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Five Weeks in a Balloon by Jules Verne
page 91 of 453 (20%)
more. The latter, then, ascends in proportion as I heat
the hydrogen.

"The descent, of course, is effected by lowering the
heat of the cylinder, and letting the temperature abate.
The ascent would be, usually, more rapid than the descent;
but that is a fortunate circumstance, since it is of no
importance to me to descend rapidly, while, on the other
hand, it is by a very rapid ascent that I avoid obstacles.
The real danger lurks below, and not above.

"Besides, as I have said, I have a certain quantity of
ballast, which will enable me to ascend more rapidly still,
when necessary. My valve, at the top of the balloon, is
nothing more nor less than a safety-valve. The balloon
always retains the same quantity of hydrogen, and the
variations of temperature that I produce in the midst of
this shut-up gas are, of themselves, sufficient to provide
for all these ascending and descending movements.

"Now, gentlemen, as a practical detail, let me add
this:

"The combustion of the hydrogen and of the oxygen
at the point of the cylinder produces solely the vapor or
steam of water. I have, therefore, provided the lower
part of the cylindrical iron box with a scape-pipe, with a
valve operating by means of a pressure of two atmospheres;
consequently, so soon as this amount of pressure
is attained, the steam escapes of itself.
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