ON AERONAUTICAL PROBLEMS. 405 
length, and twice the girth. Not wishing to waste our somewhat exceptional 
talent, we create one forthwith. But mark what follows :—The weight of 
our large-scale fox-terrier goes up as his volume,—that is, eight times ; 
like his smaller prototype, he stands on four legs ; and the cross-section of 
each leg is only increased in the ratio of four to one: therefore, each leg 
is loaded, in relation to its size, twice as heavily as before. It is clear that 
we cannot continue this process indefinitely : every time we double our 
fox-terrier, his bones will be loaded—per square inch of cross-section, as 
engineers would say—twice as heavily as before ; and unless in the begin- 
ning they were utterly over-proportioned, a time will soon come when they 
will crack under the strain. 
What is the remedy ? Clearly this, that as we increase his size we must 
also thicken his legs, and with his legs his neck, his backbone,—his whole 
skeleton. He will get steadily less lovely ; all his erstwhile friskiness will 
evaporate as he braces himself more and more earnestly to the dreadful 
task of supporting his own weight; and never, while his substance increases, 
will the demands of gravity relax their embarrassing pressure. Sooner or 
later the struggle must be given up: no matter how carefully we distribute 
his material, a time will come when, in literal truth, flesh and bone can do no 
more. And when that time comes, our fox-terrier will be looking, in size 
and shape of body, very like an elephant! To quote Professor Thompson 
again—‘ The elephant, in the dimensions of its limb bones, is already show- 
ing signs of a tendency to disproportionate thickness as compared with the 
smallermammals. . . . Itis already tending towards the limit of size which 
the physical forces permit ’.” 
If Mr. Wells had written ‘ Gulliver’s Travels’, I fancy that the men of 
Lilliput would have had legs as slender as those of our smaller birds: 
what he would have done about the Brobgingnagians I am unable to 
conjecture. For made of human bone and tissue they could not possibly 
stand erect; gravity must win. And this, in the end, will be the fate of all 
endeavours after greater size. Using steel, we succeed in building 
bridges of a span which Nature nowhere approaches ; but there is a dimen- 
sional restriction on engineering structures as on animals, and we shall never 
bridge more than some limited span. Now the body of an aeroplane, like 
that of a bird, is a bridge, transmitting weights to the lifting surfaces of its 
wings and tail; and it too is subject to the dimensional handicap. How- 
ever cunningly we distribute our loads, we make our problem harder by going 
to greater size. 
The Additional Handicap of ‘ Necessary Speed ’. 
17. As a matter of fact, the problem is made harder by another factor 
still. The larger bird flies faster than the smaller: not merely because it can, 
but because (like Old Man Kangaroo in the ‘Just-So’ story) ‘zt has to’. Let 
us again assume the power of creation, and decide to construct a sparrow 
of four times normal size. Its weight, as before, goes up as its volume,— 
that is, in this instance, it is multiplied by sixty-four. But the area of its 
wings (to which, at any given speed, their lifting power is proportional) 
is only multiplied by sixteen; so they are loaded, like our fox-terrier’s 
legs, with an intensity proportional to the scale,—that is, four times as 
7 Growth and Form, p. 21. 
