PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 263 



l)ring in the theory of power ; that is to say, of resistance multiplied by 

 the square of the distance through which it acts in a given time, admit- 

 ting a uniform rate for the downward stroke of the extremity of the 

 wing in two birds to be compared, and which have the proportion of 

 1 : li in their linear dimensions. The surface of the wings of the larger 



Fijr. 16. 



bird will be, as we have already said, four times as great as that of 

 the smaller one; now, as the resistance of the air against surfaces 

 moving at the same rate is proportionate to their extent, if we call the 

 resistance experienced by the wing of the small bird r, that for tlie 

 large bird will be 4 r. But these birds, in the downward stroke of their 

 wings, do not execute motions of equal amplitude. In the large bird, 



