1891 - 92 .] Mr R. W. Western on the Tactics of Birds. 
83 
is the extreme rapidity with which movements must he made. In 
order to demonstrate this, let us consider the rate at which an object 
of weight, W lbs., presenting to the wind an effective area of A sq uare 
feet, will acquire velocity. 
Take the velocity of the wind y feet per second, and its relative 
velocity to the object (y - v) feet per second. Then the pressure 
acting on the object will be 
g ^ ’ 
and dividing this by the mass of the body, we get its accelera-tion 
whence the equation 
inverting 
GA . , 
-( 7 -) 
2 
w 
g 
dv 
dt 
GA 
W 
(y - vY 
W dv 
GA fpAdf 
Then as,?;, the velocity of the body, can never reach y, the rate of 
the wind, we may integrate both sides 
whence 
