# Works of Jules Verne/Five Weeks in a Balloon/Chapter 7

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.
​If Dr. Ferguson had been able to make use of two balloons, his chances of success would have been increased, and if one happened to burst in the air, he could, by casting out ballast, save himself by means of the other. But the maneuvering of two balloons would have been very difficult when it was necessary to preserve an equal ascending power in both.
After much reflection, Ferguson, by an ingenious contrivance, united the utility of two balloons without their inconvenience; he constructed two of unequal size and enclosed one within the other. The exterior balloon, in which he adhered to the dimensions given above, contained a smaller one of the same shape, only forty-five and sixty-eight feet respectively, of horizontal and vertical diameter. The capacity of this interior balloon then was only 67,000 cubic feet. It floated in the fluid surrounding it. A valve opened from one balloon to the other, and admitted of communication between them.
This arrangement had this advantage viz., that if it were at any time necessary to let the gas escape, it could be let off from the larger balloon. Even if they were obliged to empty it altogether, the smaller one would remain intact; they could then detach the exterior covering a useless drag on them and the second balloon by itself would not offer the same resistance to the wind as a partially-filled balloon.
Furthermore, if by accident the outer balloon were injured, the other would be intact. Both balloons were made with striped taffetas from Lyons, coated with guttapercha. This resinous-gummy substance is perfectly waterproof, and is unaffected by acids or gas. The taffetas were placed side by side double, stretching upwards to the top, where almost all the weight was.
This envelope could retain the gas for an unlimited period. It weighed half a pound to nine square feet. Now as the surface of the exterior balloon was about. 11, 600 square feet, its envelope weighed 650 lbs. The envelope of the second balloon had a surface of 9,200 cubic feet, and weighed only 510 lbs.; altogether they weighed 1,160 lbs.
The netting to hold the car was made of the strongest hempen cord; the valves became objects of the most minute care, as if they had been the rudder of a ship. The car was ​of circular form, and fifteen feet in diameter, of osier, strengthened by a light iron covering, and fastened to the lower part by elastic springs, with a view to break the force of concussion. Its weight, including the net, did not exceed 280 pounds.
The doctor caused to be made also four chests of sheetiron about one-eighth of an inch thick. These were joined together by tubes furnished with taps. He added a coil about two inches wide, which terminated in two straight branches of unequal lengths, of which the greater was twenty-five feet high, and the shorter fifteen feet only. The chests were fitted into the car so as to occupy the least ossible space. The large tap, not easily fitted, was packed separately, as well as a large galvanic battery. This apparatus had been so ingeniously contrived that it only weighed 700 pounds, and contained as much as twenty-five gallons of water in one case alone.
The instruments prepared for the journey were two barometers, two thermometers, two compasses, a sextant, two chronometers, an artificial horizon, and an instrument to take the levels of distant and inaccessible objects. He had access to the Greenwich Observatory. He, however, did not propose to make any experiments in physics, he wished merely to become acquainted with his intended route, and to determine the position of the principal rivers, mountains, and towns.
He provided three grapnels of well-tested iron, also a silken ladder, tough and tight, about fifty feet in length.
He also estimated the weight of his provisions; they consisted of tea, coffee, biscuits, salt meat, and pemmican, a preparation which, in a very small compass, contains a great deal of nourishment. Besides a reserve of brandy, he stowed away two tanks of water,
The consumption of these viands would, by degrees, diminish the weight of the balloon. For it is very necessary to know that in the air a balloon is sensible of the least difference of weight. An almost inappreciable loss is sufficient to make a considerable difference in displacement.
The doctor had not forgotten a tent, which could cover up part of the car; neither rugs, which composed all their bed-clothes during the journey; nor the rifles and ammunition.
​The following is the statement of his different calculations:
Such was the detail of the 4,000 pounds that Doctor Ferguson proposed to raise. He only carried 200 pounds of ballast, "merely for a contingency," said he, for, thanks to his arrangements, he did not anticipate to be obliged to use it.
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