226 ANIMAL MECHANISM. 



But how is it that \\ings whose surfaces vary as to the 

 square of their linear dimensions are sufficient to move the 

 weights of birds which vary in the ratio of the cubes of these 

 dimensions? 



It can be proved that, if the strokes of the wing were as 

 frequent in large as in small birds, each stroke woiild have a 

 velocity whose value would increase with the size of the bird; 

 and as the resistance of the air increases for each element of 

 the surface of the wing, according to the square of the velo- 

 city of that organ, a considerable advantage would result to 

 the bird of large size, as to the work produced upon the air. 



Hence it follows, that it would not be necessary for large 

 birds to give such frequent strokes of the wing in order to 

 sustain themselves as would be required for those of smaller 

 size. 



Observers have not, hitherto, been able to determine very 

 accurately the number of the strokes of the wing, in order to 

 ascertain whether their frequency is in an exact inverse ratio 

 to the size of birds ; but it is easy to see that the number of 

 strokes varies in birds of different size in a proportion of this 

 kind. 



CHAPTER IV. 



OF THE MOVEMENTS OF THE WING OF THE BIRD 

 DURING FLIGHT. 



Frequency of the movements of the wing Relative durations of its rise 

 and fall Electrical determination Myographical determination. 



Trajectory of the bird's wing dining flight Construction of the instru- 

 ments which register this movement Experiment Elliptical figure 

 of the trajectory of the point of the wing. 



IN the general remarks on the form of the bird, and on the 

 deductions to be drawn from it, the reader must have sec u 

 that many hypotheses await experimental demonstration. For 

 this reason, we have been anxious to apply to the flights of 



