"The attempt has often been made, gentlemen," said
the doctor, "to rise and descend at will, without losing
ballast or gas from the balloon. A French aeronaut, M.
Meunier, tried to accomplish this by compressing air in an
inner receptacle. A Belgian, Dr. Van Hecke, by means
of wings and paddles, obtained a vertical power that would
have sufficed in most cases, but the practical results
secured from these experiments have been insignificant.
"I therefore resolved to go about the thing more directly;
so, at the start, I dispensed with ballast altogether,
excepting as a provision for cases of special emergency,
such as the breakage of my apparatus, or the necessity of
ascending very suddenly, so as to avoid unforeseen obstacles.
"My means of ascent and descent consist simply in dilating
or contracting the gas that is in the balloon by the
application of different temperatures, and here is the
method of obtaining that result.
"You saw me bring on board with the car several
cases or receptacles, the use of which you may not have
understood. They are five in number.
"The first contains about twenty-five gallons of water,
to which I add a few drops of sulphuric acid, so as to
augment its capacity as a conductor of electricity, and then I
decompose it by means of a powerful Buntzen battery.
Water, as you know, consists of two parts of hydrogen to
one of oxygen gas.
"The latter, through the action of the battery, passes
at its positive pole into the second receptacle. A third
receptacle, placed above the second one, and of double its
capacity, receives the hydrogen passing into it by the
negative pole.
"Stopcocks, of which one has an orifice twice the size
of the other, communicate between these receptacles and
a fourth one, which is called the mixture reservoir, since in
it the two gases obtained by the decomposition of the
water do really commingle. The capacity of this fourth
tank is about forty-one cubic feet.
"On the upper part of this tank is a platinum tube
provided with a stopcock.
"You will now readily understand, gentlemen, the apparatus
that I have described to you is really a gas cylinder
and blow-pipe for oxygen and hydrogen, the heat of
which exceeds that of a forge fire.
"This much established, I proceed to the second part
of my apparatus. From the lowest part of my balloon,
which is hermetically closed, issue two tubes a little
distance apart. The one starts among the upper layers of the
hydrogen gas, the other amid the lower layers.
"These two pipes are provided at intervals with strong
jointings of india-rubber, which enable them to move in
harmony with the oscillations of the balloon.
"Both of them run down as far as the car, and lose
themselves in an iron receptacle of cylindrical form,
which is called the heat-tank. The latter is closed at
its two ends by two strong plates of the same metal.
"The pipe running from the lower part of the balloon
runs into this cylindrical receptacle through the lower
plate; it penetrates the latter and then takes the form of
a helicoidal or screw-shaped spiral, the rings of which,
rising one over the other, occupy nearly the whole of the
height of the tank. Before again issuing from it, this
spiral runs into a small cone with a concave base, that is
turned downward in the shape of a spherical cap.
"It is from the top of this cone that the second pipe
issues, and it runs, as I have said, into the upper beds of
the balloon.
"The spherical cap of the small cone is of platinum, so
as not to melt by the action of the cylinder and blow-pipe,
for the latter are placed upon the bottom of the iron tank
in the midst of the helicoidal spiral, and the extremity of
their flame will slightly touch the cap in question.
"You all know, gentlemen, what a calorifere, to heat
apartments, is. You know how it acts. The air of the
apartments is forced to pass through its pipes, and is then
released with a heightened temperature. Well, what I
have just described to you is nothing more nor less than a
calorifere.
"In fact, what is it that takes place? The cylinder
once lighted, the hydrogen in the spiral and in the
concave cone becomes heated, and rapidly ascends through
the pipe that leads to the upper part of the balloon. A
vacuum is created below, and it attracts the gas in the
lower parts; this becomes heated in its turn, and is
continually replaced; thus, an extremely rapid current of gas
is established in the pipes and in the spiral, which issues
from the balloon and then returns to it, and is heated over
again, incessantly.
"Now, the cases increase 1/480 of their volume for each
degree of heat applied. If, then, I force the temperature
18 degrees, the hydrogen of the balloon will dilate 18/480 or
1614 cubic feet, and will, therefore, displace 1614 more
cubic feet of air, which will increase its ascensional power
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
is attained, the steam escapes of itself.
"Here are the exact figures: 25 gallons of water,
separated into its constituent elements, yield 200 pounds
of oxygen and 25 pounds of hydrogen. This represents,
at atmospheric tension, 1,800 cubic feet of the former and
3,780 cubic feet of the latter, or 5,670 cubic feet, in all, of
the mixture. Hence, the stopcock of my cylinder, when
fully open, expends 27 cubic feet per hour, with a flame at
least six times as strong as that of the large lamps used
for lighting streets. On an average, then, and in order to
keep myself at a very moderate elevation, I should not
burn more than nine cubic feet per hour, so that my
twenty-five gallons of water represent six hundred and
thirty-six hours of aerial navigation, or a little
more than twenty-six days.
"Well, as I can descend when I please, to replenish my
stock of water on the way, my trip might be indefinitely
prolonged.
"Such, gentlemen, is my secret. It is simple, and,
like most simple things, it cannot fail to succeed. The
dilation and contraction of the gas in the balloon is my
means of locomotion, which calls for neither cumbersome
wings, nor any other mechanical motor. A calorifere to
produce the changes of temperature, and a cylinder to
generate the heat, are neither inconvenient nor heavy. I
think, therefore, that I have combined all the elements of
success."
Dr. Ferguson here terminated his discourse, and was
most heartily applauded. There was not an objection to
make to it; all had been foreseen and decided.
"However," said the captain, "the thing may prove
dangerous."
"What matters that," replied the doctor, "provided
that it be practicable?"