340 Mr Meiklc on the Theory of 



stili, under the same reduced volume, the specific heat will just 

 regain its former value EF x 1° ; and the same may be proved 

 of EG X 1°. 



What a difference between this simple result and the com- 

 plex conclusions which a gratuitous hypothesis has enabled the 

 Marquis Laplace to state in his Mecanique Celeste, and M. Pois- 

 son to copy from him, as already quoted ! And yet, had they 

 used a diagram even with straight lines, as their hypothe- 

 sis required, they might have obtained precisely the same result 

 as I have got. For inconsistent data sometimes produce a cor- 

 rect result. This, however, only happens when opposite errors 

 destroy each other, or when part of the data is allowed to lie 

 dormant. 



The specific heat of steam is very likely independent of its 

 density ; and if so, ought it not, under a constant volume, to be 

 equal that of water ? And if the specific heats of equal volumes 

 of elastic fluids, as analogy would almost lead us to suppose, be 

 the same under equal pressures and temperatures, the specific 

 heat of air would be 'Q9^5 under a constant volume, and '883 

 under a constant pressure ; about three times the common esti- 

 mate, which is very uncertain. But this is merely thrown out as 

 a conjecture. 



M. Poisson^s memoir being nearly related to the foregoing- 

 inquiry, I have, for the better pointing out the errors into which 

 that illustrious author has fallen, kept closer to his method than 

 was otherwise necessary. It must now be sufficiently evi- 

 dent, that his hypothesis, so often mentioned, was both super- 

 fluous and at variance with his other principles. In the same 

 memoir, M. Poisson acknowledges that his theory of the ex- 

 pansive force of steam is far from accounting for the economy 

 of heat, which experiment indicates in the use of high pressure 

 engines. This furnishes a farther proof in favour of the law 

 we have investigated ; for, according to it, when the tempera- 

 ture is elevated, the force even of air having its density con- 

 stant, increases in an enormously higher ratio than the quantity 

 of heat does, viz. in geometrical progression, whilst the heat in- 

 creases in arithmetical progression. Thus, calling the heat unit, 

 which doubles the force when the density is constant, we shall 

 have the following two series : 



