Five Weeks in a Balloon Chapter 7

**Alex E. Talander** (0:26)
CHAPTER VII.
GEOMETRICAL DETAILS CALCULATION OF THE CAPACITY OF THE BALLOON THE DOUBLE RECEPTICLE THE COVERING THE CAR THE MYSTERIOUS EPRATUS 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 four thousand 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 44,847 cubic feet, or in other words, 44,847 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 essential force of the former. However, were the 44,847 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 44,847 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 50 feet and the vertical diameter 75 feet. He thus obtained a spheroid, the capacity of which amounted in round numbers to 90,000 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 to keep them both at an equal and sensual 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 enclosed 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 for 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 empty the smaller one would still remain intact. The outer envelope might then be cast off as a useless encumbrance, and the second balloon, left freer 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 outer would remain intact. The balloons were made of a strong but light leon silk, coated with gutta percha. This gummy, resonance substance is absolutely waterproof and also resists acids and gas perfectly. The silk was doubled at the upper extremity of the oval, where the most of the strain would come.
Such an envelope as this would retain the inflating fluid for any length of time. It weighed half a pound per nine square feet, hence the surface of the outside balloon being about 11,600 square feet, its envelope weighed 650 pounds. The envelope of the second, or inner balloon, having 9,200 square feet of surface, weighed only about 510 pounds, or, say, 1160 pounds for both. The network that supported the car was made of very strong hemp and cord, and the two valves were the object of the most minute and careful attention, as the rudder of a ship would be.