394 Intelligence and Miscellaneous Articles. 



applied, it is evident that the value of a and c afford us a means of 

 comparing the expansive force of heat with the extending force of a 

 weight ; or in other words, these values enable us to determine the 

 mechanical equivalent of heat. It must not here be forgotten, that 

 heat acts equally on all sides like pressure ; but Poisson has shown 

 that a weight which extends a wire to the amount of I, applied as 

 pressure acting equall)'^ in all directions, would produce a linear ex- 

 pansion equal to ^o. We have therefore to regard 



2a 

 I 

 as the relation of the mechanical elFect of the above-mentioned 

 quantity of heat to the mechanical effect of a pound weight. In order 

 to express this relation numerically, it is only necessary to know 

 the elastic constant of any given substance, its specific heat, specific 

 weight, and its expansion by heat. 



I have determined* the elastic constants of several metals by means 

 of the times of oscillation of wires 10 feet in length and 1 line in 

 diameter, which were fastened at their upper ends, and carried at 

 their lower ends a horizontal lever whose moment of inertia was 

 known, and which was loaded with weights. This method, which 

 admits of very great accuracy, gave me the following elastic con- 

 stants (o), to which I add the specific gravities (S) of the wires 

 employed : — 



S. S. 



Iron wire O'OOOOOOOlllO 7-5536 



Brass wire 2139 8-4760 



Platinum wire . . 1269 20-9624 



Silver wire .... 2854 10-4845 



These metals have the following specifiic heats (m), and coefficients 

 of expansion by heat (a) : — 



m. a. 



Iron 0-11379 0-001182 



Brass 0-09391 0-001878 



Platinum 0-03243 0-0008842 



Silver 0-05701 0001910 



All these values must correspond with the equation 



c . »J . S .\Z=a, 



in which c is the mechanical equivalent of the heat which is neces- 

 sary to raise the temperature of the cylinder of water from 32° to 

 212°, or the pressure exjiressed in pounds which this quantity of 

 heat exerts. 



If these values are inserted in the above equation, we obtain as 

 the value of c in each case the following numbers : — 



For iron wire c= 247800 



For brass wire .... c= 220600 



For platinum wire. . c= 205050 



For silver wire c= 223900 



* Man. d. Acad. St. P^tersb. VI. Se'rie, So. Math, et Phys., vol. v. 

 pp. 233-302, 



