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