PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 269 





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these limits, the experiments show that the strokes of the wings of birds 

 differ in frequency and amplitude in the difiereiit moments of flight. 

 At starting the strokes are fewer but more energetic ; they attain, after 

 the first two or three, a regular rhythm, which they lose at the moment 

 when 'the animal is about to alight. 

 "^"^""—"'■^ We shall find in other experiments more 



complete indications of the variation of the 

 movements of the wing during the different 

 jieriods of flight. 



Such are the certain indications which can be 

 derived from the method of signalizing estab- 

 lished between the flying bird and the regis- 

 tering apparatus. But if it is wise to guard 

 our conclusions by more rigorous experiments, 

 it may at least be permitted us to attempt 

 to discover whether the tracings of these mus- 

 cles cannot furnish us with further informa- 

 tion in regard to the motions from which they 

 are derived. I have elsewhere demoustrated 

 that the form of the motion produced by a 

 muscle when it is excited varies according to 

 the resistance which this motion encounters. 

 Thus, in applying the myograph to the muscle 

 of a frog, I have seen that if contraction be 

 impeded by an obstacle the duration of the 

 muscular shock becomes greater on account of 

 that obstacle. Theory, also, would foretell us, 

 that if the muscle presents certain modifica- 

 tions in the different phases of its contraction, 

 the result of unequal resistance overcome at 

 different periods, the swelling of the muscle 

 should also present the same phases. If the 

 tracing is the exact impression of the motions 

 produced by the muscle, it can inform us of 

 the nature of the resistance wdiich the wing of 

 the bird encounters in the different phases of 

 g one of its vibrations. 



5, Let us take the most simple example. As 

 ^ the median pectoral and great pectoral are very 

 ^ 2 unequal in size, we may suppose that if the 

 resistance is equal in the two periods of ele- 

 vation and depression, the duration of the for- 

 mer would much exceed that of the latter; 

 and, as exactly the contrary is the case, we 

 g I may conclude that the rising wing does not 

 £ p, strike the air but cuts it, ap])arently with its 

 edge, so that the resistance to the elevation 

 is very feeble, and is very strong to the de- 

 Now, if we examine the tracing of the depression 

 of the wing we shall find there, within certain limits, the expression of 

 the different amount of resistance which the wing encounters in the dif- 

 ferent phases of its depression. It is necessary by previous experiments 

 to determine the effect of certain special kinds of resistance, which we 

 may call elastic resistance, in order to better understand the significa- 

 tion of different forms of muscular motion. 





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pression of the wing. 



