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 ...

HORSE-POWER OF ENGINE$S 2~95
that the mechanical force of four-tenths of the water evaporated in the boiler is intercepted by the engine, and the
other six-tenths are available as a moving force. In this
calculation, however, the resistance produced in the condensing engine by the uncondensed steam is not taken into
account: the amount of this force will depend upon the
temperature at which the water is maintained in the condenser. If this water be kept at the temperature of 120~,
the vapour arising from it will have a pressure expressed by
three inches seven-tenths of mercury; if we suppose the
pressure of steam in the boiler to be measured by 37 inches
of mercury, then the resistance from the uncondensed steam
will amount to one-tenth of the whole power of the boiler;
this, added to the four-tenths already accounted for, would
show a waste amounting to half the whole power of the
boiler, and consequently only half the water evaporated
would be available as a moving power.
If the temperature of the condenser be kept down to 100~,
then the pressure of uncondensed steam will be expressed
by two inches of mercury, and the loss of power consequent
upon it would amount to a proportionally less fraction of
the whole power.
The following example will illustrate the method of estimating the effective power of an engine.
In a double-acting engine, in good working condition, the
total power of steam in the boiler being expressed by 1000,
the proportion intercepted by the engine, exclusive of the
resistance of the uncondensed steam, will be 368, and the
effective part 632. Now, suppose the pressure of steam in
the boiler to be measured by a column of 35 inches of mercury; the thousandth part of this will be seven two-hundredths of an inch of mercury, and 632 of these parts will
express the effective portion of the power. By multiplying
seven two-hundredths by 632, we obtain 22 nearly. Now,
suppose the temperature in the condenser is 1200, the pressure of steam corresponding to that temperature will be