weighed, and varies according to barometric pressure and the
temperature; but for the purposes of this example we may take it
that under normal conditions air weighs 75 lb. per 1,000 cubic
feet. Therefore, if a balloon of 1,000 cubic feet volume is
charged with air, this air contained will weigh 75 lb. It is
then manifest that a balloon filled with air would not lift,
because the air is not displaced with a lighter gas.

Hydrogen is the lightest gas known to science, and is used in
airships to displace the air and raise them from the ground.
Hydrogen weighs about one-fifteenth as much as air, and under
normal conditions 1,000 cubic feet weighs 5 lb. Pursuing our
analogy, if we fill our balloon of 1,000 cubic feet with hydrogen
we find the gross lift is as follows:

1,000 cubic feet of air weighs 75 lb.
1,000 cubic feet of hydrogen weighs 5 lb.
------
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