บทที่ บทที่ 7 · Geometrical Details.—Calculation of the Capacity of the Balloon.—The

. 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 aëronaut 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.

Such an envelope as this could 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 eleven thousand six hundred

square feet, its envelope weighed six hundred and fifty pounds. The

envelope of the second or inner balloon, having nine thousand two

hundred square feet of surface, weighed only about five hundred and ten

pounds, or say eleven hundred and sixty pounds for both.

The network that supported the car was made of very strong hempen cord,

and the two valves were the object of the most minute and careful

attention, as the rudder of a ship would be.

The car, which was of a circular form and fifteen feet in diameter, was

made of wicker-work, strengthened with a slight covering of iron, and

protected below by a system of elastic springs, to deaden the shock of

collision. Its weight, along with that of the network, did not exceed

two hundred and fifty pounds.

In addition to the above, the doctor caused to be constructed two

sheet-iron chests two lines in thickness. These were connected by means

of pipes furnished with stopcocks. He joined to these a spiral, two

inches in diameter, which terminated in two branch pieces of unequal

length, the longer of which, however, was twenty-five feet in height

and the shorter only fifteen feet.

These sheet-iron chests were embedded in the car in such a way as to

take up the least possible amount of space. The spiral, which was not

to be adjusted until some future moment, was packed up, separately,

along with a very strong Buntzen electric battery. This apparatus had

been so ingeniously combined that it did not weigh more than seven

hundred pounds, even including twenty-five gallons of water in another

receptacle.

The instruments provided for the journey consisted of two barometers,

two thermometers, two compasses, a sextant, two chronometers, an

artificial horizon, and an altazimuth, to throw out the height of

distant 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; he

merely wanted to be able to know in what direction he was passing, and

to determine the position of the principal rivers, mountains, and

towns.

He also provided himself with three thoroughly tested iron anchors, and

a light but strong silk ladder fifty feet in length.

He at the same time carefully weighed his stores of provision, which

consisted of tea, coffee, biscuit, salted meat, and _pemmican_, a

preparation 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 that the

equilibrium of a balloon floating in the atmosphere is extremely

sensitive. The loss of an almost insignificant weight suffices to

produce a very noticeable displacement.

Nor did the doctor forget an awning to shelter the car, nor the

coverings and blankets that were to be the bedding of the journey, nor

some fowling pieces and rifles, with their requisite supply of powder

and ball.

Here is the summing up of his various items, and their weight, as he

computed 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. Ferguson

proposed to carry up with him. He took only two hundred pounds of

ballast for “unforeseen emergencies,” as he remarked, since otherwise

he did not expect to use any, thanks to the peculiarity of his

apparatus.