222 
THE FLIGHT OF BIRDS AND INSECTS. 
Oct., 1889. 
there are some, at any rate, of the disputed points in the 
study of flight which can be solved by purely physical 
methods. 
The fundamental fact in all flight, from a physical point 
of view, is the tendency of every flat surface moving in air to 
move in the direction of its own plane. This arises from the 
fact that any motion towards either side at once produces a 
compression of the air on that side, and so a resultant pressure 
nearly at right angles to the surface and in the direction 
opposed to the motion. The fact may be loosely but shortly 
expressed, by saying that every flat surface forced to move in 
air tries to move edgewise. Our experience of fans, cards, 
thin books, and the like, makes this tendency quite familiar. 
One way in which it may be used to support a body in the 
air is illustrated by a child’s toy, once so popular as to earn 
the honour of the notice of “ Punch." In this toy four plane 
surfaces of paper stretched on wire are fastened like the sails 
of a windmill to a centre, and the whole can be spun like a 
top with a piece of string. The four wings or sails are so 
set that, when spun, the higher edge of each is in front, so 
that each wing, tending to move edgewise, is urged upward, 
and the toy rises in the air until its velocity is spent. Or 
similar wings attached to one pan of a pair of scales, and 
made to rotate by a twisted elastic cord, will raise the pan to 
which they are attached. To make such a model flv by 
itself in the air something more is necessary, for if left free 
the other end of the elastic cord will untwist and the wings 
remain still; but as in the toy Japanese butterflies, the other 
end of the elastic cord is fastened to large stretched surfaces 
of paper arranged at right angles to the direction in which 
the elastic cord would turn them, so that there is a great 
resistance to its uncoiling at that end, or if, as in the 
original arrangement of Sir G. Cayley, that end is attached to 
another set of wings inclined in the opposite direction, so 
that these rotating the other way, may also tend to rise, we 
have an arrangement which will fly by itself, and differs in 
principle from a small insect in only one important respect. 
An insect might, no doubt, be created which should fly 
exactly in this way ; it is, indeed, mechanically a far better 
arrangement than that of actual insect’s wings. But such 
an insect would have to be created ; it could not grow. No 
part of any animal can rotate always in the same direction ; 
the blood-vessels and nerves, which are necessary for its 
growth, would be twisted off. So, for locomotion on land, we 
have the alternate motion of legs, not the continuous rotation 
of a wheel; for the water, the oscillations of a tail, not the 
revolutions of a screw ; for the air, vibrating wings, not the 
