Chapter 7

Geometrical Details.--Calculation of the Capacity of the Balloon.--The Double Receptacle.--The Covering.--The Car.--The Mysterious Apparatus.--The Provisions and Stores.--The Final Summing up.

Dr. Ferguson had long been engaged upon the details of his expedition. It is easy to comprehend that the balloon--that marvellous vehicle which was to convey him through the air--was the constant object of his solicitude.

At the outset, in order not to give the balloon too ponderous dimensions, he had decided to fill it with hydrogen gas, which is fourteen and a half times lighter than common air. The production of this gas is easy, and it has given the greatest satisfaction hitherto in aerostatic experiments.

The doctor, according to very accurate calculations, found that, including the articles indispensable to his journey and his apparatus, he should have to carry a weight of 4,000 pounds; therefore he had to find out what would be the ascensional force of a balloon capable of raising such a weight, and, consequently, what would be its capacity.

A weight of four thousand pounds is represented by a displacement of the air amounting to forty-four thousand eight hundred and forty-seven cubic feet; or, in other words, forty-four thousand eight hundred and forty-seven cubic feet of air weigh about four thousand pounds.

By giving the balloon these cubic dimensions, and filling it with hydrogen gas, instead of common air--the former being fourteen and a half times lighter and weighing therefore only two hundred and seventy-six pounds--a difference of three thousand seven hundred and twenty-four pounds in equilibrium is produced; and it is this difference between the weight of the gas contained in the balloon and the weight of the surrounding atmosphere that constitutes the ascensional force of the former.

However, were the forty-four thousand eight hundred and forty-seven cubic feet of gas of which we speak, all introduced into the balloon, it would be entirely filled; but that would not do, because, as the balloon continued to mount into the more rarefied layers of the atmosphere, the gas within would dilate, and soon burst the cover containing it. Balloons, then, are usually only two-thirds filled.

But the doctor, in carrying out a project known only to himself, resolved to fill his balloon only one-half; and, since he had to carry forty-four thousand eight hundred and forty-seven cubic feet of gas, to give his balloon nearly double capacity he arranged it in that elongated, oval shape which has come to be preferred. The horizontal diameter was fifty feet, and the vertical diameter seventy-five feet. He thus obtained a spheroid, the capacity of which amounted, in round numbers, to ninety thousand cubic feet.

Could Dr. Ferguson have used two balloons, his chances of success would have been increased; for, should one burst in the air, he could, by throwing out ballast, keep himself up with the other. But the management of two balloons would, necessarily, be very difficult, in view of the problem how to keep them both at an equal ascensional force.

After having pondered the matter carefully, Dr. Ferguson, by an ingenious arrangement, combined the advantages of two balloons, without incurring their inconveniences. He constructed two of different sizes, and inclosed the smaller in the larger one. His external balloon, which had the dimensions given above, contained a less one of the same shape, which was only forty-five feet in horizontal, and sixty-eight feet in vertical diameter. The capacity of this interior balloon was only sixty-seven thousand cubic feet: it was to float in the fluid surrounding it. A valve opened from one balloon into the other, and thus enabled the aeronaut to communicate with both.

This arrangement offered the advantage, that if gas had to be let off, so as to descend, that which was in the outer balloon would go first; and, were it completely emptied, the smaller one would still remain intact. The outer envelope might then be cast off as a useless encumbrance; and the second balloon, left free to itself, would not offer the same hold to the currents of air as a half-inflated one must needs present.

Moreover, in case of an accident happening to the outside balloon, such as getting torn, for instance, the other would remain intact.

The balloons were made of a strong but light Lyons silk, coated with gutta percha. This gummy, resinous substance is absolutely water-proof, and also resists acids and gas perfectly. The silk was doubled, at the upper extremity of the oval, where most of the strain would come.