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The balance is the gross lift of the balloon 70 lb.

It follows, then, that apart from the weight of the structure itself
the balloon is 70 lb. lighter than the air it displaces, and provided
that it weighs less than 70 lb. it will ascend into the air.

As the balloon or airship ascends the density of the air decreases as
the height is increased. As an illustration of this the barometer
falls, as everyone knows, the higher it is taken, and it is accurate to
say that up to an elevation of 10,000 feet it falls one inch for every
1,000 feet rise. It follows that as the pressure of the air decreases,
the volume of the gas contained expands at a corresponding rate. It
has been shown that a balloon filled with 1,000 feet of hydrogen has a
lift of 70 lb. under normal conditions, that is to say, at a barometric
pressure of 80 inches. Taking the barometric pressure at 2 inches
lower, namely 28, we get the following figures:

1,000 cubic feet of air weighs 70 lb.
1,000 cubic feet of hydrogen weighs 4.67 "
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65.33 lb.

It is therefore seen that the very considerable loss of lift, 4.67 lb.
per 1,000 cubic feet, takes place with the barometric pressure 2 inches
lower, from which it may be taken approximately that 1/30 of the volume
gross lift and weight is lost for every 1,000 feet rise. From this
example it is obvious that the greater the pressure of the atmosphere,
as indicated by the barometer, the greater will be the lift of the
airship or balloon.

Temperature is another factor which must be considered while discussing
lift. The volume of gas is affected by temperature, as gases expand or
contract about 1/500 part for every degree Fahrenheit rise or fall in
temperature.

In the case of the 1,000 cubic feet balloon, the air at 30 inches
barometric pressure and 60 degrees Fahrenheit weighs 75 lb., and the
hydrogen weighs 5 lb.

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