FLIGHT OF BIRDS. So long ago as 1680 an Italian, Borelli, propounded a theory of flight which in its essential feature holds good to-day. Noting that all wings are rigid in front, flexible behind, he considered that as the wings were raised and lowered they formed obliquely directed planes alternately facing backward and forward, by which the bird was, so to speak, wedged through the air. More recently Professor Petti grew, of England. and Professor Marcy, of France, have studied the flight of birds, the latter in par ticular devoting much attention to the subject, and devising very elaborate experiments to ascer tain the action of the wings. Professor Marcy was able to photograph the position of the wing at different phases of the stroke. This shows that the wing is not merely moved up and down, but that it has a variety of movements by which, combined with the resistance of the air, the wing assumes a somewhat twisted shape. According to Marcy, the movements of the wing in a pigeon are briefly as follows: At the commencement of the downward stroke the wings are spread to the utmost, and brought downward and forward until their tips are in advance of the head. At the beginning of flight the downward movement is so great that the wing-tips NV111 le 011 their recovery they strike above the bird's back, producing the clap ping sound so noticeable when pigeons lake flight. The wing is now raised rapidly and at the same time flexed at the wrist-joint, to be again straightened as the wings are raised to their full extent. In performing these motions the long axis of the wing describes an ellipse, being ear ried first forward and downward, then backward and upward. During the downward stroke the front edge of the wing looks downward, the under surface of the wing being directed backward, these positions being exactly reversed during the upward stroke. It will be seen that by this dis position the wing has a propelling force during the downward stroke, while during the recovery, or upward movement, it, acts as an inclined plane to raise the bird. 'There is, nevertheless, a rising
and falling of the body, more or less pronounced, at every stroke, so that the forward path of the bird is not a perfectly straight line. Profewor Pettigrew insists that (luring the downward stroke of the wing its under surface is directed forward, and that the long axis of the wing describes a figure co at each stroke, and that the wing, by this screw-like action, acts much after the fashion of a propeller; but the results obtained by photography do not seem to hear out this theory.
The method of sailing flight is less easily ex plained, because the results obtained seem out of all proportion to the force expended. Having obtained its initial velocity, the bird is held to avail itself of every increase or upward eddy of the wind to rise, although in doing this onward movement is checked. The bird then sails for ward and downward, to again rise and repeat these movements. Soaring, or ascending in spi rals, is performed by this method, aided by up ward currents of air. The difficulty in the way of this theory is that birds progress for long distances almost horizontally, while we cannot suppose that convenient upward currents are everywhere to be met with. But the experiments of Lilienthal have shown how little force is necessary for gliding flight, since by jumping from an elevation he was able, with the aid of wings of his invention, to sail for upward of a quarter of a mile. For power of long-distance flight, see MIGRATION OF ANIMALS.
BIBLIOGRAPHY. Pettigrew, Anima/ Locomotion Bibliography. Pettigrew, Anima/ Locomotion (London, 1873) ; Marcy. Animal Mechanics (Lon don, 1874) ; Marcy. Le vol. Physiologic du moure meat du vol. des oiseaux (Paris, 1890) ; Newton, article "Flight," in Dictionary of Birds (Lon don, 1893-96) : Headley. The Structure and Life of Birds (London, 1S115).