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 ENGINES. 291
engine is in the direct proportion of the quantity of water
evaporated. It has also been shown that the quantity of
water evaporated, whatever be the pressure of the steam, will
be in the direct proportion of the quantity of heat received
from the fuel, and therefore in the direct proportion of the
quantity of fuel itself, so long as the same proportion of its
heat is imparted to the water.
(136.) The POWER of an engine is a term which has been
used to express the rate at which it is able to raise a given
load, or overcome a given resistance. The DUTY of an
engine is another term, which has been adopted to express
the load which may be raised a given perpendicular height,
by the combustion of a given quantity of fuel.
When steam engines were first introduced, they were
commonly applied to work pumps or mills which had been
previously wrought by horses. It was, therefore, convenient,
and indeed necessary, in the first instance, to be able to
express the performance of these machines by reference to the
effects of animal power, to which manufacturers, miners, and
others had been long accustomed. When an engine, therefore, was capable of performing the same work in a given
time, as any given number of horses of'average strength
usually performed, it was said to be an engine of so many
horses' power. This term was long used with much vagueness and uncertainty: at length, as the use of steam engines
became more extended, it was apparent that confusion and
inconvenience would ensue, if some fixed and definite meaning were not assigned to it, so that the engineers and others
should clearly understand each other in expressing the powers
of these machines. The term horse-power had so long
been in use, that it was obviously convenient to retain it. It
was only necessary to agree upon some standard by which
it might be defined. The performance of a horse of average
strength, working for eight hours a day, was, therefore, selected as a standard or unit of steam engine power. Smeaton
estimated the amount of mechanical effect which the animal