VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 379 



smaller in the latter; and being further of a large mass, and of a porous and 

 spongy substance, and therefore not quickly sensible to small variations of heat, 

 it will become of a mean temperature at a certain depth, between the heat of 

 summer, and the cold of winter, provided it contain no internal source of heat 

 within itself. This conclusion is strictly agreeable to the experiments and ob- 

 servations hitherto made, in heating and cooling bodies, or in mixing portions 

 of matter of the same kind of different temperatures*. Water, though in a 

 large mass, follows in some degree the heat and cold of our summer and winter, 

 from the mobility of its parts occasioning a more speedy diffusion of heat. Air 

 is quickly susceptible of heat, and from the expansions produced in it, and con- 

 sequent motions in the whole mass, the temperature is soon rendered uniform. 



The changes in the heat of the air are what we have measured, and we are to 

 be understood to speak of them, when we talk of the temperature of summer 

 and of winter. It may be asked then, is the heat of the sun first communicated 

 to the air, and thence to the earth? No, the air is susceptible of a very small de- 

 gree of heat from the rays of the sun passing through it; for it is well known 

 that they produce no heat in a transparent medium, and consequently, that the 

 air is only so far heated as it differs from a medium that is perfectly transparent. 

 The heat produced by the rays of the sun bears a proportion to their number, 

 their duration, and their angle of incidence; and it takes place at the points 

 where they strike an opaque and non-reflecting surface. The surface of the earth 

 may therefore be considered as the place from which the heat proceeds, which is 

 communicated to the air above, and the earth below. That this is really the 

 case, is evident from the superior degree of heat produced by the action of the 

 rays of the sun on an opaque body, which will often be heated to 150° of Fahren- 

 heit, while the temperature of the air is not above 90° -J-. It may seem therefore, 

 that to measure the heat communicated to the earth, it should be done at the 

 surface, where the action of the rays immediately takes place. But though the 

 heat be produced at the surface, it is communicated freely to the air as well as the 

 earth ; and though the apparent intensity of heat be greater in the earth, from 

 the rays of light acting for a longer time on the same 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 fresh portion of cold air to come in contact with the 

 heated surface. But still it is immaterial, whether the heat of the sun 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 so restore the balance. The same 

 observation applies to such causes of cold as may operate at the surface of the 

 earth, as evaporation, &c. The air therefore, near the surface of the earth, 



* De Luc Modifications de rAtmosphere, vol. 1, p. 285. + Martiue's Essays^ p. 309. 



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