52 MR. J. P. JOULE OK THE, ETC. 



be possible to effect the same object in a more economical 

 manner, by availing ourselves of the use of the magneto- 

 electrical machine. This machine enables us to obtain heat 

 from ordinary mechanical force, which mechanical force may 

 again be derived from the conversion of heat, as in the steam 

 engine. In a steam engine it is practically possible to convert 

 at least one-fifth of the heat due to the combustion of coal into 

 force, and one-half of this force applied to work a magneto- 

 electrical machine may be evolved in the shape of heat. 

 Hence then it is possible to arrange machinery so as to pro- 

 duce currents of electricity which shall evolve one-tenth of the 

 quantity of heat due to the combustion of the coal employed. 

 So that 5,000 grains of coal used in this way would suffice for 

 the fusion of one pound of iron. 



