MODERN SCIENCE. 243 



SEGUIN and the German physicist MAYER found a method 

 by which "the mechanical equivalent of heat" as Mayer called 

 it, could be calculated. Joule, independently of them, 

 commenced about the same time a series of experiments on 

 the relation between heat and work, upon which several 

 other physicists, among them Sir W. Thomson, Helmholtz, 

 Regnault, were soon likewise engaged. Joule, after intense 

 labour extending over a period of six years (1843-9), was 

 able to determine the TRANSFORMATION OF HEAT INTO 

 MECHANICAL ENERGY, or of mechanical energy into heat, in 

 a definite numerical ratio the mean number adopted for the 

 mechanical equivalent of one thermal unit being 1,390 foot- 

 pounds (on the Centigrade scale). The law of the relation of 

 heat to mechanical energy stands thus : " Heat and 'mechanical 

 energy are mutually convertible ; and heat requires for its 

 production, and produces by its disappearance, mechanical energy 

 in the ratio of i ^go foot-pounds for every thermal unit" ; that 

 is, the quantity of heat by which a pound of water is raised 

 through one degree Centigrade is generated by the expenditure 

 of the same amount of work as would be required to raise 1,392 

 pounds through one foot, or one pound through 1,392 feet, 

 and this is expressed by saying that the mechanical equivalent 

 of the thermal unit is 1,392 foot-pounds. But Joule's work 

 was a demonstration which had to be supplemented. He 

 had shown that mechanical energy or work can be converted 

 into heat ; it was required also to demonstrate that heat can 

 be converted into work. Carnot had already done so by 

 formulating the principle of the reversibility of energy; but 

 the real significance of this principle was proved by HlRN by 

 .a long series of experiments, the result of which established 

 the fact that heat can be converted into work in exactly the 

 same equivalent proportion as that determined by Joule, viz., 

 that one degree Centigrade (of heat) would raise one pound to 

 1,392 feet, or 1,392 pounds one foot. Great, however, as was 

 Joule's mathematical work so far, he carried it further still by 

 following up its logical, natural, and philosophical consequence 

 and formulating its corollary that THE TOTAL AMOUNT OF 

 ENERGY (or power of doing work) possessed by a bo dy ever 

 remains the same; that is, REMAINS UNALTERED whatever 



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