oj Wells and Springs in Jamaica. 57 
to the earth, it fhould be done at the furface, where the a£ion 
of the rays immediately takes place. But though the heat be 
produced at the furface, it is communicated freely to the air as 
well as the earth ; and though the apparent intenfity of heat be 
greater in the earth, from the rays of light acting for a longer 
time upon the fame parts of matter, yet there is little doubt 
that much the greater part is carried off by the air, which as 
it is heated flies off, and allows a frefh portion of cold air to 
come in contact with the heated furface. But dill it is imma- 
terial, whether the heat of the fun be excited more in the earth or 
in the air ; for whichever has the larger proportion will in the 
end communicate a part to the other, and fo redore the balance. 
The fame obfervation applies to fuch caufes of cold as may 
operate at the furface of the earth, as evaporation, and that 
taken notice of by Mr. Wilson *. The air, therefore, near the 
furface of the earth will (hew by a thermometer in the fhade 
nearly, if not exadlly, the fame degree of heat that the fun 
communicates to our terredrial globe ; and if a mean of the 
heats thus fhewn be taken for the year round, and we pene- 
trate into the earth to that depth, that it is no longer affedted 
either by the daily, monthly, or annual variations of heat, 
the temperature at fuch depth fhould be equal to the annual 
mean above mentioned. To afeertain this with the utmod pre- 
cifion, it mud be obvious, that numerous obfervations fhould 
be made every day, correfponding to the frequent changes of 
temperature, which are known to happen in the courfe of the 
twenty-four hours in all climates ; and upon thefe a daily mean 
fhould be taken, and the annual mean deduced therefrom. 
This has not yet been done, but where we have obfervations 
from which a mean temperature can be deduced with any de- 
gree of certainty, it will be found not to differ greatly from 
* Vid. Phil. Tranf. Vol. LXX. p. 451. and Vol. LXXI. p. 386. 
Vol. LXX VIII. I the 
