--What is the Admiralty rule for determining the power of an
engine?

_A._--Square the diameter of the cylinder in inches, which multiply by the
speed of the piston in feet per minute, and divide by 6,000; the quotient
is the power of the engine by the Admiralty rule.[2]

222. _Q._--The high speed engine does not require so heavy a fly wheel as
common engines?

_A._--No; the fly wheel will be lighter, both by virtue of its greater
velocity of rotation, and because the impulse communicated by the piston is
less in amount and more frequently repeated, so as to approach more nearly
to the condition of a uniform pressure.

223. _Q._--Can nominal be transformed into actual horse power?

_A._--No; that is not possible in the case of common condensing engines.
The actual power exerted by an engine cannot be deduced from its nominal
power, neither can the nominal power be deduced from the power actually
exerted, or from anything else than the dimensions of the cylinder. The
actual horse power being a dynamical unit, and the nominal horse power a
measure of capacity of the cylinder, are obviously incomparable things.

224. _Q._--That is, the _nominal_ power is a commercial unit by which
engines are bought and sold, and the _actual_ power a scientific unit by
which the quality of their performance is determined?

_A._--Yes; the nominal power is as much a commercial measure as a yard or a
bushel, and is not a thing to be ascertained by any process of science, but
to be fixed by authority in the same manner as other measures. The actual
power, on the contrary, is a mechanical force or dynamical effort capable
of raising a given weight through a given distance in a given time, and of
which the amount is ascertainable by scientific investigation.

225. _Q._--Is there any other measure of an actual horse power than 33,000
lbs. raised one foot high in the minute?

_A._--There cannot be any _different_ measure, but there are several
equivalent measures. Thus the evaporation of a cubic foot of water in the
hour, or the expenditure of 33 cubic feet of low pressure steam per minute,
is reckoned equivalent to an actual horse power, or 528 cubic feet of water
raised one foot high in the minute involves the same result.

[1] Tables of the horse power of both high and low pressure
engines are given in the Key.

[2] Example.--What is the power of an engine of 42 inches
diameter, 3-1/2 feet stroke, and making 85 strokes per