The steam engine familiarly explained and illustrated; with an historical sketch of its invention and progressive improvement; its applications to navigation and railways; with plain maxims for railway speculators. By the Rev. Dionysius Lardner ... With additions and notes by James Renwick ...

12 2 THE STEAM ENGINE.
sure of the steam must be exactly equal to that of the atmosphere; because the steam pressure on the mercury in A B
balances the atmospheric pressure on the mercury in B c.
If, however, the level of the mercury in B c be above the
level of the mercury in B A, the pressure of the steam will
exceed that of the atmosphere. The excess of its pressure
above: that of the atmosphere may be found by observing
the differenceof the levels of the mercury in the tubes B c
and B A; allowing a pressure of one pound on each square
inch for every two inches in the difference of the levels.
If, on the contrary, the level of the mercury in B C should
fall below its level in A n, the atmospheric pressure will
exceed that of the steam, and the degree or quantity of the
excess may be ascertained exactly in the same way.
If the tube be glass, the difference of levels of the mercury
would be visible: but it is most commonly made of iron;
and in order to ascertain the level, a thin wooden rod with
a float is inserted in the open end of B c; so that the portion
of the stick within the tube indicates the distance of the
level of the mercury from its mouth. A bulb or cistern of
mercury might be substituted for the leg A B, as in the
common barometer. This instrument is called the steam.
ga'ge.
If the steam gauge be used as a measure of the strength of
the steam which presses on the piston, it ought to be on the
same side of the throttle valve (which is regulated by the
governor) as the cylinder; for if it were on the same side of
the throttle valve with the boiler, it would not be affected by
the changes which the steam may undergo in passing through
the throttle valve, when partially closed by the agency of
the governor.
(69' The force with which the piston is pressed depends
on two things: 10, the actual strength of the steam which
presses onI it; and 2~, on the actual strength of the vapour
which resists it. For although the vacuum produced by
the method of separate condensation be much more perfect