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CHAPTER XIX.

THE POWER OF STEAM-ENGINES.

THE power of engines is universally stated in comparison with that of horses, and the English standard is the maximum work of which a powerful horse is found to be capable, viz.: 33,000 lbs. raised 1 foot high in 1 minute.

As a matter of practice this result is unattainable with ordinary animals for any length of time, therefore a steamengine of given effective power is equal to more than the work of the number of horses stated.

An effective horse-power, called also, brake, belt, or actual horse-power, is the real power given by it from the shaft, pulley, or belt.

An indicated horse-power is the power developed by the steam in the cylinder, and of course from it has to be deducted the power eaten up in driving the engine itself.

The French "force de cheval" is a close approximation to the English, as,

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I English horse-power 1.01385 force de cheval. Inversely stated

I Force de cheval .986337 of an English horse-power.

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The rule for finding a French indicated horse-power is as follows:

Let D= diameter of cylinder in metres.

S stroke in metres.

=

Rrevolutions per minute.

P = average pressure on piston in kilogrammes per centimetre.

French indicated horse-power = 3.49 X D2 × P x R x S.

From the result of above deduct 15 to 20 per cent. to arrive at approximate effective horse-power.

The rule for ascertaining the English indicated horsepower of an engine is,

A = area of piston in square inches. (See Table of Areas, P. 42.)

S = stroke in feet (not inches).

P = average pressure on piston in lbs. per square inch. (See following table.)

R = revolutions per minute.

Indicated horse-power =

AX PX RX 2S

33,000 ft.-lbs.

From which should be deducted an amount of 15 to 20 per cent. to allow for the force required to operate the engine itself. The net result is effective horse-power.

The standard of a horse-power has been adopted by other nations with fractional variations to suit their own standards of weight and measure. The differences are not large, being about 11⁄2 per cent. at the most, and for general purposes the three following standards will be found all that are usually required:

STANDARD HORSE-POWERS OF VARIOUS NATIONS.

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A commercial horse-power is a term in use in America, and represents an amount of 30 lbs. of water evaporated from feed-water at a heat of 100° Fahrenheit, and raised therefrom to a pressure of 70 lbs. over the atmosphere.

This rather clumsy definition of a power forms a standard for the power or duty of boilers, which is convenient in the absence of any clearer form of reference. Naturally, with boilers working under widely different conditions, a parity can be established only by proportionate calculations, and the same remark would hold good of steam-engines using more or less than the amount it fixes as a basis.

For the purpose of readily ascertaining the power of any given engine cylinder the following table will be found to save much calculation :

TABLE OF MEAN PRESSURES OF STEAM IN CYLINDERS.

Initial Pressure per Square Inch.

AVERAGE PREssure of Steam in Lbs. per Square Inch FOR THE

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Nominal Horse-Power. This term is, among engineers, happily becoming obsolete, and it would not be necessary to deal with it here but for its continued use by merchants and some manufacturers in their price-lists, rendering it necessary for a purchaser to ascertain what is intended by the term. The effective horse-power may be roughly taken at 22 to 3 times the stated nominal power of ordinary single-cylinder engines. In compound engines it may be taken at 4 times the amount, always assuming that a proper size of machine is represented by the nominal power.

The rule, if such it can be called, to produce this absurd anachronism, is as follows:

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In practice, to find approximately what cylinder a nominal horse-power represents, or, rather what it should represent, it is necessary to fall back on a list, in which I have collected the average sizes of cylinders represented by "nominal horse-power."

The figures in the list following are what should be insisted on as the proper value for an engine of a given nominal power, when comparing prices :

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AVERAGE SIZES OF CYLINDERS CORRESPONDING TO VARIOUS NOMINAL HORSE-POWERS IN VARIOUS TYPES OF ENGINES.

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These figures have been prepared from the nominal horse-powers given by first-class manufacturers, but it must be borne in mind that a common practice with mer

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