540 HISTORY OF THERMOTICS. 



sonings upon Prevost's law of exchanges ; and that, 

 in this way, the second of their laws above stated, 

 respecting the quickness of cooling, was a mathe- 

 matical consequence of the first. It may be ob- 

 served also, that their temperatures are measured 

 by means of the air-thermometer, and that if they 

 were estimated on another scale, the remarkable 

 simplicity and symmetry of their results would dis- 

 appear. This is a strong argument for believing 

 such a measure of temperature to have a natural 

 prerogative of simplicity. This belief is confirmed 

 by other considerations; but these, depending on 

 the laws of expansion by heat, cannot be here re- 

 ferred to ; and we must proceed to finish our survey 

 of the mathematical theory of heat, as founded on the 

 phenomena of radiation and conduction, which alone 

 have as yet been traced up to general principles. 



We may observe, before we quit this subject, 

 that this correction of Newton's law will materially 

 affect the mathematical calculations on the sub- 

 ject, which were made to depend on that law both 

 by Fourier, Laplace, and Poisson. Probably, how- 

 ever, the general features of the results will be 

 the same as on the old supposition. M. Libri, an 

 Italian mathematician, has undertaken one of the 

 problems of this kind, that of the armil, with 

 Dulong and Petit' s law for his basis, in a Memoir 

 read to the Institute of France in 1825, and since 

 published at Florence 21 . 



21 Mem. de Math, et de Phys. 1829. 



