# Five Weeks in a Balloon HTML version

CHAPTER SEVENTH
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.