THE STEAM-ENGINE. 



515 



It is usual to express and estimate all mechanical effect whatever by nature 

 of the resistance overcome — by an equivalent weight raised a certain height. 

 Thus, if an engine exerts a certain power in driving a mill, in drawing a car- 

 riage on a road, or in propelling a vessel on water, the resistance against which 

 it has to act must be equal to a definite amount of weight. If a carriage be 

 drawn, the traces are stretched by the tractive power, by the same tension 

 that would be given to them if a certain weight were appended to them. If 

 the paddle-wheels of a boat are made to revolve, the water opposes to them a 

 resistance equal to that which would be produced, if, instead of moving the 

 water, the wheel had to raise some certain weight. In any case, therefore, 

 weight becomes the exponent of the energy of the resistance against which 

 the moving power acts. 



But the amount of mechanical effect depends conjointly on the amount of 

 resistance, and the space through which that resistance is moved. The quan- 

 tity of this effect, therefore, will be increased in the same proportion, whether 

 the quantity of resistance, or the space through which that resistance is moved, 

 be augmented. Thus, a resistance of one hundred pounds, moved through 

 two feet, is mechanically equivalent to a resistance of two hundred pounds 

 moved through one foot, or of four hundred pounds moved through six inches. 

 To simplify, therefore, the expression of mechanical effect, it is usual to re- 

 duce it invariably to a certain weight raised one foot. If the resistance un- 

 der consideration be equivalent to a certain weight raised through ten feet, it 

 is always expressed by ten times the amount of that weight raised through one 

 foot. 



It has also been usual in the expression of mechanical effect, to take the 

 pound weight as the unit of weight, and the foot as the unit of length, so that 

 all mechanical effect whatsoever is expressed by a certain number of pounds 

 raised one foot. 



The gross effect of the moving power in a steam-engine, is the whole me- 

 chanical force developed by the evaporation of water in the boiler. A part of 

 this effect is lost by the partial condensation of the steam before it acts upon 

 the piston, and by the imperfect condensation of it subsequently ; another por- 

 tion is expended on overcoming the friction of the different moving parts, and 

 in acting against the resistance which the air opposes to the machine. If the 

 motion be subject to sudden shocks, a portion of the power is then lost by the 

 destruction of momentum which such shocks produce. But if those parts of 

 the machine which have a reciprocating motion be, as they ought to be, brought 

 gradually to rest at each change of direction, then no power is absorbed in this 

 way. 



The useful effect of an engine is variously denominated according to the re- 

 lation under which it is considered. If it be referred to the time during 

 which it is produced, it is called power. 



If it be referred to the fuel, by the combustion of which the evaporation has 

 been effected, it is called duty. 



When steam-engines were first brought into use, they were commonly ap- 

 plied to work pumps for mills which had been previously worked or driven by 

 horses. In forming their contracts, the first steam-engine builders found them- 

 selves called upon to supply engines capable of executing the same work as 

 was previously executed by some certain number of horses. It was therefore 

 convenient, and indeed necessary, to be able to express the performance of 

 these machines by comparison with the animal power to which manufacturers, 

 miners, and others, had been so long accustomed. When an engine, there- 

 fore, was capable of performing the same work in a given time as any given 

 number of horses of average strength usually performed, it was said to be an 



