British Airships, Past, Present and Future | Page 5

upon the lift of an
airship. One of the greatest difficulties to be contended with is
maintaining the hydrogen pure in the envelope or gasbags for any
length of time. Owing to diffusion gas escapes with extraordinary
rapidity, and if the fabric used is not absolutely gastight the air finds its
way in where the gas has escaped. The maximum purity of gas in an
airship never exceeds 98 per cent by volume, and the following
example shows how greatly lift can be reduced: Under mean
atmospheric conditions, which are taken at a temperature of 55 degrees

Fahrenheit, and the barometer at 29.5 inches, the lift of 1,000 cubic feet
of hydrogen at 98 per cent purity is 69.6 lb. Under same conditions at
80 per cent purity the lift of 1,000 cubic feet of hydrogen is 56.9 lb., a
resultant loss of 12.9 lb. per 1,000 cubic feet. The whole of this
statement on "lift" can now be condensed into three absolute laws: 1.
Lift is directly proportional to barometric pressure. 2. Lift is inversely
proportional to absolute temperature. 3. Lift is directly proportional to
purity. AIRSHIP DESIGN The design of airships has been developed
under three distinct types, the Rigid, the Semi-Rigid, and the
Non-Rigid. The rigid, of which the German Zeppelin is the leading
example, consists of a framework, or hull composed of aluminium,
wood, or other materials from which are suspended the cars, machinery
and other weights, and which of itself is sufficiently strong to support
its own weight. Enclosed within this structure are a number of gas
chambers or bags filled with hydrogen, which provide the necessary
buoyancy. The hull is completely encased within a fabric outer cover to
protect the hull framework and bags from the effects of weather, and
also to temper the rays of the sun. 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 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;