272 ANIMAL MECHANISM. 



two vertical oscillations of the bird, the greater and the 

 less, must thus be represented by two curves, of which 

 the summit will be placed downwards. It is easy to recognise 

 their existence in the large curve, a b c, and the smaller one 

 c d e. The bird was, therefore, rising from a to b, descending 

 from b to c ; it rose again from c to d, and descended from 

 d to e. 



But these two oscillations encroach on each oilier, which 

 produces the loop c d ; the oscillation c d e partly covers the 

 first by turning towards the head of the bird. Since the 

 indications of the curve are in a direction contrary to the real 

 motion, this is a proof that the bird, at this moment, was 

 cither carried backwards, or at least slackened the rapidity of 

 its flight. 



This figure, therefore, recalls all that the former experiments 

 have taught us concerning the movements of the bird in 

 space. We see from them, that at each revolution of its wing 

 it rises twice, followed by two descents; that these oscillations 

 are unequal : the larger one, as we know, corresponds with the 

 lowering of the wing, the smaller one with its elevation. Wo 

 see, also, that the ascent of the bird, while tho wing is rising, 

 is accompanied by the slackening of its speed, which justifies 

 the theory that this re-ascent is made at the expense of the 

 velocity acquired by the bird. 



But this is not all : fig. 115 shows us, also, that the move- 

 ments of the bird are not the same at the commencement as 

 at the end of its flight. We have already seen (figs. 95 and 

 100) that ihe strokes of the wing at its departure are more 

 extended; we see here that the oscillations produced at its 

 departure by the descent of tho wing (shown at the left hand 

 of l he figure) are also more extended. But theory enables us 

 to foresee that the oscillation of the ascent of the wing, being 

 produced by the velocity of the bird, must be very feeble at 

 the commencement of its flight, when the bird has, as yet, but 

 little rapidity. This figure shows us that this is actually tho 

 case, and that at the beginning of the flight, tho second 

 oscillation of the wing (that which forms the loop) is but 

 slight. 



Wo are now, therefore, in possession of tho principal 



f 



