237 



In the metrical system,, in which the units of quantity are one 

 kilogramme, one metre, and one degree in the centigrade scale, 

 the above equivalent is represented by 424 dynamic units, 

 which, for brevity^s sake, we may as well agree with the French 

 in calling " dynams/^ 



50. It has been found by experiment that a less amount of heat 

 is required to raise a gas maintained at a constant volume one 

 degree of temperature, than when the gas is allowed to expand 

 under a constant pressure. Suppose, for example, that the gas 

 be inclosed in a vertical cylinder under a piston of 100 square 

 inclies area, the atmospheric pressure on this piston will be 

 1,500 lb., and the raising this piston is equivalent to raising a 

 weight of that amount. Dr. J. R. Mayer, assuming that the 

 difference in the quantities of heat in the two cases above men- 

 tioned is equivalent to the work done by the expanding gas, 

 proceeds to determine the numerical value of these equivalent 

 quantities. Taking the specific heat of air to be 0*2675 as at 

 that time determined by the observations of De la Roche and 

 Berard, he found the dynamic equivalent of an unit of thermic 

 energy to be 367 dynams. But if, in the calculation of this 

 number, the more careful and accurate subsequent determina- 

 tions of the specific heat of air by Joule and Eegnault be substi- 

 tuted, namely, 0*2375, the result gives as the equivalent 

 426 dynams ; a result almost identical with that of Dr. Joule, 

 Imt based on theoretical considerations only. 



51. M. Seguin pursued a course of observation exactly the 

 reverse of that of Dr. Joule, namely, to determine the amount of 

 heat converted into work in the steam-engine. Taking it as an 

 axiom, in strict accordance with experimental facts, that the 

 difference between the heat existing in the steam as it enters 

 the cylinder, and that remaining in it after its exit, must be the 

 thermic equivalent of the work done in and by the engine, 

 (which difference, in the best constructed engines, amounts to 

 about five per cent, of the total heat due to the combustion of 

 tlie fuel,) he assigned a value to the thermic unit. Subse- 

 quently, M. G. A. Hirn, pursuing the same course, with the aid 

 of more perfect instrumental means, determined the value of 

 one thermic unit to be 425 dynams ; a remarkable result, and 

 intermediate between those previously inferred by Mayer, and 

 obtained by Joule. In the face of such overwhelming concur- 



or, in other words, the amount necessary to raise one pound one foot ; and 

 the numerical equivalent here given means that 772 dynamic units are 

 equivalent to the amount of thermic energy required to raise the temperature 

 of one pound of pure water, at or about the mean temperature of the air, 

 one degree of Fahrenheit's scale. 



