# Works of Jules Verne/Five Weeks in a Balloon/Chapter 10

CHAPTER X PRELIMINARY EXPERIMENTS
"Attempts have been made frequently, gentlemen, to ascend and descend at will, without losing the gas in a balloon. A French aëronaut, M. Meumier, attempted to do this by compressing the air. A Belgian, Dr. Van Hecke, by means of wings and paddles, gained a vertical force, which has proved ineffective in the majority of instances. The practical results obtained by the above means are insignificant.
"I then resolved to go into the question boldly, and at once put the ballast on one side, if it were not a case of absolute necessity as to the breaking of my apparatus, or in case of being obliged to rise suddenly to avoid any obstacle.
"My means of ascent and descent consist equally in the dilation or contraction by varying temperatures of the gas confined in the balloon. And this is how I manage it.
"You have already seen put on board certain chests with the car, of which you did not understand the utility. These chests are five in number.
"The first contains about twenty-five gallons of water, to which I add sulphuric acid to increase its conductibility, and I resolve it into its component parts by means of a strong Bautzen galvanic battery. Water, as you are aware, is composed of two volumes hydrogen gas to one of oxygen.
"The oxygen under the battery action goes off by the positive pole into a second chest. A third chest, placed on the top of it, and of about twice the size, receives the hydrogen which enters it by the negative pole.
"Two taps, one of which has an opening double that of the other, keep up a communication between these two cases and a fourth, which is known as the mixing chest. Here in fact the gases arising from the decomposition of the water mingle together. The capacity of this chest is about forty-one cubic feet. In the upper part of it is a platinum tube with a stop-cock.
"You will already have perceived, gentlemen, that the apparatus I have described is nothing more than an oxyhydrogen blow-pipe, the heat evolved by which by which surpasses that of a forge fire.
"That matter settled, I pass on to the second part of the apparatus.
​"From the lower part of my balloon, which is hermetically closed, two tubes pass out at a short distance from each other. One of these leads from the upper volume of hydrogen, the other from the lower. They both descend as far as the car, and terminate in a cylindrical iron chest called the heat chest. It is closed at each extremity by a strong disc of the same metal.
"The tube from the lower part of the balloon enters the cylindrical chest through the lower disc, and there assumes the shape of a coil, whose upper rings occupy nearly the entire height of the box. Before leaving the chest, the coil is led into a little cone, whose base, concave, like a round cap, is directed downwards.
"It is at the top of this cone that the second tube makes its exit, and it terminates, as I have said, in the upper folds of the balloon.
"The spherical cap of the little cone is made of platinum, so that it may not be melted under the action of the blowpipe, for this is placed at the bottom of the iron case in the center of the coil, and the flame lightly licks this cap.
"You know those stoves used for warming rooms? You know how they act? The air of the room is forced through the tubes and comes back warmer. So that what I have been describing is, after all, only a stove.
"And, in fact, what takes place? Once the blow-pipe is lighted, the hydrogen is warmed and rises rapidly by the tube to the upper part of the balloon. A vacuum is caused below, and the gas from the lower part is attracted to fill it, which, in its turn, is warmed, and is continually replaced, so that an extremely rapid current of gas is generated, leaving the balloon, returning, and being warmed without cessation.
"Now, gases increase 1/480 of their volume for every degree of heat. If, then, I create a temperature of 18°, the hydrogen in the balloon will increase 18/480, or 1,614 cubic feet; it will then displace 1,674 cubic feet of air more, which will increase its power of ascent 160 pounds. That comes, then, to the same weight of ballast. If I increase the temperature to 180°, the gas expands 180/480, it displaces 6,740 cubic feet, and the ascending force amounts to 1,600 pounds.
"You can understand, gentlemen, that I am easily able to obtain considerable changes of equilibrium. The volume of my balloon has been calculated in such a way that, when half ​inflated, it displaces a weight of air exactly equal to the envelope of hydrogen gas and of the car occupied by the travelers and their belongings. At this point of inflation it is in exact equilibrium in the air; it will neither rise nor fall.
"In order to ascend, I bring the gas to a temperature higher than the ambient temperature, by means of my blow-pipe; by this access of heat, a strong tension is created, and fills the balloon, which rises so long as I expand the hydrogen.
"The descent is, naturally, made by moderating the heat, and permitting the temperature to cool. The ascent will generally be much more rapid than the descent. But that is a very good feature, for one never wants to descend quickly, and it is, on the contrary, a quick upward movement by which I avoid danger beneath me, not above the balloon.
"However, as already hinted, I have a certain quantity of ballast which can be got rid of, and enable me to rise still more quickly if desirable. The valve at the top is only a safety-valve. The balloon itself looks after its supply of hydrogen; the variations of temperature which I can produce in the center of the gas reservoir are only applied to the ascending and descending movements.
"Now, gentlemen, I will just add a few practical details.
"The combustion of hydrogen and oxygen at the end of the blow-pipe produces only watery vapor. I have therefore provided the lower part of the cylindrical case with an escape-pipe acting with the pressure of two atmospheres. Consequently, so soon as that pressure has been reached, the vapor makes its escape of its own accord.
"Here are the exact figures.
"Twenty-five gallons of water, resolved into their constituent elements, yield 200 pounds of oxygen and 25 pounds of hydrogen. That represents, at the tension of the air, 1,890 cubic feet of the former and 3,780 cubic feet of the latter; altogether, 670 cubic feet of the mingled gases.
"Now the top of the blow-pipe, fully open, gives twenty-seven cubic feet per hour, with a flame at least six times more powerful than the largest lamp. On an average, then, and so as not to be too high up, I shall only burn nine cubic feet in the hour, so my twenty-five gallons of water represents 630 hours of aërial navigation, or rather more than twenty-six days.
​"As I can descend at pleasure and obtain water on my route, my journey is practically indefinite.
"There is my secret, gentlemen; it is very simple, and, like all simple things, it cannot but succeed. My plan is only the extension and contraction of the gas in the balloon, which necessitates no wings nor mechanical power of motion. A stove to produce changes of temperature and a blow-pipe to warm it are neither heavy nor in the way. I believe that I have overcome all the serious difficulties of the undertaking."
Here Doctor Ferguson ended his discourse, and was heartily applauded. No one had any objections to advance. Everything appeared provided for and carried out.
"Nevertheless," said the captain, "it may be very dangerous."
"What does that matter," rejoined the doctor, "if it be practicable?"
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