388 THE CONNECTION AND EQUIVALENCE OF FORCES. 



obtained for the movement of the piston. It is heat, in this 

 last form, which performs the mechanical work of the ma 

 chine. 



Every force acts by producing pressure either from or tow 

 ards the centre of motion. In every machine in operation 

 the amount of power is always measurable by the resistance 

 overcome ; and this again can be expressed by corresponding 

 weights, which that power is capable of raising to a certain 

 height. If one man raises by a pump, in one minute, 150 Ibs. 

 of water, and another 200 Ibs. ; or if one horse draws to a 

 certain distance a load of 20 cwt., and another a load of 30 

 cwt., it is evident that these numbers express the relative 

 working power of these two men or horses. In mechanics, 

 the working power of every machine is expressed in horse 

 power, that is, a force capable of elevating in each second 75 

 kilogrammes (=2 J Ibs. avoirdupois) to a height of one meter 

 (39-37 inches). 



The whole power communicated to a machine is not actu 

 ally available, but is in part lost by friction. For if two 

 machines possess the same power, it is found that the greater 

 quantity of work will be* executed by the one which has to 

 overcome the smaller amount of friction. In mechanics, 

 friction is always regarded as acting in direct opposition to 

 motion in every machine. It was believed that the working 

 power of a machine could be absolutely annihilated by it. 



As the proximate cause of the cessation of motion, friction 

 was a palpable fact, and could as such be taken into account ; 

 but a fatal error was committed in giving a theoretical view 

 of its mode of action. For if a power could be annihilated, 

 or, in other words, have nothing as its effect, then there would 

 be no contradiction involved in the belief, that out of nothing 

 also power could be created. To this erroneous idea we may 

 partly trace the belief, held for centuries by most able men, in 

 the possibility of discovering a machine which should renew 

 within itself its own power as it was expended, and thus ever 



