the temperature, having risen to 90
degrees, the lift now becomes 70.9 lb. - 4.7 lb. = 66.2 lb.

Conversely, with a fall in the temperature the lift is increased.

We accordingly find from the foregoing observations that at the start
of a voyage the lift of an airship may be expected to be greater when
the temperature is colder, and the greater the barometric pressure so
will also the lift be greater. To put this into other words, the most
favourable conditions for the lift of an airship are when the weather
is cold and the barometer is high.

It must be mentioned that the air and hydrogen are not subject in the
same way to changes of temperature. Important variations in lift may
occur when the temperature of the gas inside the envelope becomes
higher, owing to the action of the sun, than the air which surrounds
it. A difference of some 20 degrees Fahrenheit may result between the
gas and the air temperatures; this renders it highly necessary that the
pilot should by able to tell at any moment the relative temperatures of
gas and air, as 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