330 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



that this is actually the fact is now regarded as proved. The 

 law expressing this is known under the name of the law of the 

 Conservation of Force. 



For a small group of natural phenomena it had already been 

 pronounced by Newton, then more definitely and in more general 

 terms by D. Bernouilli, and so continued of recognised appli- 

 cation in the greater part of the then known purely mechanical 

 processes. Certain amplifications at times attracted attention, 

 like those of Ruinford, Davy, and Montgolfier. The first, 

 however, to compass the clear and distinct idea of this law, and 

 to venture to pronounce its absolute universality, was one whom 

 we shall have soon the pleasure of hearing from this platform, 

 Dr. Robert Mayer, of Heilbronn. While Dr. Mayer was led 

 by physiological questions to the discovery of the most general 

 form of this law, technical questions in mechanical engineering 

 led Mr. Joule, of Manchester, simultaneously, and independently 

 of him, to the same considerations ; and it is to Mr. Joule that 

 we are indebted for those important and laborious experimental 

 researches in that department where the applicability of the law 

 of the conservation of force appeared most doubtful, and where 

 the greatest gaps in actual knowledge occurred, namely, in the 

 production of work from heat, and of heat from work. 



To state the law clearly it was necessary, in contradistinction 

 to Galileo's conception of the intensity of force, that a new 

 mechanical idea should be elaborated, which we may term the 

 conception of the quantity of force, and which has also been 

 called quantity of work or of energy. 



A way to this conception of the quantity of force had been 

 prepared partly, in theoretical mechanics, through the conception 

 of the amount of vis viva of a moving body, and partly by 

 practical mechanics through the conception of the motive power 

 necessary to keep a machine at work. Practical machinists had 

 already found a standard by which any motive power could be 

 measured, in the determination of the number of pounds that 

 it could lift one foot in a second ; and, as is known, a horse-power 

 was defined to be equivalent to the motive power required to 

 lift seventy kilogrammes one metre in each second. 



