1891-92.] Mr R. W. Western on the Tactics of Birds. 
Substituting this value for a in the equation 
81 
_ / W.7 sec a 
V GA sin‘^ a 
the least value of is shown to be 14*5 x = 23'3 feet per 
*8165 ^ 
second being 14 '5 in the present instance, as found before. 
1-316 
Hence our bird would remain supported at an inclination of 
54° 44' when a wind of 23-3 feet per second was blowing against 
it. This only constitutes a light breeze, Ho. 2 in Beaufort’s scale, in 
which a well-conditioned man-of-war, with all sail set, and clean 
hull, would go about three knots an hour in smooth water. 
There is seldom less wind than this in the air, and when this 
velocity is exceeded, an upward force is developed which is greater 
than the weight of the bird ; the latter being thus raised to a higher 
level. 
Let us now consider the effect of the horizontal force 
GA 
— sin^ a, which is hurrying the bird along in the direction of the 
wind. 
If the velocity of the wind is y feet per second, as has been 
shown, our bird will no longer be supported after its own velocity 
in the same direction becomes y- 23-3 feet per second. 
Let us suppose the gust touches the rate of 50 feet per second, 
which is not much for its momentary highest velocity. Then our 
bird, when the support fails, will be travelling at (50-23*3) feet 
per second, or 26-7 feet per second. If the gust then comes to an 
end, and the wind falls, the bird can avail itself of this energy to 
rise in the air. 26*7 feet per second is equivalent to an increase of 
^.2 
level of 1 1 -7 feet, according to the relation H = ^ . 
It will be necessary, of course, to discount this by loss due to 
skin friction or resistance produced by the air in passing over the 
feathers, also by loss of energy involved in changing the direction, 
neither of which sources of discrepancy are easily estimated. 
It was only desired to demonstrate one case in which the bird 
20/5/92 
VOL. XTX. 
F 
