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