118 



ANIMAL LOCOMOTION. 



The same may be said of the hollow bones, — some really 

 admirable fliers, as the swifts, martins, and snipes, having 

 their bones filled with naarrow, while those of the wingless 

 running birds alluded to have air. Furthermore and finally, 

 a living bird weighing 10 lbs. weighs the same when dead, 

 plus a very few grains ; and all know what eff'ect a few grains 

 of heated air would have in raising a weight of 10 lbs. from 

 the ground. 



How Balancing is effected in Flight, the Sound produced hy 

 the Wing, etc—Th.^ manner in which insects, bats, and birds 

 balance themselves in the air has hitherto, and with reason, 

 been regarded a mystery, for it is difiicult to understand how 

 they maintain their equilibrium when the wings are beneath 

 their bodies. Figs. 67 and 68, p. 141, throw considerable 

 light on the subject in the case of the insect. In those 

 figures the space {a, g) mapped out by the wing during its 

 vibrations is entirely occupied by it ; i.e. the wing (such is 

 its speed) is in every portion of the space at nearly the same 

 instant, the space representing what is practically a solid 

 basis of support. As, moreover, the wing is jointed to the 

 upper part of the body (thorax) by a universal joint, which 

 admits of every variety of motion, the insect is always sus- 

 pended (very much as a compass set upon gimbals is sus- 

 pended) ; the wings, when on a level with the body, vibrating 

 in such a manner as to occupy a circular area {vide r dbf of 

 fig. 56, p. 120), in the centre of which the body {a e c) is 

 placed. The wings, when vibrating above and beneath the 

 body occupy a conical area ; the apex of the cone being directed 

 upwards when the wings are below the body, and downwards 

 when they are above the body. Those points are well seen 

 in the bird at figs. 82 and 83, p. 158. In fig. 82 the in- 

 verted cone formed by the wings when above the body is repre- 

 sented, and in fig. 83 that formed by the wings when below 

 the body is given. In these figures it will be observed that 

 the body, from the insertion of the roots of the wings into its 

 upper portion, is always suspended, and this, of course, is equi- 

 valent to suspending the centre of gravity. In the bird and 



the swan and goose, as prepared by me, may be seen in the Museum of the 

 Royal College of Surgeons of England. 



