OUTLINES OF NATURAL PHILOSOPHY. 



The distribution of heat, therefore, if only the direct 

 influence of the sun were to act, would be very diffe- 

 rent from that which takes place in nature. 



The intensity of the solar heat is less in low eleva- 

 tions than is supposed in these calculations, on ac- 

 count of the rays coming through a large mass of 

 air, and of air more loaded with vapour, so that a 

 great quantity of them is intercepted. BOUGUER, 

 Traitc cTOptique^ sur la Gradation de la Lumiere, 

 Liv. in. Sect. iv. 



398. The effects of the direct influence of the 

 sun, are greatly modified by the transportation of 

 the temperature of one region into another, in con- 

 sequence of that disturbance in the equilibrium of 

 the atmosphere which the action of those rays ne- 

 cessarily produces. 



In order that there may be an equilibrium in a fluid 

 like the air, every stratum of air that is level or ho- 

 rizontal all round, ought to be of the same density. 

 It ought, therefore, also to be everywhere of the 

 same temperature, which, not being the case, the 

 constant motion of the air is the necessary conse- 

 quence of heat being unequally distributed. The 

 columns of air that are lighter, are displaced hy 

 those that are heavier, and hence a general tenden- 

 cy in the air to move from the poles toward the 

 equator. This general tendency, which is calcula- 

 ted to moderate the extremes of temperature, is al- 

 so greatly modified by local circumstances. 



399. The 



