this
difficulty is insurmountable would seem to be a very fair deduction,
not only from the failure of all attempts to surmount it, but from the
fact that Maxim has never, so far as we are aware, followed up his
seemingly successful experiment.

There is, indeed, a way of attacking it which may, at first sight, seem
plausible. In order that the aeroplane may have its full sustaining
power, there is no need that its motion be continuously forward. A
nearly horizontal surface, swinging around in a circle, on a vertical
axis, like the wings of a windmill moving horizontally, will fulfil all
the conditions. In fact, we have a machine on this simple principle in
the familiar toy which, set rapidly whirling, rises in the air. Why
more attempts have not been made to apply this system, with two sets of
sails whirling in opposite directions, I do not know. Were there any
possibility of making a flying-machine, it would seem that we should
look in this direction.

The difficulties which I have pointed out are only preliminary ones,
patent on the surface. A more fundamental one still, which the writer
feels may prove insurmountable, is based on a law of nature which we
are bound to accept. It is that when we increase the size of any
flying-machine without changing its model we increase the weight in
proportion to the cube of the linear dimensions, while the effective
supporting power of the air increases only as the square of those
dimensions. To illustrate the principle let us make two flying-machines
exactly alike, only make one on double the scale of the other in all
its dimensions. We all know that the volume and therefore the weight of
two similar bodies are proportional to the cubes of their dimensions.
The cube of two is eight. Hence the large machine will have eight times
the weight of the other. But surfaces are as the squares of the
dimensions. The square of two is four. The heavier machine will
therefore expose only four times the wing surface to the air, and so
will have a distinct disadvantage in the ratio of efficiency to weight.

Mechanical principles show that the steam pressures which the engines
would bear would be the same, and that the larger engine, though it
would have more than four times the horse-power of the other, would
have less than eight times. The larger of the two machines would
therefore be at a disadvantage, which could be overcome only by
reducing the thickness of its parts, especially of its