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Geometrical details—Estimate of the balloon's capacity—The double balloon—The envelope—The car—The mysterious apparatus—The stores—Total weight
DR FERGUSSON had long been busy with the details of his expedition. It can be imagined that the balloon, that wonderful vehicle destined to carry him through the air, was the object of his constant solicitude.
First of all, in order to avoid giving the balloon too big dimensions, he decided to inflate it with hydrogen, which is fourteen and a half times lighter than air. The gas is easy to produce, and has given the best results in aerostatic experiments.
After very careful calculation, the doctor found that in order to have with him all the things indispensable to his journey and apparatus, he would have to carry a weight of 4000 lbs. He therefore had to find out what would be the lifting force capable of raising this weight and consequently what would be its volume.
A weight of 4000 lbs represents an air displacement of 44,847 cubic feet, which means that 44,847 cubic feet of air weigh, approximately, 4000 lbs. By giving the balloon this capacity of 44,847 cubic feet and filling it, not with air but with hydrogen, which is fourteen and a half times lighter and weighs only 276 lbs, a change of equilibrium is caused amounting to a difference of 3780 lbs. It is this difference between the weight of the gas contained in the balloon and the weight of the air surrounding it that constitutes the lifting force of the balloon.
Yet if he put into the balloon the 44,847 cubic feet of gas we have mentioned, it would be completely filled. Now this would not do, for as the balloon rises into the less dense layers of the atmosphere, the gas contained in it tends to expand, and would soon burst the envelope. It is therefore usual only to fill the balloon up to two-thirds of its capacity. The doctor, however, according to a certain plan known to himself alone, decided to fill his balloon to only one-half of its capacity and, since he had to carry 4447 cubic feet of hydrogen, to double, approximately, the capacity of his balloon.
He designed it in that elongated form which is known to be the best. Its horizontal diameter was fifty feet and the vertical diameter seventyfive.
This gave him a spheroid with a capacity in round figures of 90,000 cubic feet.
Had Dr Fergusson been able to employ two balloons his chances of success would have been increased, for, had one happened to break down in the air, he could have thrown out ballast and kept up by means of the other. But it would have been very difficult to handle two balloons when it came to a question of keeping their lifting force equal.
After long reflection, Fergusson, by an ingenious device, obtained the advantages of two balloons without the inconvenience. He constructed two of unequal size and enclosed one within the other. His outer balloon, to which he gave the dimensions already stated, contained inside it a smaller one of the same shape, having a horizontal diameter of only forty-five feet and a vertical diameter of sixty-eight feet. The capacity of this inner balloon was therefore only 67,000 cubic feet. It was to float in the gas surrounding it. A tube connected one balloon with the other and, in case of need, allowed of connection between them.
This arrangement afforded the advantage that, should it be necessary to emit gas in order to come down, the gas of the outer balloon could be emitted first. Should it be necessary to deflate completely, the smaller one would remain intact. The outer envelope could then be removed as an unnecessary burden, and the second balloon, left free, would only offer to the wind the resistance of a half-deflated balloon.
Further, in case of an accident, such as a tear in the outer envelope, the second envelope would remain uninjured.
The two balloons were constructed of twilled taffeta from Lyons, treated with gutta-percha. This resinous substance has the advantage of being completely gas-tight. It is entirely proof against the action of acid or gas. The taffeta at the upper pole of the spheroid, where practically all the strain is concentrated, was of double thickness.
This envelope was capable of retaining the gas indefinitely. It weighed half a pound per nine square feet. Now the surface of the outer balloon having an area of about 11,600 square feet, the envelope weighed 650 lbs. The envelope of the second balloon, with a surface area of 9200 square feet, only weighed 510 lbs, making altogether 1160 lbs.
The net to support the car was made of hemp rope of great strength.
The two tubes were the object of minute care, such as would be given to the steering gear of a ship. The car, circular in form and with a diameter of fifteen feet, was constructed of wicker-work strengthened by a light casing of iron and fitted below with shock-absorbing springs. Its weight, together with that of the net, did not exceed 280 lbs.
In addition, the doctor had four sheet-iron containers constructed of double thickness. They were connected together by pipes, to which taps were fitted. There was also a spiral tube two inches in diameter, ending in two straight pieces of unequal length, the longer measuring twenty-five feet and the other only fifteen. These containers could be packed into the car in such a way as to occupy the minimum of space. The spiral, which was not to be fitted until later, was packed separately, as was also a very powerful Bunsen electric battery. This apparatus was so cleverly contrived that it weighed no more than 700 lbs, even including twenty-five gallons of water in a special container.
The instruments selected for the journey consisted of two barometers, two thermometers, two compasses, a sextant, two chronometers, a theodolite, and an altazimuth for distant and inaccessible objects. The Greenwich Observatory had placed itself at the doctor's disposal. He did not, however, intend to make physical experiments, but merely wanted to determine his direction and the position of the principal rivers, mountains, and towns. He also furnished himself with three carefully tested iron anchors and a ladder of strong light silk, about fifty feet long.
He also calculated the precise weight of his stores, which consisted of tea, coffee, biscuits, salt meat, and pemmican, a preparation combining small volume with great nutritive properties. Apart from an adequate reserve of brandy, he provided two water containers, each having a capacity of twenty-two gallons. The consumption of these various stores would gradually diminish the weight carried by the balloon. For it must be realised that the equilibrium of a balloon in the air is extremely sensitive. The loss of an almost insignificant weight can produce a very appreciable displacement.
The doctor did not forget an awning to cover over a section of the car, nor the blankets which were to compose the sole bedding for the journey, nor the sporting guns, nor his supplies of powder and shot.
Here is a table giving his calculations:
Such were the details of the 4000 lbs which Dr Fergusson proposed to carry. They included only 200 lbs of ballast, 'for emergencies only,' he said, for, thanks to his special device, he was counting on not using it at all.