406 REPORTS ON THE STATE OF SCIENCE, ETC. 
heavily as before. And since the efficiency of the wings is, for practical 
purposes, independent of their scale, they cannot develop the required 
lifting power except by moving faster through the air: to increase their 
‘lift’ four times they must move twice as fast. 
This is the ‘ principle of necessary speed ’,—that for the maintenance of 
level flight a bird must fly at a speed proportional to the square root of its 
linear dimensions. And, as Helmholtz showed, the rate at which it has 
to do work will increase as the power 34 of its linear dimensions,® whilst 
its capacity for doing work, which depends on the mass of its muscles, 
only varies as the third power.® On creatures which aspire to fly, 
increasing size brings to bear an ever-increasing handicap : life in the air, | 
a thing of buccaneering adventure for the midge and the mosquito, has 
already proved too strenuous for the ostrich ! 
Borelli and the Aeroplane. 
18. Borelli, who wrote in 1685, recognised these facts; in one of his 
chapters he develops the proposition—‘ It is impossible that man should 
ever fly under his own power’.!° And the coming of mechanical flight has 
done nothing to disprove the existence of the dimensional handicap, 
although the extraordinary efficiency of the internal combustion engine, 
as compared with Nature’s equivalent of lungs and muscle, has greatly 
increased the size at which that handicap is felt. I do not say that we have 
yet reached a limit in respect of size of aeroplanes: new materials, new 
principles of construction, and above all, new types of engine, may relieve 
the pressure of the laws which I have been discussing. All that I am 
concerned to show is that this pressure will be merely ‘ postponed’ ; that 
it is idle to talk gaily of size as an advantage which nothing but our 
ignorance withholds from our grasp to-day. 
The Giant Airship. 
19. Outside the pages of the monthly magazines these principles, of 
course, are generally recognized. At all events, there is recognition of an 
element of risk,—of adventure, if you are in favour of taking it: of 
gambling, if you are not,—which attends every advance to greater size. 
And in particular, doubts are expressed as to the wisdom of the Air 
Ministry policy which, after four years of complete stagnation in airship 
construction, is now embarking on the adventure of ships just twice as 
large as any that have been built hitherto. 
As to that four years’ stagnation, the less said the better. A nation 
which looks to attain economy by the road of Anti- Waste agitations in the 
illustrated newspapers, culminating in a Geddes Committee and the 
Geddes Axe, gets nothing but its plainest deserts when these stupidities 
result. What concerns us now is not the fact that we have thrown away 
the potential advances of those four years, but that we are trying to repair 
our blunder: are we wise to make our first step one of definite advance ? 
8 128 for the large-scale sparrow. 
® 64 for the large-scale sparrow. 
0 De Motu Animalium, I, prop. cciv., ed. 1685, p. 243. (Growth and Form, p. 28.) 
