5+ Dr. Hunter’s Obfcrvations on the Heat 
THE great difference between the temperature of the open air, 
and that of deep caverns or mines, has long been taken notice 
of, both as matter of curiofity and furprize. After thermo* 
meters were brought to a tolerable degree of perfection, and 
meteorological regifters were kept with accuracy, it became a 
problem, to determine what the caufe was of this difference 
between the heat of the air, and the heat of the earth ; for it 
was loon found, that the temperature of mines and caverns 
did not depend upon any thing peculiar to them ; but that a 
certain depth under ground, whether in a cave, a mine, or a 
well, was fufficient to produce a very fen Able difference in 
the heat. In obfervations of this kind, there was perhaps 
nothing more ffriking, than that the heat in fuch caves was 
nearly the fame in iummer and winter ; and this even in 
changeable climates, that admitted of great variation between 
the extremes of heat in fummer, and cold in winter. There 
is an example of this in the cave of the Royal Obfervatory at 
Paris. The explanations, which have been attempted of this 
phenomenon, have turned chiefly upon a fuppofition, that 
there was an internal fource of heat in the earth itfelf, totally 
independent of the influence of the fun *. M. de Mairan has 
beftowed much labour on this fubjeT, and by obfervation and 
calculation is led to conclude, that of the 1026° of heat (by 
Reaumur’s fcale), which he finds to be the heat of fummer 
at Paris, 34°,02 only proceed from the fun, and the remaining 
99i°,98 from the earth, by emanations of heat from the cen- 
ter t. The proportion therefore of heat derived from this latter 
fource is to that of the fun, as 29,16 to 1. It muff be evident, 
that an hypothefis of this kind, which renders the influence of 
the fun of fmall account, is dire&ly contrary to the general 
* Vid. Mar tine’s Effliys, p. 319. 
f Memoir, de l’Acad, des Sciences, An. 1719 et 1765. 
experience 
