ight varies in almost inverse ratio to the square of the speed, and
therefore becomes almost insignificant.

It is also known that as the weight of a flying animal increases, the
less is the proportional increase in the surface beaten by the wings
in order to sustain it, although the motion of the wings becomes
slower.

A flying machine must therefore be constructed to take advantage of
these natural laws, to imitate the bird, "that admirable type of
aerial locomotion," according to Dr. Marcy, of the Institute of
France.

In short the contrivances likely to solve the problem are of three
kinds:--

1. Helicopters or spiralifers, which are simply screws with vertical
axes.

2. Ornithopters, machines which endeavour to reproduce the natural
flight of birds.

3. Aeroplanes, which are merely inclined planes like kites, but towed
or driven by screws.

Each of these systems has had and still has it partisans obstinately
resolved to give way in not the slightest particular. However, Robur,
for many reasons, had rejected the two first.

The ornithopter, or mechanical bird, offers certain advantages, no
doubt. That the work and experiments of M. Renard in 1884 have
sufficiently proved. But, as has been said, it is not necessary to
copy Nature servilely. Locomotives are not copied from the hare, nor
are ships copied from the fish. To the first we have put wheels which
are not legs; to the second we have put screws which are not fins.
And they do not do so badly. Besides, what is this mechanical
movement in the flight of birds, whose action is so complex? Has not
Doctor Marcy suspected that the feathers open during the return of
the wings so as to let the air through them? And is not that rather a
difficult operation for an artificial machine?

On the other hand, aeroplanes have given many good results. Screws
opposing a slanting plane to the bed of air will produce an
ascensional movement, and the models experimented on have shown that
the disposable weight, that is to say the weight it is possible to
deal with as distinct from that of the apparatus, increases with the
square of the speed. Herein the aeroplane has the advantage over the
aerostat even when the aerostat is furnished with the means of
locomotion.

Nevertheless Robur had thought that the simpler his contrivance the
better. And the screws--the Saint Helices that had been thrown in
his teeth at the Weldon Institute--had sufficed for all the needs of
his