.S7A' WILLIAM. SIEMENS, FJLS. 31 



The specific heat and temperature of a body determine the 

 vihmtiim velocity of the material particles, the square of which 

 multiplied by the weight of the particles, gives their inherent 

 force, or "vis viva." 



In solids the "vis viva" is least remarkable, in fluids it is 

 greater, and in gaseous fluids it predominates so strongly over 

 the gravitation, that the latter force becomes practically inap- 

 preciable. 



Joule explains the elastic pressure of a gas by the rebound of 

 its particles against the side of the vessel containing it,* and proves 

 the correctness of his views by calculation. If one side of a vessel 

 gradually yields to the pressure, as is the case with a working 

 piston, then the rebound of the particles will be less than their 

 impact, and consequently the length of their vibrations must 

 diminish, in proportion to the outward motion produced. 



It is thus shown, that vibratory motion, or heat, is converted 

 into its equivalent of onward motion, or dynamical effect. 



To express this equivalent by number, it is necessary to agree, in 

 the first place, on an arbitrary unit of heat, which is usually the 

 heat required to raise the temperature of 1 Ib. of water through 1 

 Centigrade, or Fahrenheit, and also, on a unit of mechanical effect, 

 which is usually the " foot-pound," or the power required to lift 

 1 Ib. through 1 foot. The data for these calculations may be 

 taken from any materials, the specific heat and density of which 

 are well known. 



The nature of the material cannot affect the result, for if one 

 should be more favourable to the production of mechanical force 

 by heat than another, and the second be more favourable to the 

 reverse process, it would follow that by a judicious selection of 

 materials, a machine might be devised, which would reproduce 

 more than its own cause of motion. This would be " to ascribe 

 creative power to matter," in contradiction to the laws of nature. 



The limits of this paper do not permit of the train of reasoning, 

 by which the numerical equivalent of power for heat has been 

 ascertained, in different ways, by English, French, and German 

 natural philosophers, within the last ten years ; the author must, 

 therefore, content himself with merely mentioning their names and 



* Vide Transactions of Philosophical Society of Manchester, 1848. 



