1 86 THE STRUCTURE AND LIFE OF BIRDS chap. 



horizon and moved forward as before, the resistance 

 will be only f lb. in a direction perpendicular to its 

 surface. If the velocity is then increased to 37*3 feet 

 per second, the resistance will be equal to 1 lb., 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 $7'$ 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 air 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 f lb. 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 § lb. x 37 '3 * 37'3 = f x I (nearly) = lib. A mistake in 

 23-6 x 23-6 5 " 3i 



the figures made by Sir George Cayley has been corrected by 



Professor Roy, See Newton's Dictionary of Birds, vol. i., p. 



263. 



