tance is estimated at
about .14 of that resistance; but it will be a sufficiently near
approximation to the power consumed by friction in high pressure engines,
if we make a deduction of a pound and a half from the pressure on that
account, as in the case of low pressure engines. High pressure engines, it
is true, have no air pump to work; but the deduction of a pound and a half
of pressure is relatively a much smaller one where the pressure is high,
than where it does not much exceed the pressure of the atmosphere. The
rule, therefore, for the actual horse power of a high pressure engine will
stand thus: square the diameter of the cylinder in inches, multiply by the
pressure of the steam in the cylinder per square inch less 1-1/2 lb., and
by the speed of the piston in feet per minute, and divide by 42,017; the
quotient is the actual horse power.

217. _Q._--But how do you ascertain the nominal horse power of high
pressure engines?

_A._--The nominal horse power of a high pressure engine has never been
defined; but it should obviously hold the same relation to the actual power
as that which obtains in the case of condensing engines, so that an engine
of a given nominal power may be capable of performing the same work,
whether high pressure or condensing. This relation is maintained in the
following rule, which expresses the nominal horse power of high pressure
engines: multiply the square of the diameter of the cylinder in inches by
the cube root of the length of stroke in feet, and divide the product by
15.6. This rule gives the nominal power of a high pressure engine three
times greater than that of a low pressure engine of the same dimensions;
the average effective pressure being taken at 21 lbs. per square inch
instead of 7 lbs., and the speed of the piston in feet per minute being in
both rules 128 times the cube root of the length of stroke.[1]

218. _Q._--Is 128 times the cube root of the stroke in feet per minute the
ordinary speed of all engines?

_A._--Locomotive engines travel at a quicker speed--an innovation brought
about not by any process of scientific deduction, but by the accidents and
exigencies of railway transit. Most other engines, however, travel at about
the speed of 128 times the cube root of the stroke in feet; but some marine
condensing engines of recent construction travel at as high a rate as 700
feet per minute. To mitigate the shock of the air pump valves in cases in
which a high speed h