* 516 THE STEAM-ENGINE. 



engine of so many horses' power. Steam-engines had been in use for a con- 

 siderable time before this term had acquired -any settled or uniform meaning, 

 and the nominal power of engines was accordingly very arbitrary. At length, 

 however, the use of steam-engines became more extended, and the confusion 

 and inconvenience arising out of all questions respecting the performance of 

 engines, rendered it necessary that some fixed and definite meaning should be 

 assigned to the terms by which the powers of this machine were expressed. 

 To have abandoned the term horse-power, which had been so long in use, 

 would have been obviously inconvenient ; nor could there be any objection to 

 its continuance, provided all engine-makers, and all those who used engines, 

 could be brought to agree upon some standard by which the unit of horse- 

 c power might be defined. The performance of a horse of average strength, 

 | > working for eight hours a day, was therefore selected as a standard, or unit, 

 \ of steam-engine power. Smeaton estimated that such an animal, so working, 

 was capable of performing a quantity of work equal in its mechanical effect to 

 22,916 Ibs. raised one foot per minute, while Desaguliers estimated the same 

 power at 27,500 Ibs. raised through the same height in the same time. The 

 discrepancy between these estimates probably arose from their being made 

 from the performances of different classes of horses. Messrs. Boulton and 

 Watt caused experiments to be made with the strong horses used in the brew- 

 eries in London, and from the result of these trials they assigned 33,000 Ibs. 

 raised one foot per minute, as the value of a horse's power. This is the unit 

 of engine-power now universally adopted ; and when an engine is said to be 

 of so many horses' power, what is meant is, that that engine, in good working 

 order and properly managed, is capable of moving a resistance equal to 33,000 

 Ibs. through one foot per minute. Thus, an engine of ten-horse power is one 

 that would raise 330,000 Ibs. weight one foot per minute. 



Whether this estimate of an average horse's power be correct or not, in 

 reference to the actual work which the animal is capable of executing, is a 

 matter of no present importance in its application to steam-power. The steam- 

 engine is no longer used to replace the power of horses, and therefore no con- 

 tracts are based upon such a comparison. The term horse-power, therefore, 

 as applied to steam-engines, must be understood to have no reference what- < 

 ever to the actual animal power, but must be taken as a term having no other 

 meaning than the expression of the ability of the machine to move the amount 

 of resistance above mentioned through one foot per minute. 



It has been already explained that the conversion of a given volume of wa- 

 ter into steam is productive of a certain definite amount of mechanical force, 

 Sthis amount depending on the pressure under which the water is evaporated, 

 and the extent to which the expansive principle is used in working the steam. 

 It is evident that this amount of mechanical effect is a major limit, which can- 

 not be exceeded by the power of the engine. 



If the steam be not worked expansively, then the whole power of the water, 

 transmitted in the form of steam from the boiler to the working machinery, will be 

 a matter of easy calculation, when the pressure at which the steam is worked is 

 known. The following table exhibits the mechanical power of a cubic foot of wa- 

 ter converted into steam at various pressures, expressed in an equivalent number 

 of pounds' .weight raised one foot high. Where much accuracy is sought for, 

 the pressure at which the steam is used must be taken into account ; but by 

 reference to the table it will be seen, that when steam is worked without ex- 

 pansion, its mechanical effect varies very little with the pressure. It may 

 therefore be assumed, as has already been stated, that for every cubic inch of 

 water transmitted in the form of steam to the cylinders, a force is produced, 

 represented by a ton weight raised a foot high. Now, as 33,000 Ibs. is very 



