APPLICATION OF FORCE. 91 



Is Perpctnal 1^5. PERPETUAL MoTION, Or the COnstrUC- 



cMne^'poTsi' ^^^^ of niachines which shall produce power 

 *''*' sufficient to keep themselves in motion con- 



tinually, is, therefore, an impossibility, since no combi- 

 nation of machinery can create, or increase, the quantity 

 of power applied, or even preserve it without diminution. 



What era 1 ^^ nature we have an example of continued and undimin* 



of continued ished motion in the revolution of the earth upon its axis, and 

 we ia natill-e7 °^ ^^^ planets around the sun. These bodies have been mov- 

 ing with undiminished velocity for ages past, and, unless pre- 

 vented by the agency which created them, will continue so to do for ages to come. 



„ ^ , 186. We derive advantasres from machines 



Ilow do ire de- , '-' 



rive advantages in thrcc different ways; Ist, from the addi- 



from machines ? i rt i r- 



tions they make to human power ; 2d, from 

 the economy they produce of human time ; 3d, from the 

 conversion of substances apparently worthless and com- 

 mon into valuable products. 



How do ma- 18T. Machiucs make additions to human 

 additioni™"^o power, because they enable us to use the 

 human power? power of natural agents, as wind, water, steam. 

 They also enable us to use animal power with greater ef- 

 fect, as when we move an object easily with a lever, which 

 we could not with the unaided hand. 

 How do ma- 1^8. Machiucs produce economy of human 

 economVofiir- tlmc, becausc they accomplish with rapidity 

 man ume t what would rcquire the hand imaided much 

 time to perform. 



A machine turns a gun-stock in a few minutes ; to shape it by hand would 

 be the work of hours. 



189. Machines convert objects apparently 

 chines convert worthless iuto valuablc products, because by 



worthless ob- - . - • t , '/• 



jccts into vai- their great power, economy, ana rapidity of 

 epro ucs ^^^Iqj^^ ^\^qj make it profitable to use objects 

 for manufacturing purposes which it would be unprofit- 

 able or impossible to use if they were to be manufactured 

 by hand. 



Without machines, iron could not be forged into sliafls for gigantic engines; 

 fibers could not be twisted into cables ; granite, in large masses, could not be 

 transported from the quarries. 



