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 ...

PROPORTION OF CYLINDER. 303
boiler of an engine is capable of producing a mechanical effect
through the means of that engine, which, when expressed
in an equivalent number of pounds' weight lifted a foot
high, is called the duhty of the engine. If all the heat (leveloped in the combustion of the fuel could be imparted to
the water in the boiler, and could be rendered instrumental
in producing its evaporation; and if, besides, the steam thus
produced could be all rendered mechanically available at the
working point; then the duty of the engine would be the
entire undiminished effect of the heat of combustion; but it
is evident that this can never practically be the case. In
the first place, the heat developed by the combustion can
never be wholly imparted to the water in the boiler: some
part of it will necessarily escape without reaching the boiler
at all; another portion will be consumed in heating the
metal of the boiler, and in supplying the loss by radiation
from its surface; another portion will be abstracted by the
various sources of the waste and leakage of steam; another
portion will be abstracted by the reaction of the condensed
steam; and another portion of the power will be consumed
in overcoming the friction and resistance of the engine itself.
It is apparent that all these sources of waste will vary according to the circumstances and conditions of the machine,
and according to the form and construction of the furnace,
flues, boilers, &c. The duty, therefore, of different engines
will be different; and when such machines are compared,
with a view to ascertain their economy of fuel, it has been
found necessary carefully to register and to compare the
fiel consumed with the weight or resistance overcome. In
engines applied to manufactures generally, or navigation, it
is not easy to measure the amount of resistance which the
engine encounters, but when the engine is applied to the
pumping of water, its performance is more easily deter
mined.
In the year 1811, several of the proprietors of the mines
n Cornwall, suspecting that some of their engines might not