The question is, what weight it can be made to carry--including the materials out of which it may be constructed." "But how are you to get the heated air into it ?" "Simply by making a fire under an aperture left open below." "But would not this air soon become cold again ?" "Yes; and then the balloon would sink back to the earth from the air inside getting cooled, and becoming as heavy as that without.

Of course," continued the philosopher, "you are aware that heated air is much lighter than the ordinary atmosphere; and that is why a balloon filled with the former, rises, and will continue rising, till it has reached that elevation, where the rarefied atmosphere is as light as the heated air.

Then it can go no further, and the weight of the balloon itself will bring it down again.

A bladder of ordinary air sunk in water, or a corked bottle, will illustrate this point to your comprehension." "I comprehend it well enough," rejoined Karl, rather piqued at being treated too much _a l'enfant_ by his learned brother.

"But I thought that, in a balloon, it was necessary to keep a fire constantly burning-- a sort of grate or fire-basket suspended below.