24 INTRODUCTION, 



Metals of all kinds are good conductors of heat 5 

 while glafs, in a heavy, folid, homogeneous body, 

 is an extremely bad conduftor, even when a metallic 

 calx enters largely into its, compofition, as in flint 

 glafs. 



A ftate of fluidity greatly promotes the diffufion of 

 heat ; for a body in a fluid flate, by the particles 

 readily moving among each other, from their dif^ 

 ferent denfities or other caufes, mixes the cold and 

 warm parts together, which occafions a quick com-? 

 munication of the heat. To apply thefe obfervations 

 to the prefent fubjedl : The fur face of the earth being 

 expofed to the great heats of fummer, and the colds 

 of winter 5 or, more properly, the low degree of heat 

 in winter ; will receive a larger proportion of heat in 

 the former feafon, and a f mailer in the latter : And 

 bjeing, further, of a large maft, and of a porous and 

 fpungy fubflance ; and, therefore, not quickly fenfible 

 to fmall variations of heat ; it will become of a mean 

 temperature at a certain depth between the heat of 

 fummer and cold of winter, provided it contain no 

 internal fource of heat v^ithin itfelf. 



Water, though in a larger mafs, follows in fome 

 degree the heat and cold of our fummer and winter, 

 from the fluidity of its parts occafioning a more fpeedy 

 efl'ufion of heat, 



,Air is quickly fufceptible of heat ; and from the 

 expanrions produced in it, and confequent motions 

 in the whole mafs, the temperature is rendered foon 

 TOiform. The changes in the heat of the air are 

 ^^hat we have meafured ^ and we are to be under- 



ftood 



