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