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