he aperture would
be below. Under this a fire was to be kindled--not, however, until
everything else should be ready; and the hot air rising up into the
aperture would enter the balloon, and cause it to swell out to its full
globular dimensions. More hot air being admitted, the cooler atmosphere
within would be expelled, the balloon would become lighter than the
surrounding air, and by the simple principle of atmospheric pressure it
would ascend into the air. It was expected it would do so--it was hoped
it would.

To say the truth, the hopes of the engineer were far from being high--
his expectations anything but sanguine. He had observed all along,
that, notwithstanding the process employed for lightening the eel-skins,
they were still far heavier than silk; and perhaps, after all, the
experiment might not succeed. There was another circumstance that had
as much _weight_ on the mind of Karl as the eel-skins; and that was
quite as likely to have a _bearing_ upon the balloon. He had not
overlooked the fact, that the spot, from which they proposed making the
ascent, was nearly ten thousand feet above the level of the sea. He
knew that the atmosphere in such a situation would be extremely
rarefied, and that a balloon, which might easily ascend many thousand
feet into the air starting from the level of the sea, would not stir
from the ground if carried to the top of a mountain ten thousand feet
high. This was the circumstance which preyed upon the spirit of the
young philosopher, and hindered him from entertaining any very sanguine
hopes of success in the experiment they were making.

The philosophical truth had been before his mind from the first, and at
times had almost determined him to abandon the project. But as he was
not sufficiently acquainted with the laws of aerostation as to be
certain of failure, he had worked on with the determination to seek
success, though it must be acknowledged with but faint hopes of finding
it.

Thus stood matters on the morning when it was finally arranged to launch
their great aerial ship, and ascertain whether it would swim.

All things were made ready at an early hour. The huge bag was set up
between the supporting stakes the _car_ was attached to it, as also
several ropes to keep the balloon from being carried away; and these
were fastened at their other ends to stout pegs, driven firmly into the
ground; while a little furnace of stones was built underneath to hold
the