# A Catechism of the Steam Engine eBook

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224. Q.—­That is, the nominal power is a commercial unit by which engines are bought and sold, and the actual power a scientific unit by which the quality of their performance is determined?

A.—­Yes; the nominal power is as much a commercial measure as a yard or a bushel, and is not a thing to be ascertained by any process of science, but to be fixed by authority in the same manner as other measures.  The actual power, on the contrary, is a mechanical force or dynamical effort capable of raising a given weight through a given distance in a given time, and of which the amount is ascertainable by scientific investigation.

225. Q.—­Is there any other measure of an actual horse power than 33,000 lbs. raised one foot high in the minute?

A.—­There cannot be any different measure, but there are several equivalent measures.  Thus the evaporation of a cubic foot of water in the hour, or the expenditure of 33 cubic feet of low pressure steam per minute, is reckoned equivalent to an actual horse power, or 528 cubic feet of water raised one foot high in the minute involves the same result.

[1] Tables of the horse power of both high and low pressure
engines are given in the Key.

[2] Example.—­What is the power of an engine of 42 inches
diameter, 3-1/2 feet stroke, and making 85 strokes per minute?  The
speed of the piston will be 7 (the length of a double stroke) x 85 =
595 feet per minute.  Now 42 x 42 = 1,764 x 595 = 1,049,580 / 6,000 =
175 horses power.

## DUTY OF ENGINES AND BOILERS.

226. Q.—­What is meant by the duty of a engine?

A.—­The work done in relation to the fuel consumed.

227. Q.—­And how is the duty ascertained?

A.—­In ordinary mill or marine engines it can only be ascertained by the indicator, as the load upon such engines is variable, and cannot readily be determined; but in the case of engines pumping water, where the load is constant, the number of strokes performed by the engine will represent the work done, and the amount of work done by a given quantity of coal

represents the duty.  In Cornwall the duty of an engine is expressed by the number of millions of pounds raised one foot high by a bushel, or 94 lbs. of Welsh coal.  A bushel of Newcastle coal will only weigh 84 Lbs.; and in comparing the duty of a Cornish engine with the performance of an engine in some locality where a different kind of coal is used, it is necessary to pay regard to such variations.

228. Q.—­Can you tell the duty of an engine when you know its consumption of coal per horse power per hour?

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