262 
THE FLIGHT OF BIRDS AND INSECTS. 
Nov., 1889. 
again. (The widely prevailing idea that insects are enormously 
strong in proportion to their weight can be shown, I think, 
to be a fallacy, resulting from a misconception of what 
increased strength, with proportionately increased mass, 
ought to effect.) So that it is certain that the bird must use 
the most economical method of flight available. Now, we 
found that a fly, forced by the reduction of its wing-surface to 
greater economy of strength in flying, was still able to fly if 
it turned its wing-surfaces into an oblique position, moved 
them obliquely instead of horizontally, and, using them thus 
as propellers as well as supporters, took advantage of the 
upward pressure arising from its forward motion. It is 
evidently only a further step in the same direction to place 
the wing-surfaces nearly horizontal, and move them vertically. 
In this case the pressure of the wings on the air is directly 
backwards, and does not support the insect at all; the whole 
support is derived from the gliding of the wings, sloping a 
little up in front, on the air. This seems to be the principle 
of the flight of the larger insects, of bats, and of birds. 
If we examine the wing of a bird, we find that it consists 
of two distinct parts, though they are not always separated 
by a distinct line. The part nearer the body of the bird 
consists of feathers which are divided nearly into equal parts 
by the ribs, are directed backwards along the bird, and over¬ 
lap so that no air can pass between, at least from below. 
The outer part, that furthest from the body, consists of 
feathers which, when the wing is outstretched, are nearly at 
right angles to the bodv, and senarate from one another like 
the outspread fingers of a hand, and consist each of a flexible 
surface supported by a stiff rib which runs very nearly to its 
front edge. Each of these outstretched feathers, then, is stiff 
along its front edge, and flexible along its hinder one, and it 
is easy to see that, as it is moved up and down, the resistance 
of the air will always set it in such a position that, if it tries 
to move edgewise, it will tend forwards. That, as a matter of 
fact, each of these feathers does act as a propeller, can easily 
be shown by fixing two of them to a clockwork arrangement 
which makes them move up and down as they do on the bird, 
and which is suspended from a small carriage which can run 
along a horizontal string. As soon as the feathers begin to 
move up and down, the carriage runs forward along the string; 
and that such a propelling power as these feathers furnish is 
all that is necessary for flight, is shown both by the fact that 
a bird can support itself in the air simply by holding its 
wings outspread as long as its forward motion lasts, and 
could therefore do so indefinitely with the aid of a propelling 
