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 
