vil FLIGHT 185 



vacuum made by the last stroke and is already in 

 motion downwards. The predicament in which his 

 wings find themselves may be illustrated by the screw 

 of a steamer ; till the vessel begins to move on, it 

 churns the same water round and round and gets very 

 little grip. Another illustration will help to explain 

 why the large body surface proves so poor a parachute. 

 Thin ice will often bear a skater who moves rapidly 

 over it, when it would break if he stopped for an 

 instant. A particular square yard has not time to 

 break before he has transferred his weight to another. 

 Or we may put it thus, that he is, as it were, supported 

 by long skates that spread the pressure over a great 

 area. This fact is turned to account by all birds. 

 Though the principle is always at work except when 

 they ascend almost vertically, we see it most clearly 

 when they get up pace and then glide onwards with- 

 out moving their wings which are fully or partly 

 extended, the body being sloped at a slight angle 

 upwards ; when this position is adopted, the resistance 

 offered by the air is very little. The bird cuts edge- 

 ways through it, and of the little resistance there is, 

 the greater part acts in an upward direction, or, in 

 other words, supports the bird's weight. Sir George 

 Cayley made some very interesting calculations with 

 regard to this. 1 Experiments had shown that if a 

 flat surface one foot square, a piece of board for 

 instance, be moved forward horizontally at the rate 

 of 2 3 6 feet a second, the resistance is one pound ; 

 whereas if it be held at an angle of 6° to the 



1 " On Aerial Navigation," Journal of Natural Philosophy, 

 Chemistry, and the Arts (Nicholson's), xxiv., p. 164 (1809). 



