180 DISCOVERY CH. 



mechanical energy and heat were reciprocally convert- 

 ible. A French philosopher, Sadi Carnot, saw the 

 connection between the two forms of energy represented 

 by heat and mechanical work, and he announced in 

 1824 that the expenditure of a given amount of work 

 would produce a definite quantity of heat, but his 

 conclusions received no recognition at the time. The 

 problem was to measure exactly the amount of work 

 which must be done to obtain a given quantity of heat ; 

 that is, to determine the mechanical equivalent of heat. 



Seguin in France calculated this relationship by observ- 

 ing the fall of temperature of steam when expanding 

 against external pressure. From 1842 to 1847 J. E. 

 Mayer, K. F. Mohr, and H. von Helmholtz, in Germany, 

 and L. A. Colding, in Denmark, all arrived indepen- 

 dently, by various lines of reasoning and experiment, at 

 the great generalisation of the conservation of energy, 

 but the work which established the principle upon the 

 solid ground of accurate measurement was done by a 

 scientific amateur, James Prescott Joule, a brewer at 

 Salford, near Manchester. 



By a number of careful experiments Joule showed 

 that a definite quantity of heat was always produced 

 by the expenditure of a definite amount of mechanical 

 work. When a pebble is dropped from a bridge into a 

 stream flowing beneath, the impact with the water pro- 

 duces a certain quantity of heat, the quantity depending 

 upon the weight of the pebble and the distance of the 

 fall. Joule found that the fall of a pound weight 

 through a distance of 778 feet always generated sufficient 

 heat to raise the temperature of a pound of water one 

 degree Fahrenheit. 



Probably this conclusion does not strike you as ve 

 remarkable ; neither did it appear so to the membe 



ne 



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