14 APPLICATION OF THE LAWS OF HYDRAULICS
with the chimney, a jet of incandescent gas or air will escape with
considerable force. What is it that sets this in motion?
Open the bell of a gas producer working with natural draft;
the producer gas will escape. What is the force which causes the
air to pass through the bed of wood or coal, where it is transformed
into gas, and which, in addition, has sufficient pressure to produce
the jet of gas?
From the fact that enclosures filled with a cold gas, but one
which is lighter than the air, always exert a pressure higher than
that of the atmosphere, it follows that the preceding phemonena
are due not to the temperature of the gas, but to the fact that it
weighs less than the air.
Take, for example, a balloon 10 m in diameter, filled with
hydrogen (Fig. 11). Com-
pute the hydrostatic pressure
of the gas at the top of the
sphere. This balloon being
open at the bottom, at the
level A A the gas is in equilib-
rium with the air, their pres-
sures being equal.
Consequently the weight
of a column of cold air 10 m
in height (1 kg 29 X10 = 12 kg
90) is held in equilibrium by
the weight of a column of
hydrogen 10 m in height plus
a certain hydrostatic pressure 5 which may be determined.
The weight of a cubic meter of hydrogen being equal to 1.29X
0.06927, the following equation is obtained:
1.29X10 = 1.29X0.06927X10+5; 5 = 12 kg 006,
which is in the neighborhood of 12 mm of water.
It is on account of this hydrostatic pressure of the hydrogen
at the top of the balloon that it is necessary to employ a very
strong material in the making of this envelope. It is this pressure
which causes the balloon filled with hydrogen to ascend and
which furnishes the energy for the flow of the hydrogen when the
valve at the top of the balloon is opened to permit it to escape.
Furthermore, children are often amused by making small hot-air