall always by their fall mechanically elevate the two which are down. Such an arrangement is calculated greatly to con serve the driving power, and, as a consequence, to reduce the weight. That the weight of the body plays an important part in the production of flight may be proved by a very simple experiment. If two quill feathers are fixed in an ordinary cork, and so arranged that they expand and arch above it, it is found that if the apparatus be dropped from a vertical height of three yards it does not fall vertically downwards, but downwards and forwards in a the forward amounting in some in- J fnnpoa fa i -vnrrl orirl Flfl - 2 ~- &quot;, fr, Quill feathers; c, cork ; &amp;lt;/, e, f. g. stances to a jam ana downwanl and forv&amp;gt;ard cur ; e( a half. Here the cork, in falling, acts upon curve, travel * d forward curved trajectory made by the feathers nnd cork before reaching the ground (A, i). (1 ettigrew, 1870.) the feathers (which are to all intents and purposes wings), and these in turn act upon the air, in such a manner as to produce a horizontal transference. The apparatus which formed the subject of the present experiment is repre sented at fig. 27. In order to utilise the air as a means of transit, the body in motion, whether it moves in virtue of the life it possesses, or because of a force superadded, must be heavier than air. It must tread and rise upon the air as a swimmer upon the water, or as a kite upon the wind. This is necessary for the simple -reason that the body must be active, the air passive. The flying body must act against gravitation, and elevate and carry itself forward at the expense of the air and of the force which resides in it, whatever that may be. If it were otherwise if it were rescued from the law of gravitation on the one hand, and bereft of independent movement on the other, it would float about uncontrolled and uncontrollable like an ordinary balloon. In flight one of two things is necessary. Either the wings must attack the air with great violence, or the air in rapid motion must attack the wings : either suffices. If a bird attempts to fly in a calm, the wings must be made to smite the air after the manner of a boy s kite with great vigour and at a higli speed. In this case the wings fly the bird. If, however, the bird is fairly launched in space and a stiff breeze is blowing, all that is required in many instances is to extend the wings at a slight upward angle to the horizon so that the under parts of the wings present kite-like surfaces. Under these circumstances the rapidly moving air flies the bird. The flight of the albatross supplies the necessary illustration. If by any chance this magnificent bird alights upon the sea he must flap and beat the water and air with his wings with tremendous energy until he 1 The other forces which assist in elevating the wings are (a) the elevator muscles of the wings, (I) the elastic properties of the wings, and (c) the reaction of the compressed air on the under surfaces of the wings. gets fairly launched. This done he extends his enormous pinions 3 and sails majestically along, seldom deigning to flap his wings, the breeze doing the work for him. A familiar illustration of the same principle may be witnessed any day when children are engaged in their favourite pastime of kite-flying. If two boys attempt to fly a kite in a calm, the one must hold up the kite and let go when the other runs. In this case the under surface of the kite is made to strike the still air. If, however, a stiff autumn breeze be blowing, it suffices if the boy who formerly ran when the kite was let go stands still. In this case the air in rapid motion strikes the under surface of the kite and forces it up. 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. The area of the insect, bird, and bat, when the wings are fully expanded, is greater than that of any other class of animals, their weight being proportionally less. As already stated, however, it ought never to be forgotten that even the lightest insect, bird, or bat is vastly heavier than the air, and that no fixed relation exists between the weight of body and expanse of wing in any of the orders. We have thus light-bodied and large-winged insects and birds, as the butterfly and heron ; and others with heavy bodies and small wings, as the beetle and partridge. Similar remarks are to be made of bats. Those apparent incon sistencies in the dimensions of the body and wings are readily explained by the greater muscular development of the heavy-bodied, small-winged insects, birds, and bats, and the increased power and rapidity with which the wings in them are made to oscillate. This is of the utmost import ance in the science of aerostation, as showing that flight may be attained by a heavy powerful animal with com paratively small wings, as well as by a lighter one with greatly enlarged wings. While, therefore, there is ap parently no correspondence between the area of the wing and the animal to be raised, there is, unless in the case of sailing insects, birds, and bats, an unvarying relation as to the weight and number of oscillations; so that the problem of flight would seem to resolve itself into one of weight, power, velocity, and small surfaces, versus buoy ancy, debility, diminished speed, and extensive surfaces, weight in either case being a sine qua non. That no fixed relation exists between the area of the wings and the size and weight of the body is evident on comparing the dimensions of the wings and bodies of the several orders of insects, bats, and birds. If such com parison be made, it will be found that the pinions in some instances diminish while the bodies increase, and the con verse. No practical good can therefore accrue to aerosta tion from elaborate measurements of the wings and body of any flying thing ; neither can any rule be laid down as to the extent of surface required for sustaining a given weight in the air. The statements here advanced are borne out by the fact that the wings of insects, bats, and birds may be materially reduced without impairing their powers of flight. In such cases the speed with which the wings are driven is increased in the direct ratio of the mutilation. The inference to be deduced from the foregoing is plainly this, that even in large-bodied, small-winged insects and birds the wing-surface is greatly in excess, the surplus wing area supplying that degree of elevating and sustaining power which is necessary to prevent undue exertion on the part of the volant animal. In this we have a partial explanation of the buoyancy of insects, and the great lifting power possessed by bats and birds, the bats carrying their young without inconvenience, the birds 2 The wings of the albatross, when fully extended, measure some 14 feet. They are exceedingly narrow, being sometimes under a foot in width.