CATARACTS, AND INUNDATIONS. 10Q 



heat within itself. This conclusion is strictly agreeable to the expe- 

 riments and observations hitherto made, in heating and cooling bo- 

 dies, or in mixing portions of matter of the same kind of different 

 temperatures *. Water, though in a large mass, follows in sonic 

 degree the heat and cold of our summer and winter, from the mo- 

 bility of its parts occasioning a more speedy diffusion of heat. Air 

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

 it, and consequent motions in the whole mass, the temperature i 

 soon rendered uniform. 



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

 and we are to be understood to sp?ak of them, when we talk of 

 the temperature of summer and of winter. It maybe asked then, 

 is the heat of the sun first communicated to the air, and thence to 

 the earth 1 No, the air is susceptible of a very small degree of beat 

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

 that they produce no heat in a transparent medium, and conse- 

 quently, that the air is only so far heated as it differs from a me. 

 dium that is perfectly transparent. The heat produced by the rays 

 of the sun bears a proportion to their number, their duration, ami 

 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 1 50 of Fahrenheit, 

 while the temperature of the air is not above 90 t. It muv seem, 

 therefore, that to measure the heat communicated to the eartli, it 

 should be done at the surface, where the action of the rays imme- 

 diately takes place. But though the heat be produced at the sur- 

 face, 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 mat. 

 ler, 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 i'resh portion 

 of cold air to come in contact with the heated surface. But sUH it 



* De Luc Modifications de 1' Atmosphere, vol. I. p. 283= 

 t Marline'* Essays, p, 30'.. 



