378 I'HILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



was sufficient to produce a very sensible difference in the heat. In observations 

 of this kind, there was perhaps nothing more striking, than that the heat in 

 such caves was nearly the same in summer and winter; and this even in change- 

 able climates, that admitted of great variation between the extremes of heat in 

 summer, and cold in winter. There is an example of this in the cave of the 

 Royal Observatory at Paris. The explanations, which have been attempted of 

 this phaenomenon, have turned chiefly on a supposition, that there was an in- 

 ternal source of heat in the earth itself, totally independent of the influence 

 of the sun*. M. de Mairan has bestowed much labour on this subject, and by 

 observation and calculation is led to conclude, that of the 1026° of heat, 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 Qgi°.g8 from the earth, by emanations 

 of heat from the centre-|-. The proportion therefore of heat derived from this 

 latter source is to that of the sun, as 29.16 to 1. It must be evident that an 

 hypothesis of this kind, which renders the influence of the sun of small 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 operation of those laws of heat with which we are ac- 

 quainted. 



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 become of the same temper- 

 ature. Again, bodies of a large mass are both cooled and heated slowly. 

 Besides the mass of matter, there are two other considerations of much im- 

 portance in the slow or quick transmission of heat through bodies; these are 

 their different conducting powers, and their being 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, depending either on 

 the density or chemical properties of matter. Metals of all kinds are good con- 

 ductors of heat, while glass, a heavy, solid, homogeneous 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 communication of heat. To apply these observations 

 to the present subject ; the surface of the earth being exposed to the great heats 

 of summer, and the colds of winter, or more properly the low degree of heat of 

 winter, will receive a larger proportion of heat in the former season, and a 



» Vid. Martine's Essays, p. 319. f Memoir, de I'Acad. des Sciences, An. 1/19 et 1765, 



