376 AIM AND PKOGRESS OF PHYSICAL SCIENCE. 



tions at times attracted attention, like those of Kumford, 

 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. Eobert 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 con- 

 tradistinction to Galileo's conception of the intensity 

 of force, that a new mechanical idea was 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 ID 

 each second. 



