452 POPULAR LECTURES AND ADDRESSES. 



supply of energy from wind or water, or other 

 external source, the perpetual motion was dead to 

 science long before Newton's time : and on the 

 negation of it Stevinus founded a beautiful proof 

 of the parallelogram of forces, which is celebrated 

 in the history of dynamics, and is still justly 

 admired. But the doctrine of the " Conservation 

 of Energy," which has grown up since the end of 

 last century, has given a fresh lease of life to the 

 idea of the perpetual motion revived in a more 

 subtle form. 



From Rumford, Davy, and Joule we have 

 learned that the reason why every machine, even 

 though not called upon to give out work done by 

 it, must come to rest, is not, as was generally 

 supposed by contemporary and preceding philo- 

 sophers, because the friction that stops the 

 machine implies annihilation of energy, but 

 because it converts into heat the energy given 

 initially in the motion of the machine. Suppose 

 now we could guard perfectly against loss of heat 

 by nidiation, or by cooling currents of air, or by 

 conduction along the supports of the machine, 



