49 4, FLIGHT AND FLYING MACHINES, 
the same piece of paper, 1s a square of twice the length and twice the 
breadth of the first, and therefore, of four times the area ; it is, there- 
fore, four times the weight of the first. You will see that these two 
squares fall to the ground in the same time. But the two squares are of 
the same thickness, and therefore their dimensions are not all propor- 
tional as they should be, according to M.de Lucy’s law. Accordingly, 
I have taken another piece of paper of double the thickness of the first, 
and have cut outa square twice the length, twice the breadth, and twice 
the thickness of the smaller square. This square is, therefore, eight 
times the weight of the small one, although it is only four times the area. 
When I pull this string and release these three pieces of paper, what 
we ought to see (if the experiment comes off properly) is two squares 
of paper, one large and one small one, fluttering down at about the 
same rate, and another large square coming down a good bit faster than 
either of them. I now pull the string. This square which fell a little 
before the others we shall find is the paper of double thickness; but 
the other two squares fell almost together. So that supposing an 
insect were learning to fly and it were to drop through 20 feet, turning 
over and over in its descent as did my squares of paper, it would strike 
the ground with lesser velocity,and therefore, would not hurt itself as 
much, in proportion to its size, as we should do if we were experi- 
menting with a flymg machine, and were suddenly lose control of the 
apparatus and to drop through 20 feet. The tiny insects, called “ fairy 
flies,” well show what very slender wings are sufficient to enable a very 
small insect to fly. ‘These insects are microscopic flies, you can hardly 
see them with the naked eye, and a highly magnified representation 
shows that the wings are really hardly thicker than the legs. 
Now, although insects found it easier to learn to fly than larger 
animals have done, small insects like these fairy flies would, of course, 
stand no chance of holding their own against a strong wind, and con- 
sequently we find that as msects have developed they have gradually 
grown heavier, and their proportion of wing area to weight has 
diminished. Finally, one order of insects, (namely, flies or diphera) 
have found that for getting through the wind, two wings are better than 
four, and they have accordingly lost the hind pair of wings altogether. 
Moreover, these flies are much heavier in proportion to the size of the 
wings than other insects ; but then, of course, they require enormously 
greater muscular power to maintain them in the air. 
The flying fish and flying gurnard may be said to represent another 
type of animals in the stage of “learning to fly.” They can go through 
the air for short distances, but they have not developed sufficient 
muscular power to fly continuously. The reason why this flying 
gurnard and also this flying fish can fly imperfectly without danger, 
although they cannot fly long distances is because they start off the 
surface of the water, and if they fall in again they do not receive a 
large shock that injures them like an animal of the same size would 
experience if it fell on dry land. For this reason it is very easy for 
them to practise flying, and we have good reason to believe that the 
birds are descended from animals which were amphibious in their 
habits, and that they acquired the power of flight in this way. It 
