otherwise a false impression will be gained of the lifting
capacity of the airship.

The lift of an airship is also affected by flying through snow
and rain. A considerable amount of moisture can be taken up by
the fabric and suspensions of a large airship which, however, may
be largely neutralized by the waterproofing of the envelope.
Snow, as a rule, is brushed off the surface by the passage of the
ship through the air, though in the event of its freezing
suddenly, while in a melting state, a very considerable addition
of weight might be caused. There have been many instances of
airships flying through snow, and as far as is known no serious
difficulty has been encountered through the adhesion of this
substance. The humidity of the air may also cause slight
variations in lift, but for rough calculations it may be ignored,
as the difference in lift is not likely to amount to more than
0.3 lb. per 1,000 cubic feet of gas.

The purity of hydrogen has an important effect 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