ell aware. Although I have taught you a
little of their lore, it was when I had nothing to do, and you were
equally idle; otherwise I should have considered that both of us were
wasting time. You already know my opinions on that subject--which are:
that a knowledge of what is usually termed `the classics' is of about as
much use to a reasoning man as might be an equally profound knowledge of
Chinese _mnemonics_. The time I have spent in the study of the _dead_
languages has been sheer waste; and all I have learnt wont raise us a
foot higher here. My knowledge of Jupiter and Juno is not likely to
gain us the means of getting out of our difficulty, no more than my
acquaintance with Mercury will help me to a pair of wings. So a truce
to classical ideas, and let us see whether scientific ones may not serve
us better just now. You have a quick invention, brother Caspar; can you
think of anything--I mean anything within our reach--that would make the
air-bag of a balloon?"

"But could you make the balloon, if you had the stuff?" inquired Caspar,
still in doubt whether any other than an experienced aeronaut could
construct so wonderful a machine.

"Pooh!" replied the philosopher, "the making of a balloon is almost as
easy as making a soap-bubble. Any air-tight bag, filled with heated
atmosphere, becomes a balloon. 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 burni