The Kenosha telegraph. [volume] (Kenosha, Wis.) 1860-1888, 1869-06-10
Measuring the Horse-Powero Of
Steam Engines. rrer.r-
From the American Journal of Mining.

we had to do was to take the pressure of
the steam In the boiler IN bounds per
square inch, multiply this into the surface
of the piston, also in square, inches, and
finally to multiply this into the distance
in feet passed over by the piston in mir al :.
ute. II, for instance, the pressure IN the
boiler was 20 pounds, the surface of the
piston 200 square inches, and the length
of stroke 2 feet, with 100 strokes per
minute, the number of foot-pounds 60 pei
obtained was supposed to be 2OxlOox2x!ioo
100000 per minute. This quantlCy
divided by 80 900, the number ot1foot-
pounds adopted as equal to one horse pow
ef.
Experience proved however, that the
actual amount Of horse power was lar De
Low this result, and, moreover, that the
engine, by having less work to perform
was caused to run faster, the same calcula
ton gave greater power-a result which
seemed to be absurd This old obserya
ton has been quite recently republished
as a novelty.
70 satisfy. the conditions of practice,
the following empirical rule was adopted:
On the authority of Watt, the speed of a
piston with a two-foot stroke was assumed
to be 100 feet per minute; with two
feet six inch stroke no; three feet, 1870
three feet six inches, 189 four feet 200e;1
Eve feet, 215; six feet, 228 seven feet,
245, eight feet, 266. These numbers are
founded upon the experimental obserya
tion, that the of the piston varies nearly
as the cube root of the length of the
stroke. It was also assumed that the Ell
fective pressure on the piston is on only 81
pounds per square inch; and the horse
power was determined by multiplying the
diameter of the cylinder in inches into
the velocity of the piston in feet per min
ute, dividing the product by 5000. Inthe
illustration already given, the piston Of
200 square inches surface, or Is inches
diameter and 2 feet stroke, would exert
162x160 divided by 5000, or nearly Z
horsepower. t.x fsY n Il
This process gayesa reasonably accurate
standard of measurement as. compared
with the results of practice, and it was,
therefore adopted as a rule for finding
the so called nominal horse power.
Later Improvements, however, combined
with stronger boilers and higher initial
pressure increased the power Of steam.
engines considerably above that indicated
by the above rule; and @ distinction was
made between nominal horse power and
actual horse-poWe the latter having be-
come two three, and, sometimes, even
four or five times as great as the former,
The nominal morse power is, in ifac6, at
the present day, a mere conventional ex-
preston, serving to convey by a simple
number an approximate idea of the size
of steam engine.
Until about half century ago, it was
supposed that the disagreement between
the practical results, as Indicate by the
nominal horse power, and the theoretical
results obtained by calculating the amount
of steam pressure in the boiler and the
motion of the piston. Zone being, in the
example given, 7-horse power, and the
other 24), was due to friction, inperfec7
tion of machinery, loss of power by the
use of the crank, etc. aaa ~ 8 so.
1t was De Pambour, in France, who,
half century ago, first pointed out the
absurdity of the supposition, that, in we'll
made steam engines, the loss by friction
and imperfection of the mechanical ar
rangements could amount to three-fourtls
of the power employed. He demonstrated
the fact, overlooked by his predecessors,
that the pressure on the piston never can
equal the pressure Of the steam in the
boiler, because the steam has to follow the
receding piston. He also showed
that, the longer the connecting tubes be
tween boiler and cylinder, or the greater
the number of bends in them, the more
serious is the proportion of loss. To this
again, must be added the cooling of the
steam in the cylinder, the temperature of
which is always considerably below that
II. /. = 4
that when the pressure of steam, not in
the boiler, but in the cylinder, during the
motion of the piston, is taken as the basis
of calculation, the results from the theo-
retical rule agreed with the actual power
obtained from the engine.
In order, now, to ascertain this pressure
of steam en- the receding piston, an in stru-
ment called the indicator is used 1t is a
miniature cylinder and piston, attached to
the cylinder of the engine; and it indi-
Gates and registers the pressure of the
steam on the miniature piston by COM
pressing spring. The amount of press
ure in the main cylinder at every part of
the stroke, when the engine is in motion,
is thus recorded.
The principle on which this Indicator
is constructed was discovered by the i'm
mortal James Watt. who at a very early
period of his inventions, employed it
it chiefly toascertain the proportion which
the vacuum in the cylinder bore, at differ
ent parts of the stroke, to that in the con-
denser, in order that he might determine
the dimensions of the cylinder required
for any desired power, as we'll as the pro-
portion to be given to the steam and ex-
haust parts etc, Succeeding mechanics
seem to have despised the unpretending
little instrument, by the assistance of
which Watt had been led to suohsplendid
results, and for half century have trust