At the same pressure, but with the temperature increased to 90
degrees Fahrenheit, the air will be expanded and 1,000 cubic feet
of air will weigh only 70.9 lb., while 1,000 cubic feet of
hydrogen will weigh 4.7 lb.

The lift being the difference between the weight of the volume of
air and the weight of the hydrogen contained in the balloon, it
will be seen that with the temperature at 60 degrees Fahrenheit
the lift is 75 lb. - 5 lb. = 70 lb., while 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