358 NINETEENTH CENTURY. FT. in. 



mechanical energy of the weight, derived from the man who 

 wound it up, is expended in fluid friction in the box and 

 thus converted into heat. 



Hirn's Experiments on Heat converted into 

 Mechanical Energy. If you have understood this ex- 

 planation, you will have some idea of the theory that 

 mechanical energy may be converted into heat ; but to 

 complete the proof we require not only to turn work into 

 heat, but also to turn heat into work. This had already 

 been done many years before by a French engineer, M. 

 Carnot, though he did not understand its real significance, 

 but it has now been most beautifully proved by a long 

 series of experiments made by M. Him, of Colmar, in 

 Alsace. It was already known exactly how much heat 

 could be obtained from every ton of coal consumed. What 

 M. Him did was to find out how much of that heat was 

 converted into useful work in an engine. This was by no 

 means a simple task, for much heat is lost in various ways 

 in passing through the engine ; and even when he thought 

 he had allowed for all this, it was found that some of the steam 

 had turned back into water on its way, and thus used up 

 some of the heat. At last, however, when all was carefully 

 measured and calculated, he found that if it had been 

 possible to turn all the heat passing through the engine into 

 work, then for every i Fahr. added to the temperature of a 

 pound of water, enough work would have been done to raise a 

 weight of i Ib. to a height of 772 feet. 



Conservation of Energy. And thus we arrive at one 

 of the grandest discoveries of modern science, namely, that 

 the whole amount of energy, or power of doing work, pos- 

 sessed by any set of bodies, remains unaltered whatever 

 transformations it may undergo. It may exist in one of 

 two forms either as potential or stored-up energy, which is 



