Page:Works of Jules Verne - Parke - Vol 1.djvu/256

222 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.