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 meves 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.! 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 236 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). 
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