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56 Dr. Hunter’s Obfervations on the Heat 
and the cold of winter, provided it contain no internal fourceof 
heat within itfelf. This conclufion is ftriclly agreeable to the 
experiments and obfervations hitherto made, in heating and 
cooling bodies, or in mixing portions of matter of the fame 
kind of different temperatures *. Water, though in a large 
mafs, follows in fome degree the heat and cold of our fummer 
and winter, from the mobility of its parts occafioning a more 
fpeedy diffufion of heat. Air is quickly fufceptible of heat, and 
from the expanfions produced in it, and confequent motions 
in the whole mafs, the temperature is foon rendered uniform. 
The changes in the heat of the air are what we have meafured, 
and we are to be underffood to fpeak of them, when we talk of the 
temperature of fummer and of winter. It may be afked then, is 
the heat of the fun firft communicated to the air, and thereby to 
the earth ? No, the air is fufceptible of a very fmall degree of 
heat from the rays of the fun paffing through it ; for it is well 
known, they produce no heat in a tranfparent medium, and 
confequently, that the air is only fo far heated as it differs from 
a medium that is perfectly tranfparent. The heat produced by 
the rays of the fun bears a proportion to their number, their 
duration, and their falling more or lefs perpendicularly ; and 
it takes place at the points where they ftrike an opaque and 
non-refledting furface. The furface of the earth may there- 
fore be confidered as the place, from whence the heat proceeds, 
which is communicated to the air above, and the earth below. 
That this is really the cafe is evident from the fuperior degree of 
heat, produced by the adlion of the rays of the fun upon an 
opaque body, which will often be heated to 150° (Fahren- 
heit), while the temperature of the air is not above 90° f. 
It may feem, therefore, that to meafure the heat communicated 
* Vid. De Luc Modifications de l’Atmofphere, Vol, I. p. 285* 
f Mar tine’s E flays, p. 309. 
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