14:6 SCIENTIFIC THOUGHT. 



principle, one which allows us to deal with the grand 

 total or outcome — mathematically called the integral — 

 of physical processes and changes without necessarily 

 possessing a detailed knowledge of the minute elements 

 or factors — mathematically called differentials — out of 

 which they are compounded. Inasmuch as what we 

 actually observe are always integral effects — -i.e., sum- 

 mations or aggregates of great numbers of indi^ddual 

 and unobservable processes — this line of reasoning is 

 not infrequently very useful, and has been in many 

 cases applied to arrive at important conclusions. In 

 fact, it is the analogue in science of the method accord- 

 ing to which practical men very often succeed in carrying 

 on extensive business transactions, of which they possess 

 a merely external though accurate knowledge ; or of the 

 balance-sheet of an industrial undertaking which exhibits 

 and guarantees the correct result, though only the profit 

 and loss account and the ledgers would show how this 

 result has been arrived at. 

 33. Faraday had taught us how to look upon any given 



Faraday. . . . ^ • -i t ■ -i-i 



portion or space m which electric, magnetic, chemical, 

 and thermal changes were going on as a connected system, 

 which he termed the electro-magnetic field. He and 

 others — notably Oersted, Ohm, Weber, Lenz, and Joule 

 — had shown how the different occurrences in such a 

 system could be reduced to a common measure, and how 

 they were observably connected. Maxwell brought all 

 these phenomena together under the term " energy of 

 the electro-magnetic field," and set himself to study the 

 possible forms and changes of this quantity under the 

 law of the conservation of energy — i.e., as the preser- 



