26 AMID THE HIGH HILLS 
Wind, of course, varies in two ways (1) direction 
and (2) velocity, and is uniform only at a given 
height. 
The direction of the wind must necessarily be 
either along the line of flight of the bird, against 
it, or at an angle with it. In the first of these 
instances the speed of the bird over the ground is 
determined merely by adding the velocity of the 
wind to, and in the second by subtracting it from 
the air speed of the bird, in the same way as a 
swimmer's speed is increased or reduced by the 
speed of the current. The third case is more 
complicated, as in this calculation allowance must 
be made for " drift," i.e. the tendency of a bird 
under such circumstances to deviate from its 
desired course. It is, however, unnecessary to 
say anything further as to this third case, as the 
comparison of speeds of various birds can only 
be made satisfactorily by ascertaining their 
speeds under identical conditions in horizontal 
flight. 
2. The path of flight must he horizontal. 
In the words ^ of Captain C. F. A. Portal, 
D.S.O. : " If any one has seen a peregrine stooping 
1 Field, February 18, 1922, p. 233. 
