1872.] Natural and Artificial Flight. 35 
Experiments with several birds—ducks, harriers, hen- 
hawks, and owls—have shown that, in relation to the inten- 
sity of the oscillations in the vertical plane, very varied 
types exist. Fig. 6 shows tracings furnished by these birds. 
The upper crenulated line is produced by a tuning-fork vi- 
brating roo times per second. Thus the tracings may be 
estimated absolutely, or in relation to each other. A still 
clearer view will be obtained of the actions, during the bird’s 
flight, if the oscillations of the bird vertically and ‘the move- 
ments of its muscles are recorded at the same time. The 
tracings represented by Fig. 7 will then be obtained. The 
duck presents two energetic oscillations at each revolution 
Fie 6. 
; Tracings, 
I. From the Tuning-Fork. II. Wild Duck. III. Buzzard. IV. Screech Owl. V. Harrier. 
of its wing; the one at 0, at the moment when the wing re- 
laxes, is easily understood; the other, at a, at the moment 
when the wing rises. ‘Yo explain the ascension of the bird 
during the time of the elevation of the wing, it seems to me, 
says M. Marey, indispensable to call in the action of a boy’s 
kite. The bird, moving forward with acquired velocity, pre- 
sents its wings to the air in an inclined position, similar to 
that of the kite, and thus transforms its horizontal force into 
an ascending one. 
The preceding experiment furnishes a very precious lesson 
in the theory of flight. In fact, if the bird executes a series 
of ascents and descents, the duration of the descending 
period will approximately inform us of the amount of 
positive work which the bird must perform to rise again to 
the height from which it fell; and we see that the duck, 
which makes nine vibrations of the wing per second, executes 
