the great pressure exerted by the compressed air within. What is true
of air is true of all gases. Dangerous boiler explosions have occurred
because the boiler walls were not strong enough to withstand the
pressure of the steam (which is water in the form of gas). The
pressure within the boilers of engines is frequently several hundred
pounds to the square inch, and such a pressure needs a strong boiler.

87. How Pressure is Measured in Buildings. In the preceding Section
we saw that undue pressure of a gas may cause explosion. It is
important, therefore, that authorities keep strict watch on gases
confined within pipes and reservoirs, never allowing the pressure to
exceed that which the walls of the reservoir will safely bear.

[Illustration: FIG. 52.--A pressure gauge.]

Pressure in a gas pipe may be measured by a simple instrument called
the pressure gauge: The gauge consists of a bent glass tube containing
mercury, and so made that one end can be fitted to a gas jet (Fig.
52). When the gas cock is closed, the mercury stands at the same level
in both arms, but when the cock is opened, the gas whose pressure is
being measured forces the mercury up the opposite arm. If the pressure
of the gas is small, the mercury changes its level but very little. It
is clear that the height of a column of mercury is a measure of the
gas pressure. Now it is known that one cubic inch of mercury weighs
about half a pound. Hence a column of mercury one inch high indicates
a pressure of about one half pound to the square inch; a column two
inches high indicates a pressure of about one pound to the square
inch, and so on.

This is a very convenient way to measure the pressure of the