velope
expands. If the envelope were hermetically sealed, the higher the ship
rose the greater would become the internal pressure, until the envelope
finally burst. To avoid this difficulty in a balloon, a valve is
provided through which the gas can escape. In a balloon, therefore,
which ascends from the ground full, gas is lost throughout its upward
journey, and when it comes down again it is partially empty or flabby.
This would be an impossible situation in the case of the airship, for
she would become unmanageable, owing to the buckling of the envelope
and the sagging of the planes. Ballonets are therefore fitted to
prevent this happening.

Ballonets are internal balloons or air compartments fitted inside the
main envelope, and were originally filled with air by a blower driven
either by the main engines or an auxiliary motor. These blowers were a
continual source of trouble, and at the present day it has been
arranged to collect air from the slip-stream of the propeller through a
metal air scoop or blower-pipe and discharge it into an air duct which
distributes it to the ballonets.

The following example will explain their functions:

An airship ascends from the ground full to 1,000 feet. The ballonets
are empty, and remain so throughout the ascent. By the time the
airship reaches 1,000 feet it will have lost 1/30th of its volume of
gas which will have escaped through the valves. If the ship has a
capacity of 300,000 cubic feet it will have lost 10,000 cubic feet of
gas. The airship now commences to descend; as it descends the gas
within contracts and air is blown into the ballonets. By the time the
ground is reached 10,000 cubic feet of air will have been blown into
the ballonets and the airship will have retained its shape and not be
flabby.

On making a second ascent, as the airship rises the air must be let out
of the ballonet instead of gas from the envelope, and by the time 1,000
feet is reached the ballonets will be empty. To ensure that this is
always done the ballonet valves are set to open at less pressure than
the gas valves.

It therefore follows in the example under consideration that it will
not be necessary to lose gas during flight, provided that an ascent is
not made over 1,000 feet.

Valves are provided to prevent the pressure in the envelope from
exceeding a certain determined maximum and are fitted both to ballonets
and the gaschamber. They are automatic in action, and, as we