186 THE STRUCTURE AND LIFE OF BIRDS cHap. 
horizon and moved forward as before, the resistance 
will be only ? Ib. in a direction perpendicular to its 
surface. Ifthe velocity is then increased to 37°3 feet 
per second, the resistance will be equal to 1 Ib., since, 
as we have seen, it increases, speaking roughly, as the 
square of the velocity.1 Sir George Cayley estimated - 
that the weight and wing area of a Rook were in 
the proportion of 1 lb. to the square foot. The Rook 
would therefore maintain his level, if having a velocity 
of 37°3 feet per second he placed himself at an angle 
of 6° to the horizon. This calculation has great value 
since it emphasizes and connects three important 
facts : (1) if a bird inclines his body at a small angle to 
the horizon the resistance of the air is much less ; (2) 
the resistance being at right angles to the plane of his 
body and wings will tend to support his weight ; (3) 
the support given increases enormously with increase 
of velocity. But it is easy to show that it is inaccurate. 
To begin with, a Rook does not present a plane 
surface like a piece of board. It presents irregular 
concavities which cause the dir to offer a far greater 
resistance ; but how much greater, has not been dis- 
covered either by calculation or by experiment. 
Besides this, the whole 2 Ib. resistance offered by 
the air will not go to the bird’s support. A small 
fraction of it will oppose his onward progress, the pro- 
portion which is thus a hindrance and not a help 
1 373 X* 373 _ 2 = : A 
2 lb. x S50 3 256 x § (nearly) = 1lb. A mistake in 
the figures made by Sir George Cayley has been corrected by 
Professor Roy, See Newton’s Dictionary of Birds, vol. i., p. 
263. 
