108 SPTUNGS, RIVERS, CANALS, LAKES, 



vation and calculation is led to conclude, that of the 1026 of hfat, 

 by Reaumur's scale, which he finds to be the heat of summer at 

 Paris, 34.02 only proceed from the sun, and the remaining y91.9S 

 from ilie earth, by emanations of heat from the centre *. The pro- 

 portion therefore of heat derived from this latter source is to that 

 of the sun, as 29.16 to J. It must be evident that an hypothesis 

 of this kind, which renders the influence of the sun of sina'l ac- 

 count, is directly contrary to the general experience and conviction 

 of mankind. Without entering however into any discussion of the 

 data from whence M. de Mairan draws his conclusions, it will be 

 more satisfactory to consider what would be the effect of the opera- 

 tion of those laws of heat with which we are acquainted. 



And first, it is well known, that heat in all bodies has a tendency 

 to diffuse itself equally through every part of them, till they be- 

 come of the same temperatuie. Again, bodies of a large mass are 

 both cooled and heated slowly. Besides the mass of matter, there 

 are two other considerations of much importance in the slow or 

 quick transmission of heat through bodies ; these are their different 

 conducting powers, and their bein^j in a state of solidity or fluidity. 

 The conducting powers of heat are well known to be very various 

 in different bodies; nor are they hitherto reducible to any law, de- 

 pending either on the density or chemical properties of matter. 

 Metals of all kinds are good conductors of heat, while glass, a 

 heavy, solid, bom'ogeueoiM body, is an extremely bad conductor, 

 even when a metallic calx enters largely into its composition, as in 

 flint-glass. A state of fluidity greatly promotes the diffusion of heat ; 

 for a body in a fluid state, by the particles moving readily among 

 each other from their different densities or other causes, mixes the 

 warm and cold parts together, which occasions a quick commu- 

 nication of heat. To apply these observations to the present sub- 

 ject ; the surface of tht> ?arth being exposed to the great heats of 

 summer, and the colds of \vinter, or more properly the low degree 

 of heat of winter, will receive a larger proportion of heat in the 

 former season, and a smaller in the latter; and being further of a 

 large mass, and of a porous and spongy substance, and therefore 

 not quickly sensible to small variations of heat, it will become of a 

 mean temperature at a certain depth, between the heat of summer 

 and the cold of winter, provided it contain no internal source of 



* Memoir de I'Acad. des Sciences, An; 1719 et 1165. 



