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.
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