430 Hydrodynamics. 



take account of the tension of the steam which remains on the 

 other side when the vacuum is not perfect. After all, this esti- 

 mate fails of giving the primitive energy of the power employed, 

 and much Jess the part which remains to be disposed of after all the 

 friction is overcome and the different parts of the machine are put 

 in motion. This useful part of the force can be measured a pos- 

 teriori by the effect which the whole engine produces. Ordina- 

 rily we compare the effective power of an engine with that of a 

 certain number of horses of a medium strength, and the force of 

 the engine is estimated accordingly. By a great number of ex- 

 periments of this kind Watt and Bolton supposed that a horse 

 of ordinary strength, working 8 hours a day, would raise 3200 Ib. 

 avoirdupois one foot per hour. Smeaton made the estimate 

 2300, and Clement about 1300. If, therefore, we divide the 

 number of pounds that a steam engine will raise one foot (or, 

 which is the same thing, the product of the number of pounds 

 into the number of feet elevation), by the number representing a 

 horse power, the quotient will be the number of horses to which 

 the engine is equivalent. There are engines of the power of 20, 

 30, &c., horses. The most powerful engine that has yet been 

 constructed is that employed in the mines of Cornwall. It has the 

 power of 1010 horses, and serves to drain by pumps a mine 590 

 feet deep. It is evident, that the power in question is all that 

 needs to be estimated ; lor we can apply it to the raising of wa- 

 ter, the turning of spindles, or to any other purpose that requires 

 such a force. The transmission of the primitive motion can be 

 effected by mechanical means and instruments of which we have 

 already spoken, and which need not be again described. 



