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 and on the landing ground, and at present there
are partisans of both types.

Thirdly, there is the streamline envelope with tangential suspensions,
which has been adopted for all classes of the S.S. airship, and which
has proved for its purpose in every way highly satisfactory.

Of these three types the rigid has the inherent disadvantage of not
being able to be dismantled, if it should become compelled to make a
forced landing away from its base. Even if it were so fortunate as to
escape damage in the actual landing, there is the practical certainty
that it would be completely wrecked immediately any increase occurred
in the force of the wind. On the other hand, for military purposes, it
possesses the advantage of having several gas compartments, and is in
consequence less susceptible to damage from shell fire and other causes.

Both the semi-rigid and the non-rigid have the very great advantage of
being easily deflated and packed up. In addition to the valves, these
ships have a ripping panel incorporated in the envelope which can
easily be torn away and allows the gas to escape with considerable
rapidity. Innumerable instances have occurred of ships being compelled
to land in out-of-the-way places owing to engine failure or other
reasons; they have been ripped and deflated and brought back to the
station without incurring any but the most trifling damage.

Experience in the war has proved that for military purposes the large
rigid, capable of long hours of endurances and the small non-rigid made
thoroughly reliable, are the most valuable types for future
development. The larger non-rigids, with the possible exception of the
North Sea, do not appear to be likely to fulfil any very useful
function.

Airship design introduces so many problems which are not met with in
the ordinary theory of structures, that a whole volume c