CHAPTER VII
THE MYSTERIOUS APPARATUS
Doctor Ferguson had been occupied for a long time in the details of his expedition. One can quite understand that the balloon, the wonderful vehicle destined to transport him through the air, was the object of his solicitude.
To begin with, and so as not to have the balloon too large, he resolved to inflate it with hydrogen gas, which is 14½ times lighter than the atmospheric air. This gas is easily made, and by its use has been the means of obtaining the best aerostatic observations.
The doctor, after careful calculation, found that, with the indispensable articles of the journey, clothes, &c., it would be necessary to carry a weight of 4,000 lbs. He must therefore provide an ascensional power capable of lifting this weight, and also ascertain what its capacity would be.
A weight of 4,000 lbs. is represented by a displacement of 44,877 cubic feet of air; in other words, that amount of air weighs about 4,000 lbs.
By giving to his balloon the capacity of 44,877 cubic feet of air, and filling it, in lieu of air, with hydrogen gas (which, being 14½ times lighter than air, would not weigh more than 275 lbs.), there would remain a difference in the equilibrium to the amount of 3,724 lbs. This is the difference between the weight of the gas in the balloon and the weight of the exterior air, which difference constitutes the ascensional power of the balloon.
Now, if we were to introduce the said 44,877 cubic feet of gas into the balloon it would be completely filled, and that would never do, because the higher the balloon rises into the atmosphere, the less dense is the air, and the gas would very quickly burst the covering. So a balloon is usually filled to the extent of two-thirds its capacity.
But the doctor, following out an idea of his own, resolved to fill the balloon only half full, and, inasmuch as he was obliged to carry 44,877 cubic feet of hydrogen, to make his balloon almost double the usual size.
He designed it of an elongated form, which appeared to be the best. The horizontal diameter was fifty feet, the vertical diameter seventy-five. He thus obtained a spheroid capable of containing (in round numbers) 90,000 cubic feet of gas.
