NATURAL PHILOSOPHY. 161 



part, and the unit of work, or the foot-pound, on the other. M. Clapeyron, 

 in his treatise on the moving power of heat, and M. Noltzman, of Manheim, 

 in 1845, who availed himself of the labors of M. Clapeyron and M. Garnet in 

 the same field, grounding their investigations on the received laws of Boyle 

 or Marriotte, and Gay-Lussac, which express the observed relations of heat, 

 elasticity, and volume in steam and other gaseous matter, concluded that 

 the unit of heat was capable of raising a weight, between the limits of six 

 hundred and twenty-six pounds and seven hundred and eighty-two pounds, 

 one foot high; that is to say, that one unit of heat was equivalent to from 

 six hundred and twenty-six to seven hundred and eighty-two foot-pounds. 

 By this mode of investigation, they suppose a given weight of steam or 

 gaseous matter to be contained in a vertical cylinder formed of non-conduct- 

 ing material, in which is fitted an air-tight but freely moving piston, which 

 is pressed downward by a weight equal to the elasticity of the gas. Now, 

 the weight, initial temperature, pressure, and volume, being known, a defi- 

 nite quantity of heat from without is supposed to be imparted to the vapor; 

 and the result is partly an elevation of the temperature of the vapor, and 

 partly a dilation or increase of volume ; or, in other words, an exertion of 

 pressure through space, the elasticity remaining the same. But the result 

 may be represented entirely by dilation, so that there shall not be any final 

 alteration of temperature; and for this purpose it is only necessary to allow 

 the vapor to dilate without any loss of its original or imparted heat until it 

 reacquires its initial temperature. In this case, the ultimate effect is purely 

 dilatation, or motion against pressure; and the work done is represented by 

 the product of that pressure into the space moved through. 



Mr. Joule, of Manchester, in 1843-47, proceeded, by entirely different, 

 independent, and, in fact, purely experimental methods, to investigate the 

 relation of heat and work. 1st. By observing the calorific effects of magneto- 

 electricity. He caused to revolve a small compound electro-magnet immersed 

 in a glass vessel containing water between the poles of a powerful magnet; 

 heat was proved to be excited by the machine by the change of temperature 

 in the water surrounding it, and its mechanical effect was measured by the 

 motion of such weights as by their descent were sufficient to keep the 

 machine in motion at any assigned velocity. 2d. By observing the changes 

 of tempci'ature produced by the rarefaction and condensation of air. In this 

 case, the mechanical force producing compression being known, the heat 

 excited was measured by observing the changes of temperature of the water 

 in which the condensing apparatus was immersed. 3d. By observing the 

 heat evolved by the friction of fluids. A brass paddle-wheel, in a copper 

 can containing the fluid, was made to revolve by descending weights. Sperm 

 oil and water yielded the same results. Mr. Joule considered the third 

 method the most likely to afford accurate results ; and he arrived at the con- 

 clusion that one unit of heat was capable of raising seven hundred and 

 seventy-two pounds one foot in height; or that the mechanical equivalent of 

 heat was expressible by seven hundred and seventy -two foot-pounds for one 

 unit of heat known as "Joule's equivalent." 



The following are the values of Joule's equivalent for different thermomet- 

 ric scales, and in English and French units : 



1 English thermal unit, or 1 deg. Fah. in 1 Ib. of water, 772 foot-pounds. 

 1 centigrade degree in 1 Ib. of water, .... 1389.6 " 

 1 French thermal unit, or 1 centigrade degree in a 



kilogramme of water, 423.55 kilogrammetres. 



14* 



