CHANGES IN THE PLANE OF THE BIKD's WING. 257 



By constructing thus the whole figure, we see that this 

 curve, after pioceeding downwards and forwards, rises and 

 returns back again. 



By comparing this figure \\iih that which we have obtained 

 by means of another apparatus (fig 100), on another kind of 

 bird, and by examining the n-ovement of another part of the 

 wing, we shall find striking 'resemblances between the two 

 curves, which show that birds proceed in their flight by 

 movements which are almost identical. In fact, the bone of 

 the wing in each describes a kind of irregular ellipse, with 

 its greater axis inclined downward and forward. The im- 

 portance of this determination is so great, that we trust we 

 shall be pardoned for the long and minute details of the 

 experiments which have furnished these results. 



OF THE CHANGES IN THE PLANE OF THE WING. 



We have seen in Chapter I. that the wing of the insect is 

 subject to torsions under the influence of the resistance of the 

 air, and that the inclination of the plane of its wing is 

 changed at every moment. These movements, which are 

 entirely passive, constitute the essence of the mechanism of the 

 insect's flight; the wing, in each of its alternate movements, 

 acts on the resistance of the air, and gains from it a force 

 which is exerted on the membrane by the side of the main- 

 rib, thus serving to sustain the insect and propel it forward. 

 The structure of the bird's wing does not allow the existence 

 of a similar mechanism. Its wing during its ascent does not 

 present to the air a resisting plane, because the feathers which 

 fold over each other would open to allow it to pass through. 

 The depn snon of the wing is therefore the only phase in the 

 flight of the bird which has any analogy with that of the 

 insect. Besides, the curve described by the point of the bird's 

 wing is sufficiently different from that of the insect, to prove 

 that their mechanical conditions are very dissimilar. 



It was indispensable to determine by experiment the dif- 

 ferent inclinations of the plane of the bird's wing at each 

 phase of its revolutions. In fact, to estimate the resistance 

 which the air presents at each moment of the flight, we must 

 know the two elements of this resistance : first, the angle 



