vii FLIGHT 215 



observation — to watch a bird flying and count the 

 strokes per minute; (2) to determine the note made by 

 the vibration of the wings, and from that to calculate 

 the velocity ; (3) to apply machinery by which the 

 bird or insect registers each stroke. These three 

 methods may be called, respectively, the method of 

 observation, the acoustic, and the graphic method. 



(1) The first can only be employed when the bird 

 is flying slowly, and even then it often happens that 

 two observers do not agree. But it is impossible to 

 bring any machinery to bear upon a bird in a state 

 of liberty, so that the third method gives us the wild 

 wing-beats of some poor wretch, the subject of alarm- 

 ing experimentation. I have repeatedly counted the 

 strokes of Gulls making long flights, and find 120 per 

 minute to be a common rate. Friends whom I have 

 got to count for me have come to conclusions 

 not far different. With birds like the Pigeon, whose 

 stroke is much more rapid, the estimates are far from 

 dependable. The Puffin's wings move so rapidly that 

 you only see a shimmer in the air, and you can no 

 more count the strokes than you can see the individual 

 spokes of a wheel in rapid motion. 



(2) The acoustic method depends on the fact that 

 a tuning-fork when its vibrations have a certain fre- 

 quency gives off a certain note, and in the same way 

 the wings of an insect beating, as they sometimes do, 

 20,000 times per minute. But the exact tone varies as 

 the insect flies towards or away from us. An engine 

 whistle sounds shriller as the train approaches. In 

 our present investigations there is besides this the in- 

 superable difficulty that the whirring of a bird's wings 



