CHAP, vn.1 VARIOUS TYPES OF STEAM-ENG1XES. 435 



Account must be taken of the dimensions of the cylinder and the 

 number of strokes of the piston that the engine gives per minute 

 or per hour, a number which itself depends on the quantity of steam 

 regularly furnished by the boiler. In this way the work of the 

 steam on the piston may be estimated. But this work in being 

 transmitted to the shaft and the fly-wheel is partly absorbed by the 

 friction and resistance of the machinery of transmission, so that it 

 must be reduced according to experimental rules to obtain the real 

 work done, or the effective force of the engine. 



This work is estimated in horse-powers. Thus we speak of an 

 engine as being of 3, 4, 10, 50, or 500 horse-power. 



Before going further we will explain clearly what is meant by this 

 expression, " horse-power." 



Horse-poiver is the unit introduced by Watt for the measurement 

 of the rate at which work is being done. One horse-power (1 H. P.) 

 is equal to 33,000 foot-pounds of work done in one minute. We 

 have thus three units involved in the definition. A foot pound is 

 the amount of work done in raising a pound weight one foot high 

 against the action of gravity. Work is measured in foot-pounds. 

 To take an example. The traction of a horse drawing a carriage is 

 measured and found to be 37'5 pounds when going at the rate of 

 ten miles an hour. To find the H. P. of the horse. 



10 miles per hour = 3 x 1760 x 10 



-TT. =880 feet per minute 



Traction x feet traversed per mi n. 37.5 x 880 

 H.P.ofhorse= "=' 33,000 



Thus a horse going at the rate of 10 miles per hour and exercising 

 a constant traction force of 37i Ibs would be doing work at the rate 

 of 1 II. P. 



The use of this term arose in this way. 



When Watt had adapted to his first steam-engines such improve- 

 ments as enabled them to be used in mines and manufactories, the 

 constructors of the engines found themselves obliged to guarantee to 

 their customers the power of the new engines. In mines, horses were 

 generally employed to turn the windlasses. The mean daily work of 

 these animals was taken as a term of comparison, and an estimation 

 made experimentally by Watt of the power of, the engines sold w r as 

 expressed in horse-powers. An amount of work was thus arrived 



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