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by 160 pounds. This is equivalent to throwing out that
weight of ballast. If I augment the temperature by 180
degrees, the gas will dilate 180/480 and will displace 16,740
cubic feet more, and its ascensional force will be augmented
by 1,600 pounds.
"Thus, you see, gentlemen, that I can easily effect
very considerable changes of equilibrium. The volume of
the balloon has been calculated in such manner that, when
half inflated, it displaces a weight of air exactly equal to
that of the envelope containing the hydrogen gas, and of
the car occupied by the passengers, and all its apparatus
and accessories. At this point of inflation, it is in exact
equilibrium with the air, and neither mounts nor descends.
"In order, then, to effect an ascent, I give the gas a
temperature superior to the temperature of the surrounding
air by means of my cylinder. By this excess of heat
it obtains a larger distention, and inflates the balloon
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
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