f 200 horse power, it is meant
that the impelling efficacy is equal to that of 200 horses, each lifting
33,000 lbs. one foot high in a minute?

_A._--No, not now; such was the case in Watt's engines, but the capacity of
cylinder answerable to a horse power has been increased by most engineers
since his time, and the pressure on the piston has been increased also, so
that what is now called a 200 horse power engine exerts, almost in every
case, a greater power than was exerted in Watt's time, and a horse power,
in the popular sense of the term, has become a mere conventional unit for
expressing a certain size of engine, without reference to the power
exerted.

211. _Q._--Then, each nominal horse power of a modern engine may raise much
more than 33,000 lbs. one foot high in a minute?

_A._--Yes; some raise 52,000 lbs., others 60,000 lbs., and others 66,000
lbs., one foot high in a minute by each nominal horse power. Some engines
indeed work as high as five times above the nominal power, and therefore no
comparison can be made between the performances of different engines,
unless the power actually exerted be first discovered.

212. _Q._--How is the power actually exerted by engines ascertained?

_A._--By means of an instrument called the indicator, which is a miniature
cylinder and piston attached to the cylinder cover of the main engine, and
which indicates, by the pressure exerted on a spring, the amount of
pressure or vacuum existing within the cylinder. From this pressure,
expressed in pounds per square inch, deduct a pound and a half of pressure
for friction, the loss of power in working the air pump, &c.; multiply the
area of the piston in square inches by this residual pressure, and by the
motion of the piston, in feet per minute, and divide by 33,000; the
quotient is the actual number of horses power of the engine. The same
result is attained by squaring the diameter of the cylinder, multiplying by
the pressure per square inch, as shown by the indicator, less a pound and a
half, and by the motion of the piston, in feet per minute, and dividing by
42,017.

213. _Q._ How is the nominal power of an engine ascertained?

_A._--Since the nominal power is a mere conventional expression, it is
clear that it must be determined by a merely conventional process. The
nominal power of ordinary condensing engines may be ascertained by the
following rule: multiply the square of the diameter of the cylinder in
inches, by t