268 



ANIMAL MECHANISM. 



second question which we have to solve relates to the deter- 

 mination of the various phases in the rapidity of flight. It 

 may receive its solution by the employment of the same 

 method. If the drum, loaded with a piece of lead, be placed 

 on the back of the bird so as to present its membrane in a 

 vertical plane — that is, at right angles to the direction of flight, 



















^^^^^^^ — ^^^v!^^^^^^^^^^^ ^^^^^y^^ 









Fig. 114. — 111 the upper part we see, placed above each other, the muscular 

 . tracing (see p. 232), and that of the vertical oscillations in a wild duck. 

 Under the undulation a, which shows the elevation of the wing, is seen a 

 vertical oscillation ; another is seen under 6, the tracing corresponding 

 with the depression of the wing. In the lower half of the figure are 

 tracings collected from a buzzard ; the oscillation at which corresponds 

 with the elevation of the wing, is less marked than that from the duck. 



the apparatus would be insensible to vertical oscillations, and 

 would only give the signal of those which are made backwards 

 and forwards. Let us turn the membrane of the drum in 

 jfront ; it is evident that if the bird quickens its speed, the 

 retarding influence of the inertia of the mass of lead will 

 produce a pressure on the membrane of the drum ; the air 

 will be compressed, and the registering lever will rise ; while 



