156 SCIENTIFIC THOUGHT. 



when supplanting manual labour on a large scale by the 

 introduction of his perfected steam-engine, had suggested 

 the term " horse-power " as the common measure of both ; 

 and the French mathematicians, who treated mechanics 

 with a view to practical application, had introduced the 

 term " work," In the general industries, however, — out- 

 side of special branches, notably marine engineering, — 

 these measures were very crudely applied ; they became 

 unintelligible and meaningless where other agencies — 

 notably those of chemistry and electricity — had to be 

 employed. It is only since the terms " power " and 

 " work " have been enlarged and the more general con- 

 ception of energy introduced that it has become possible 

 to measure the new forces or agencies in terms applic- 

 able to all alike. Practically as well as theoretically 

 the system of measurement remained imperfect so long 

 as the energy of chemical combination could not be 

 measured in the same way as Watt measured the 

 energy of heat, and as Joule and others taught us 

 how to measure the energy of an electric current. The 

 term " energy " has thus become as important a con- 

 ception for practical as it has been long recognised to 

 be for purely scientific purposes. If the only power 

 we use is manual labour or steam power, there exists 

 a crude way of measuring both by the hands employed 

 and the weight 'of coal burnt ; but electrical power is 

 not so exclusively dependent on a personal or material 

 item, and thus it can only be measured by a system in 

 which the several items of cost are reduced to a common 

 term. It is through the wholesale introduction of the 

 electric current as a practical agent that the thing called 



