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.
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.