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 fire, whose ascending caloric was to expand the balloon, and raise it into the air.
The fuel out of which this fire was to be made had been already collected near the spot.

It was not wood, nor faggots of any kind; for although these might have served after a fashion, Karl was acquainted with a better material.