of Wells and Springs in Jamaica. 5 - 
experience and conviction of mankind. Without entering, 
however, into any difeuffion of the data from whence M. de 
Matran draws his conclufions, it will be more fatisfa&ory to 
confider what the effect of the operation of thofe laws of 
heat, with which we are acquainted, would be. 
And fir ft, it is well known, that heat in all bodies has a 
tendency to diffufe itfelf equally through every part of them, 
till they become of the fame temperature. Again, bodies of 
a large mafs are both cooled and heated {lowly. Befides the 
mafs of matter, there are two other confiderations of much 
importance in the flow or quick tranfmiffion of heat through 
bodies ; thefe are their different conducting powers, and their 
being in a {fate of folidity 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 
upon the denfity, or chemical properties of matter. Metals of 
all kinds are good conductors of heat, while glafs, an heavy, 
folid, homogeneous body, is an extremely bad conductor, even 
when a metallic calx enters largely into its compofition, as in 
flint-glafs. A fate of fluidity greatly promotes the diffufl on of 
heat ; for a body in a fluid {fate, by the particles moving rea- 
dily among each other from their different denfities or other 
caufes, mixes the warm and cold parts together, which occa- 
fions a quick communication of heat. To apply thefe offer- 
vations to the prefent fubjeCt ; the furface of the earth being 
0 
expofed to the great heats of fummer, and the colds of win- 
ter, or more properly the low degree of heat of winter, 
will receive a larger proportion of heat in the former feafon, 
and a fmaller in the latter ; and being further of a large 
mafs, and of a porous and fpongy fubffance, 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, 
3 and 
