This had been already prepared; and only needed to be attached to the bottom of the air-bag.
In the present case, the "bottom of the bag" is quite a figure of speech--_lucus a non lucendo_.

Strictly speaking, it had no bottom; but, where this should have been, there was a round aperture, formed by a stout hoop of ringall bamboo, to which the skin covering was lashed, and to which, also, the cords intended to sustain the afore-mentioned basket, as also the stay-ropes, were to be attached.
The object of this aperture will be easily understood.

It was by it that the hot air was to be admitted inside the balloon, for the purpose of inflating it.
And how was this hot air to be obtained?
That was a question which Karl alone could answer.

Of course, fire was to be the agent for producing it: but how was it to be got into the bag?
Karl could tell that, and Karl only; and, now that the time had arrived for trying the experiment, he condescended to explain to his coadjutors how he meant to proceed.
The bag was to be propped up between tall stakes set in the ground; its bottomless bottom turned towards the earth, so that the 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.