36 Natural and Artificial Flight. [January, 
two vertical oscillations during each vibration, or eighteen 
ina second. Each oscillation is composed of arise anda 
fall, so that each descent of the bird cannot last more than 
1-36th of a second. Now, if we subtract the effect produced 
(as in a parachute) by the outspread wings of the bird, we 
find that a body which falls during 1-36th of a second tra- 
verses only 52 millimetres. This fall, repeated eighteen 
times a second, constitutes a total rise of 9°36 centimetres, 
necessary to maintain the bird in the same horizontal plane 
during one second. In the tracings of the harrier the 
descents are less than in that of the wild duck, probably on 
account of the large surface of the wings of this bird. 
Fic. 7: 
The two upper lines are the muscular tracing and the tracing of the oscillations of 
the duck. The lower lines are tracings from a harrier. Underneath the undula- 
tion a, indicating the elevation of the wing, is seen a vertical oscillation ; and 
again beneath 8, indicating the lowering of the wing. The oscillation at a inthe 
lower tracings of the harrier, corresponding to the elevation of the wing, is less 
marked than in the duck, 
The remaining question to be solved relates to the deter- 
mination of the phases of rapidity of flight. The solution 
can be found by placing the weighted drum vertically upon 
the bird’s back. The tracings furnished are, however, so 
analogous to the oscillations in the vertical plane, that they 
show very little more than a proof that during the descent 
of the wing the speed of the bird is accelerated. 
At last, then, we are in possession of the principal facts 
upon which the study of the mechanical power developed by 
the bird during flight can be established, and we see that it 
is during the descent of the wing that the entire motive 
force which sustains and directs the bird in space is deve- 
