CHAPTER SEVENTH.
Geometrical Details.--Calculation of the Capacity of the Balloon.--TheDouble Receptacle.--The Covering.--The Car.--The MysteriousApparatus.--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 vehiclewhich was to convey him through the air--was the constant object of hissolicitude.
At the outset, in order not to give the balloon too ponderousdimensions, he had decided to fill it with hydrogen gas, which isfourteen and a half times lighter than common air. The production ofthis gas is easy, and it has given the greatest satisfaction hitherto inaerostatic 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 tofind out what would be the ascensional force of a balloon capable ofraising such a weight, and, consequently, what would be its capacity.
A weight of four thousand pounds is represented by a displacement of theair amounting to forty-four thousand eight hundred and forty-sevencubic feet; or, in other words, forty-four thousand eight hundred andforty-seven cubic feet of air weigh about four thousand pounds.
By giving the balloon these cubic dimensions, and filling it withhydrogen gas, instead of common air--the former being fourteen anda half times lighter and weighing therefore only two hundred andseventy-six pounds--a difference of three thousand seven hundred andtwenty-four pounds in equilibrium is produced; and it is this differencebetween the weight of the gas contained in the balloon and the weight ofthe surrounding atmosphere that constitutes the ascensional force of theformer.
However, were the forty-four thousand eight hundred and forty-sevencubic feet of gas of which we speak, all introduced into the balloon, itwould be entirely filled; but that would not do, because, as the ballooncontinued 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 carryforty-four thousand eight hundred and forty-seven cubic feet of gas,to give his balloon nearly double capacity he arranged it in thatelongated, oval shape which has come to be preferred. The horizontaldiameter was fifty feet, and the vertical diameter seventy-five feet.He thus obtained a spheroid, the capacity of which amounted, in roundnumbers, to ninety thousand cubic feet.
Could Dr. Ferguson have used two balloons, his chances of success wouldhave been increased; for, should one burst in the air, he could, bythrowing out ballast, keep himself up with the other. But the managementof two balloons would, necessarily, be very difficult, in view of theproblem how to keep them both at an equal ascensional force.
After having pondered the matter carefully, Dr. Ferguson, by aningenious arrangement, combined the advantages of two balloons, withoutincurring their inconveniences. He constructed two of different sizes,and inclosed the smaller in the larger one. His external balloon, whichhad the dimensions given above, contained a less one of the same shape,which was only forty-five feet in horizontal, and sixty-eight feetin vertical diameter. The capacity of this interior balloon wasonly sixty-seven thousand cubic feet: it was to float in the fluidsurrounding it. A valve opened from one balloon into the other, and thusenabled 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 remainintact. The outer envelope might then be cast off as a uselessencumbrance; and the second balloon, left free to itself, would notoffer the same hold to the currents of air as a half-inflated one mustneeds present.
Moreover, in case of an accident happening to the outside balloon, suchas getting torn, for instance, the other would remain intact.
The balloons were made of a strong but light Lyons silk, coated withgutta percha. This gummy, resinous substance is absolutely water-proof,and also resists acids and gas perfectly. The silk was doubled, at theupper extremity of the oval, where most of the strain would come.
Such an envelope as this could retain the inflating fluid for any lengthof time. It weighed half a pound per nine square feet. Hence the surfaceof the outside balloon being about eleven thousand six hundred squarefeet, its envelope weighed six hundred and fifty pounds. The envelopeof the second or inner balloon, having nine thousand two hundred squarefeet of surface, weighed only about five hundred and ten pounds, or sayeleven hundred and sixty pounds for both.
The network that supported the car was made of very strong hempencord, and the two valves were the object of the most minute and carefulattention, as the rudder of a ship would be.
The car, which was of a circular form and fifteen feet in diameter, wasmade of wicker-work, strengthened with a slight covering of iron, andprotected below by a system of elastic springs, to deaden the shock ofcollision. Its weight, along with that of the network, did not exceedtwo hundred and fifty pounds.
In addition to the above, the doctor caused to be constructed twosheet-iron chests two lines in thickness. These were connected by meansof pipes furnished with stopcocks. He joined to these a spiral, twoinches in diameter, which terminated in two branch pieces of unequallength, the longer of which, however, was twenty-five feet in height andthe shorter only fifteen feet.
These sheet-iron chests were embedded in the car in such a way as totake up the least possible amount of space. The spiral, which was not tobe adjusted until some future moment, was packed up, separately, alongwith a very strong Buntzen electric battery. This apparatus had beenso ingeniously combined that it did not weigh more than seven hundredpounds, even including twenty-five gallons of water in anotherreceptacle.
The instruments provided for the journey consisted of two barometers,two thermometers, two compasses, a sextant, two chronometers, anartificial horizon, and an altazimuth, to throw out the height ofdistant and inaccessible objects.
The Greenwich Observatory had placed itself at the doctor's disposal.The latter, however, did not intend to make experiments in physics; hemerely wanted to be able to know in what direction he was passing, andto determine the position of the principal rivers, mountains, and towns.
He also provided himself with three thoroughly tested iron anchors, anda light but strong silk ladder fifty feet in length.
He at the same time carefully weighed his stores of provision, whichconsisted of tea, coffee, biscuit, salted meat, and pemmican, apreparation which comprises many nutritive elements in a small space.Besides a sufficient stock of pure brandy, he arranged two water-tanks,each of which contained twenty-two gallons.
The consumption of these articles would necessarily, little by little,diminish the weight to be sustained, for it must be remembered thatthe equilibrium of a balloon floating in the atmosphere is extremelysensitive. The loss of an almost insignificant weight suffices toproduce a very noticeable displacement.
Nor did the doctor forget an awning to shelter the car, nor thecoverings and blankets that were to be the bedding of the journey, norsome fowling pieces and rifles, with their requisite supply of powderand ball.
Here is the summing up of his various items, and their weight, as hecomputed it:
Ferguson........................... 135 pounds. Kennedy............................ 153 " Joe................................ 120 " Weight of the outside balloon...... 650 " Weight of the second balloon....... 510 " Car and network.................... 280 " Anchors, instruments, awnings, and sundry utensils, guns, coverings, etc................... 190 " Meat, pemmican, biscuits, tea, coffee, brandy................... 386 " Water.............................. 400 " Apparatus.......................... 700 "
Weight of the hydrogen............. 276 " Ballast............................ 200 " ----- 4,000 pounds.
Such were the items of the four thousand pounds that Dr. Fergusonproposed to carry up with him. He took only two hundred pounds ofballast for "unforeseen emergencies," as he remarked, since otherwise hedid not expect to use any, thanks to the peculiarity of his apparatus.