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 ...

124 THE STEAM ENGINE.
of the atmospheric pressure above the uncondensed steam.
tIence, if these two heights be added together, we shall obtain the excess of the impelling force of the steam from the
boiler on the one side of the piston, above the resistance of
the uncondensed steam on the other side. This will give
the effective impelling force. Now, if one pound be allowed
for every two inches of mercury in the two columns just
mentioned, we shall have the number of pounds of impelling
pressure on every square inch of the piston. Then if the
number of square inches in the section of the piston be fbund,
and multiplied by the number of pounds on each square inch,
the whole effective force with which it moves will be obtained.
In the computation of the power of the engine, however,
all this force, thus computed, is not to be allowed as the
effective working power. For it requires some force, and
by no means an inconsiderable portion, to move the engine
itself, even when unloaded; all this, therefore, which is spent
in overcoming friction, &c. is to be left out of account, and
only the balance set down as the effective working power.
From what we have stated, it appears that in order to
estimate the effective force with which the piston is urged,
it is necessary to refer to both the barometer and the steam
gauge. This double computation may be obviated by making one gauge serve both purposes. If the end c of the
steam gauge, (fig. 38,) instead of communicating with the
atmosphere, were'continued to the condenser, we should have
the pressure of the steam acting upon the mercury in the
tube B A, and the pressure of the uncondensed vapour which
resists the piston acting on the mercury in the tube -C.
Hence the difference of the levels of the mercury in the
tubes will at once indicate the difference between the force
of the steam and that of the uncondensed vapour, which is
the effective force with which the piston is urged.
(70., To secure the boiler from accidents arising from the
steam becoming too strong, a safety valve is used, similar to