Mathemntks and General Physics. 857 



being then faster than the law would indicate. It was also sooa 

 discovered that cold radiated as well as heat. The Professoc 

 then entered into a most interesting examination of the labours 

 of Diilong and Petit ; of Fourier, Poisson, and Cochet. He 

 eulogized justly the sagacity with which Laplace and Biot fir^ 

 saw the difficulty, that the cooling of bodies depended in part 

 upon this conducting power, while the heat was traversing tlie 

 internal parts, as upon the radiating power, while thrown fron? 

 the surface. This difficulty was solved by Fourier, who found 

 that the quantity of heat thus conducted from particle to par- 

 ticle within the body, varied as the difference of temperature df 

 the two directly, and inversely as the distance at which they !av 

 from each other. He also showed that the radiating intensit}' at 

 the surface of a body, varied as the sine of the angle in whicia 

 that ray came to the surface. Professor Whewell then discuss- 

 ed the second head, but any attempt to give a clear view of the 

 formuljB required for the mathematical discussion of the questioiii 

 would be impossible within the limits to which we must confine 

 ourselves ; and we the less regret this, as the report will be 

 printed at length in the next volume of the reports of the Bri- 

 tish Association. Under the third general head, he entered 

 upon an interesting comparison of the observed and calcu- 

 lated results, and showed the harmony that prevailed betweea 

 them. Under the fourth general head, he considered — " the 

 application of these speculations to practically interesting cases." 

 Amongst the numerous interesting topics discussed, the follow-- 

 ing may be selected. The sun, from day to day, is pouring u|xjr 

 the earth a quantity of heat ; this, as it descends, by the conduct- 

 ing powers of the parts of the earth, follows certain laws of ia- 

 crease and decrease ; and the entire quantity of each year de- 

 scends to a certain depth, where it is succeeded by the quantity 

 thrown upon the earth in the preceding year, which had not 

 yet been dissipated ; below that lies the stratum occupied by the 

 solar heat of the preceding year, and so on, until at length, at a 

 certain depth, this solar heat ceases to be perceptible. Ho 

 showed, that the mean annual quantity of this solar heat was 

 such as would melt fourteen metres of ice encircling the entire 

 surface of the earth. He next considered the central heat of tlva 



