vir FLIGHT 263 
giving what I consider to be the explanation, I shall 
first try to show that what is accepted as the solution 
of the problem by some noted ornithologists in reality 
leaves it just where it was. 
If you take two cubes, a side of one of which is 
twice the length of a side of the other, the larger 
one is in bulk eight times the smaller, but its surface 
area is only four times as great (see figure 29, p. 113). 
This will hold of other figures of three dimensions. 
Magnify a bird till it is eight times its former size: 
yet you will only have multiplied the surface area by 
four. This is no doubt a true principle in geometry, 
and it might be applied to the present case if 
symmetry were the only thing under consideration. 
We should then be doing right if we took the pro- 
portion of bulk (not area) of wing to bulk of body, and 
it would turn out that the build of big birds and small 
is not very different! But the present question is 
really one of dynamics. The problem which nature 
has to solve when she increases a bird’s size is: “If 
the weight of the bird be multiplied by so many, how 
much will the area of the supporting surfaces have to 
be increased?” In other words, how much must be 
added to the wing area, in order that the bird may be 
able to fly? And the answer is (except for modifying 
circumstances which I shall pass on to soon), that if 
the weight be doubled the supporting surfaces must 
also be doubled. 
There is an undeniable interest in the fact that 
wheh we compare birds’ bodies and wings (the legs are 
1 Those who wish to apply this principle should take the cube 
root of the bird’s weight and the square root of the wing area, 
