tapers at either end to a point which is slightly more
accentuated aft. Owing to the shape, these ships, in the early
days until experience had been gained, were extremely difficult
to handle, both on the landing ground and also in the air. They
were extremely unstable both in a vertical and horizontal plane,
and were slow in answering to their rudders and elevators.

The envelope is composed of rubber-proofed fabric doped to hold
the gas and resist the effects of weather. Four ballonets are
situated in the envelope, two in each of the lower lobes, air
being conveyed to them by means of a fabric air duct, which is
parallel to the longitudinal centre line of the envelope, with
transverse ducts connecting each pair of ballonets. In earlier
types of the Coastal, the air scoop supplying air to the air duct
was fitted in the slip stream of the forward engine, but later
this was fitted aft of the after engine.

Six valves in all are used, four air valves, one fitted to each
ballonet, and two gas valves. These are situated well aft, one
to each of the lower lobes, and are fitted on either side of the
rudder plane. A top valve is dispensed with because in practice
when an Astra-Torres envelope loses shape, the tendency is for
the tail to be pulled upwards by the rigging, with the result
that the two gas valves always remain operative.

Crabpots and non-return valves are employed in a similar manner
to S.S. airships.

The Astra-Torres system of internal rigging must now be described
in some detail. The envelope is made up of three longitudinal