The present standard method of improving altitude regulation involves
the use of the ballonet, or compartment air bag, inside the main
envelope. For stability and effective propulsion, it is important that
the balloon preserve its shape, no matter how much gas be allowed to
escape. Dirigible balloons are divided into two types, according to
the method employed for maintaining the shape. In the Zeppelin type, a
rigid internal metal framework supports the gas envelope. This forms
a series of seventeen compartments, each isolated from the others. No
matter what the pressure of gas, the shape of the balloon is unchanged.
In the more common form of balloon, the internal air ballonet is empty,
or nearly so, when the main envelope is full. As gas is vented from the
latter, air is pumped into the former. This compresses the remaining
gas and thus preserves the normal form of the balloon outline.
[Illustration: BALLOON WITH BALLONETS]
But the air ballonet does more than this. It provides an opportunity
for keeping the balloon on a level keel, for by using a number of
compartments the air can be circulated from one to another as the case
may require, thus altering the distribution of weights. Besides this,
if the pressure in the air ballonet be initially somewhat greater
than that of the external atmosphere, a considerable ascent may be
produced by merely venting this air ballonet. This involves no loss of
gas; and when it is again desired to descend, air may be pumped into
the ballonet. If any considerable amount of gas should be vented, to
produce quick and rapid descent, the pumping of air into the ballonet
maintains the shape of the balloon and also facilitates the descent.
[Illustration: CONSTRUCTION OF THE ZEPPELIN BALLOON]