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 ...

22.2 THE STEAM ENGINE.
could produce at 22,916 pounds, raised one foot per minute;
Desaguiliers makes it 27,500 pounds, raised through the
same height, Messrs. Bolton and Watt caused experiments
to be, made with the strong horses used in the breweries in
London; and from the result of these they assigned 33,000
pounds raised one foot per minute, as the value of a horse's
power: this is, accordingly, the estimate now generally
adopted; and, when an engine is said to be of so many
horses': power, it is meant that, when in good working order
and properly managed, it is capable of overcoming a resistance equivalent to so many times 33,000 pounds raised one
foot per minute. Thus, an engine of ten-horse power
would be capable of raising 330,000 pounds one foot per
minute.
As the same quantity of water converted into steam will
always:produce the same mechanical effect, whatever be the
density. of the steam produced from it, and at whatever rate
the evaporation may proceed, it is evident that the power of
a steam engine will depend on two circumstances: first, the
rate at. which the boiler with its appendages is capable of
evaporating water; and, secondly, the rate at which the
engine is capable of consuming the steam by its work. We
shall consider these two circumstances separately.
The rate at which the boiler produces steam will depend
upon the rate at which heat can be transmitted from the fire
to the water which it contains. Now this heat is transmitted
in twvo ways: either by the direct action of the fire radiating
heat against the surface of the boiler, or by the flame and
heatedair which escapes from the fire passing through the
flues, as already explained. The surface of the boiler exposed to the direct radiation of the fire is technically called
fire surface; and that which takes heat from the flame and
air, on its:way to the chimney, is called flzue svrface. Of
these the most efficient in the generation of steam is the
former. In stationary boilers, used for condensing engines,
where magnitude and weight are matters of little importance,