of necessity lie at a very flat angle and exert a serious
longitudinal compression. This must be resisted by a high
internal pressure, which demands a stouter fabric for the
envelope and, therefore, increased weight. It follows that the
tendency of the envelope to deform is decreased as the distance
of the car from the gas compartment is increased.

One method of overcoming this difficulty is found by using the
Astra-Torres design. As will be seen from the diagram of the
North Sea airship, the loads are excellently distributed by the
several fans of internal rigging, while external head resistance
is reduced to a minimum, as the car can be slung close underneath
the envelope. Moreover, the direct longitudinal compression due
to the rigging is applied to a point considerably above the axis
of the ship. In a large non-rigid many of these difficulties can
be overcome by distributing the weight into separate cars along
the envelope itself.

We have seen that as an airship rises the gas contained in the
envelope 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.