idered that the
amount of steam necessary to produce the same power in prime movers of
different types and sizes varies over very wide limits.

To do away with the confusion resulting from an indefinite meaning of
the term boiler horse power, the Committee of Judges in charge of the
boiler trials at the Centennial Exposition, 1876, at Philadelphia,
ascertained that a good engine of the type prevailing at the time
required approximately 30 pounds of steam per hour per horse power
developed. In order to establish a relation between the engine power and
the size of a boiler required to develop that power, they recommended
that an evaporation of 30 pounds of water from an initial temperature of
100 degrees Fahrenheit to steam at 70 pounds gauge pressure be
considered as _one boiler horse power_. This recommendation has been
generally accepted by American engineers as a standard, and when the
term boiler horse power is used in connection with stationary
boilers[58] throughout this country,[59] without special definition, it
is understood to have this meaning.

Inasmuch as an equivalent evaporation from and at 212 degrees Fahrenheit
is the generally accepted basis of comparison[60], it is now customary
to consider the standard boiler horse power as recommended by the
Centennial Exposition Committee, in terms of equivalent evaporation from
and at 212 degrees. This will be 30 pounds multiplied by the factor of
evaporation for 70 pounds gauge pressure and 100 degrees feed
temperature, or 1.1494. 30 x 1.1494 = 34.482, or approximately 34.5
pounds. Hence, _one boiler horse power is equal to an evaporation of
34.5 pounds of water per hour from and at 212 degrees Fahrenheit_. The
term boiler horse power, therefore, is clearly a measure of evaporation
and not of power.

A method of basing the horse power rating of a boiler adopted by boiler
manufacturers is that of heating surfaces. Such a method is absolutely
arbitrary and changes in no way the definition of a boiler horse power
just given. It is simply a statement by the manufacturer that his
product, under ordinary operating conditions or conditions which may be
specified, will evaporate 34.5 pounds of water from and at 212 degrees
per definite amount of heating surface provided. The amount of heating
surface that has been considered by manufacturers capable of evaporating
34.5 pounds from and at 212 degrees per hour has changed from time to
time as the art has progressed. At t