1875.] Aérial Locomotion. 247 
Again :— 
“Confining the question within these limits, experiment shows that the 
strokes of the bird’s wing differ in amplitude and in frequency from one mo- 
ment to another as they fly. When they first start the strokes are rather fewer, 
but much more energetic; they reach, after two or three strokes of the wing, 
a rhythm almost regular, which they lose again when they are about to settle.” 
—(P. 234, op. cit.) 
Dr. Pettigrew lays especial emphasis on the elliptical 
movements made by the wing of the bird (Trans. Linn. Soc., 
vol. xxvi.) Thus he remarks :— 
“During extension the elbow and bones of the fore-arm, particularly their 
distal extremities, describe an upward curve. During flexion the elbow and 
bones referred to describe another but opposite curve. The movements 
described by the elbow-joint during extension and flexion may consequently 
be represented by an ellipse or ovoid.’’—(P. 248, op. cit., Diagrams 8 to 13 
more especially.) 
Prof. Marey follows Dr. Pettigrew in the matter of these 
elliptical movements. He says :— 
“During the whole of the bird’s flight the registering lever described a kind 
of ellipse.” . . . ‘All our experiments have shown that birds of different spe- 
cies describe with their wings an elliptical trajectory.”—(P. 242.) 
Again :— 
“In tact, the bone of the wing in each describes a kind of irregular ellipse, 
with its greater axis inclined downwards and forwards.” 
Dr. Pettigrew represents the wing of the bird as oscillating 
on two separate axes,—the one running parallel with the 
bird, the other at right angles to it,—and adds (in his 1867 
memoir to the Linnean Society, p. 243)— 
‘* The wing may be said to agitate the air in two principal directions, viz., 
from within outward or the reverse, and from behind forward or the reverse, 
the agitation in question producing two powerful pulsations—a longitudinal 
and a lateral.” 
Prof. Marey gives, more briefly but yet in very similar 
terms, the same statement. He says (p. 247)— 
“That a bird passes through a double oscillatory movement, in a vertical 
plane, for each revolution of its wings.” 
Dr. Pettigrew describes and delineates the wing of the 
bird as advancing in a curved line, both when it rises and 
falls. He observes (Trans. Linn. Soc., vol. xxvi., pp. 214 
and 233)— 
“Tn the water the wing strikes downwards and backwards (and aés as an 
auxiliary of the foot), whereas in the air it strikes downwards and forwards. 
. . . To counteract the tendency of the bird in motion to fall in a downward 
and forward direction, the stroke is delivered in the direction in which falling 
would naturally occur,—the kite-like action of the wing, and the rapidity with 
which it is moved, causing the mass of the bird to pursue a more or less hori- 
zontal direction. I offer this explanation of the action of the wing in and out 
of the water after repeated and careful observation in tame and wild birds, 
and, as I am aware, in opposition to all previous writers on the subject.” 
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