PRELIMINARY DISCOURSE. 3 



In fact, the radiation of the sun in which this planet is 

 incessantly plunged, penetrates the air, the earth, and the waters ; 

 its elements are divided, change in direction every way, and, 

 penetrating the mass of the globe, would raise its mean tem 

 perature more and more, if the heat acquired were not exactly 

 balanced by that which escapes in rays from all points of the 

 surface and expands through the sky. 



Different climates, unequally exposed to the action of solar 

 heat, have, after an immense time, acquired the temperatures 

 proper to their situation. This effect is modified by several ac 

 cessory causes, such as elevation, the form of the ground, the 

 neighbourhood and extent of continents and seas, the state of the 

 surface, the direction of the winds. 



The succession of day and night, the alternations of the 

 seasons occasion in the solid earth periodic variations, which are 

 repeated every day or every year: but these changes become 

 less and less sensible as the point at which they are measured 

 recedes from the surface. No diurnal variation can be detected 

 at the depth, of about three metres [ten feet] ; and the annual 

 variations cease to be appreciable at a depth much less than 

 sixty metres. The temperature at great depths is then sensibly 

 fixed at a given place : but it is not the same at all points of the 

 same meridian ; in general it rises as the equator is approached. 



The heat which the sun has communicated to the terrestrial 

 globe, and which has produced the diversity of climates, is now 

 subject to a movement which has become uniform. It advances 

 within the interior of the mass which it penetrates throughout, 

 and at the same time recedes from the plane of the equator, and 

 proceeds to lose itself across the polar regions. 



In the higher regions of the atmosphere the air is very rare 

 and transparent, and retains but a minute part of the heat of 

 the solar rays : this is the cause of the excessive cold of elevated 

 places. The lower layers, denser and more heated by the land 

 and water, expand and rise up : they are cooled by the very 

 fact of expansion. The great movements of the air, such as 

 the trade winds which blow between the tropics, are not de 

 termined by the attractive forces of the moon and sun. The 

 action of these celestial bodies produces scarcely perceptible 

 oscillations in a fluid so rare and at so great a distance. It 



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