ich should be kept in motion. Thirdly, by
rockets, which, going off successively, would drive up the balloon by
the force of projection. Fourthly, by an octahedron of glass, heated by
the sun, and of which the lower part should be allowed to penetrate the
dense cold air, which, pressing up against the rarefied hot air, would
raise the balloon. Fifthly, by a car of iron and a ball of magnetised
iron, which the aeronaut would keep throwing up in the air, and which
would attract and draw up the balloon. The wiseacre who invented these
modes of flying in the air seems, some would say, to have been more in
want of very strict confinement on the earth than of the freedom of the
skies.

In 1670 Francis Lana constructed the flying-machine shown on the next
page. The specific lightness of heated air and of hydrogen gas not
having yet been discovered, his only idea for making his globes rise was
to take all the air out of them. But even supposing that the globes were
thus rendered light enough to rise, they must inevitably have collapsed
under the atmospheric pressure.

As for the idea of making use of a sail to direct the balloon, as one
directs a vessel, that also was a delusion; for the whole machine,
globes and sails, being freely thrown into the air, would infallibly
follow the direction of the wind, whatever that might be. When a ship
lies in the sea, and its sails are inflated with the wind, we must
remember that there are two forces in operation--the active force of
the wind and the passive force of the resistance of the water; and in
working these forces the one against the other, the sailor can turn
within a point of any direction he pleases. But when we are subjected
wholly to a single force, and have no point of support by the use of
which to turn that force to our own purposes, as is the case with the
aeronaut, we are entirely at the mercy of that force, and must obey it.

After the flying-machine of Lana there was constructed by Galien (who,
like the former, was an ecclesiastic) an air-boat, less chimerical in
its form, looked at in view of the conditions of aerial navigation,
but much more singular. Galien describes his air-boat, in 1755, in his
little work entitled, "The Art of Sailing in the Air." His project was
a most extraordinary one, and its boldness is only equalled by the
seriousness of the narrative. According to him, the atmosphere is
divided into two horizontal layers, the upper of which is much light