The semi-rigid, which has been exploited principally by the
Italians with their Forlanini airships, and in France by Lebaudy,
has an envelope, in some cases divided into separate
compartments, to which is attached close underneath a long girder
or keel. This supports the car and other weights and prevents
the whole ship from buckling in the event of losing gas. The
semi-rigid type has been practically undeveloped in this country.

The non-rigid, of which we may now claim to be the leading
builders, is of many varieties, and has been developed in several
countries. In Germany the chief production has been that of
Major von Parseval, and of which one ship was purchased by the
Navy shortly before the outbreak of war. In the earliest
examples of this type the car was slung a long way from the
envelope and was supported by wires from all parts. This
necessitated a lofty shed for its accommodation as the ship was
of great overall height; but this difficulty was overcome by the
employment of the elliptical and trajectory bands, and is
described in the chapter dealing with No. 4.

A second system is that of the Astra-Torres. This envelope is
trilobe in section, with internal rigging, which enables the car
to be slung very close up to the envelope. The inventor of these
envelopes was a Spaniard, Senor Torres Quevedo, who manufactured
them in conjunction with the Astra Company in Paris. This type
of envelope has been employed in this country in the Coastal, C
Star, and North Sea airships, and has been found on the whole to
give good results. It is questionable if an envelope of
streamline shape would not be easier to handle, both in the air