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 ...

POWER OF BOILERS. 297
equivalent to 2160 lbs. raised a foot high. If wne divide
1,980,000, therefore, by 2160, we shall find the number of
cubic inches of water which must be evaporated per hour,
in order to produce the mechanical effect expressed by one
horse-power; the result of this division will be 91 66, which
is therefore the number of cubic inches of water per hour,
whose evaporation is equivalent to one horse-power. But
it has been shown that, for every 6 cubic inches of water
evaporated in the boiler which are available as a moving
power, there will be 4 cubic inches intercepted by the engine.
To find, then, the quantity of waste corresponding to 916
cubic inches of water, it will be necessary to divide that
number by 6, and to multiply the result by 4: this process
will give 610 as the number of cubic inches of water wasted.
The total quantity of water, therefore, which must be evaporated per hour, to produce the effect of one horse-power, will
be found by adding 610 to 916, which gives 1526.
This result, however, being calculated upon a supposition
of a degree of efficiency in the engines which is, perhaps,
somewhat above their average state, it has been customary
with engineers to allowv a cubic foot of water per hour for
each horse-power, a cubic foot being 1728 cubic inches, or
above 11 per cent. more than the above estimate.
(137.) It has been stated, that to evaporate a cubic foot of
water per hour requires 9 square feet of surface exposed to
the action of the fire and heated air. This, therefore, is the
quantity of surface necessary for each horse-power, and we
shall find the total quantity of fire and flue surface necessary
for a boiler of a given power, by multiplying the number of
horses in the power by 9; the product will express, in square
feet, the quantity of boiler surface which must be exposed to
the fire, one-half of this being fire surface and the other half
flue surface.
Since the supply of heat to the boiler must be proportionate to the quantity of fuel maintained in combustion, and the
quantity of that fuel must depend on the extent of grate
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