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 "
--------
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.