[62] In some cases the posterior margin is slightly elevated above the horizon (fig. [53], g).
[63] Weight, as is well known, is the sole moving power in the clock—the pendulum being used merely to regulate the movements produced by the descent of the leads. In watches, the onus of motion is thrown upon a spiral spring; and it is worthy of remark that the mechanician has seized upon, and ingeniously utilized, two forces largely employed in the animal kingdom.
[64] Sappey enumerates fifteen air-sacs,—the thoracic, situated at the lower part of the neck, behind the sternum; two cervical, which run the whole length of the neck to the head, which they supply with air; two pairs of anterior, and two pairs of posterior diaphragmatic; and two pairs of abdominal.
[65] “On the Functions of the Air-cells and the Mechanism of Respiration in Birds,” by W. H. Drosier, M.D., Caius College.—Proc. Camb. Phil. Soc., Feb. 12, 1866.
[66] “An Account of certain Receptacles of Air in Birds which communicate with the Lungs, and are lodged among the Fleshy Parts and in the Hollow Bones of these Animals.”—Phil. Trans., Lond. 1774.
[67] According to Dr. Crisp the swallow, martin, snipe, and many birds of passage have no air in their bones (Proc. Zool. Soc., Lond. part xxv. 1857, p. 13). The same author, in a second communication (pp. 215 and 216), adds that the glossy starling, spotted flycatcher, whin-chat, wood-wren, willow-wren, black-headed bunting, and canary, five of which are birds of passage, have likewise no air in their bones. The following is Dr. Crisp’s summary:—Out of ninety-two birds examined he found “air in many of the bones, five (Falconidæ); air in the humeri and not in the inferior extremities, thirty-nine; no air in the extremities and probably none in the other bones, forty-eight.”
[68] Nearly allied to this is the great gular pouch of the bustard. Specimens of the air-sac in the orang, emu, and bustard, and likewise of the air-sacs of the swan and goose, as prepared by me, may be seen in the Museum of the Royal College of Surgeons of England.
[69] In this diagram I have purposely represented the right wing by a straight rigid rod. The natural wing, however, is curved, flexible, and elastic. It likewise moves in curves, the curves being most marked towards the end of the up and down strokes, as shown at m n, o p. The curves, which are double figure-of-8 curves, are obliterated towards the middle of the strokes (a r). This remark holds true of all natural wings, and of all artificial wings properly constructed. The curves and the reversal thereof are necessary to give continuity of motion to the wing during its vibrations, and what is not less important, to enable the wing alternately to seize and dismiss the air.
[70] In birds which skim, sail, or glide, the pinion is greatly elongated or ribbon-shaped, and the weight of the body is made to operate upon the inclined planes formed by the wings, in such a manner that the bird when it has once got fairly under weigh, is in a measure self-supporting. This is especially the case when it is proceeding against a slight breeze—the wind and the inclined planes resulting from the upward inclination of the wings reacting upon each other, with this very remarkable result, that the mass of the bird moves steadily forwards in a more or less horizontal direction.
[71] “On the Physiology of Wings, being an Analysis of the Movements by which Flight is produced in the Insect, Bat, and Bird.”—Trans. Roy. Soc. of Edinburgh, vol. xxvi.