oj Wells and Springs in Jamaica. 63 
higher than a mean : and did the central heat bear as high a 
proportion to that of the fun as M. de Mairan alledges, the 
heat of the earth itfelf ought to be a great deal above the mean 
temperature of the air, which xrom obfervation theiv_ ib no 
ground for believing. It is eafy to fee the iource of M. de 
M a Iran’s error ; he has founded his calculations upon the 
fcale of Reaumur, and confiders the degrees of his thermo- 
meter as marking the real proportions, and ablolute quantity of 
heat*. It is a matter that cannot be denied, that we know 
nothing of the abfolute quantities of heat; and that the de- 
grees of our thermometers are only to be confidered as a few 
of the middle links of a chain, the length of which we are 
totally ignorant of, and therefore in no condition to compare 
its proportional parts. It deferves, however, to be remarked,, 
that obfervations of a late date have (hewn, that the notions 
of cold upon which Reaumur’s fcale was conftru&ed, and 
upon which M. de M a Iran’s calculations are founded, are. 
imaginary and without foundation "f'« 
Hot fprings and volcanos may be produced as proofs of the. 
exiftence of an internal fource of heat in the earth ; but their 
operation appears to be limited to a very fmall extent, an 
fcarcely deferving of notice in the prefent difeuffion. It is no 
uncommon thing to find fprings of the ufual temperature 
clofe by hot fprings ; and no volcano, with which we, are yet. 
acquainted, appears to have raifed the temperature of the 
country immediately adjoining to it. , 
The l'ea admits of change of temperature more quickly than 
the earth, particularly near the fhore. The mean heat of the . 
* Vid. Memoir, de 1 ’Acad. des Sciences, An. 1765. P- H 3 
t Vid. Phil. Tranf. Vol. LXXUI. p. *303- 3 ° 3 - ^ 3 * 9 - 
fea 
