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 
inches 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*267, 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 Regnault be substi- 
tuted, namely, 0*2375, the result gives as the equivalent 
426 dynams ; a result almost identical with that of Dr. Joule, 
but based on theoretical considerations only. 
51. M. Seguin pursued a course of observation exactly the 
reverse of that of Dr. J oule, 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 
the 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. 
