Oct., 1889. 
THE FLIGHT OF BIRDS AND INSECTS. 
221 
* 
THE FLJGHT OF BIRDS AND INSECTS.* 
BY EDMUND CATCHPOOL, B.SC. 
The question, “ How do animals fly ? ”—a difficult one to 
answer in any case—has been made much more difficult by 
two facts connected with it. One of these is that the study 
of flight lies on the border-land between the sciences of 
biology and physics, while those who have devoted them¬ 
selves to it have usually been specialists in one only of these 
sciences. The other is the fact that there are two methods of 
flight, as distinct from each other as walking and jumping, 
and that from want of distinctive names these are often 
confused together. As an instance, one among many, of the 
first of these causes, let us take the common idea that birds 
are at least partly supported in the air by the possession, in 
the bones and other parts, of cavities filled with warm air. 
It is, of course, certain that this warm air, acting like that 
in a hot-air balloon, does to some extent support the bird ; 
and equally certain that this support never amounts to as 
much as the weight of a single quill feather on the bird. 
This result of a simple calculation from physical data disposes 
at once of the air-sacs as a practicallv important element in 
flight. 
I will give another instance, which will help to clear the 
way to an understanding of the flight of birds. It is quite 
commonly believed that the feathers of the wings of birds 
open to let the air pass between them during the upstroke, 
and I have had it shown to me, by directing a jet of oxygen 
on the upper side of a wing, that when the pressure on this 
side exceeds that on the lower side, the feathers do open in 
this manner. This is quite true, but in flight the pressure 
never is greater on the upper surface, as a simple physical 
calculation proves. If the pressure on the upper surface was 
greater than on the lower, the bird would, of course, be 
unsupported during the upstroke, and would fall with the 
same velocity as any other unsupported body. In an upstroke, 
lasting J sec., like that of the buzzard, the body would fall at 
least 3in., and, as observation shows that it does not, the 
pressure must always be greater on the lower surface. 
I have begun with these two illustrations because in this 
paper I shall treat the subject of flight purely as a physicist. 
I do not pretend to be anything else, and I wish to show that 
* ltead before the Birmingham Natural History and Microscopical 
Society, January 8tli, 1889. 
