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.
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