THE NEW SCIENCE OF METEOROLOGY 



oration and condensation of humidity ; and, in this con- 

 trivance of the seasons, there must appear an ample 

 provision for those alternate operations in every part; 

 for as the place of the vertical sun is moved alternately 

 from one tropic to the other, heat and cold, the original 

 causes of evaporation and condensation, must be car- 

 ried over all the globe, producing either annual sea- 

 sons of rain or diurnal seasons of condensation and 

 evaporation, or both these seasons, more or less that 

 is, in some degree. 



" The original cause of motion in the atmosphere is 

 the influence of the sun heating the surface of the earth 

 exposed to that luminary. We have not supposed 

 that surface to have been of one uniform shape and 

 similar substance; from whence it has followed that 

 the annual propers of the sun, perhaps also the diurnal 

 propers, would produce a regular condensation of rain 

 in certain regions, and the evaporation of humidity in 

 others ; and this would have a regular progress in cer- 

 tain determined seasons, and would not vary. But 

 nothing can be more distant from this supposition, that 

 is the natural constitution of the earth; for the globe 

 is composed of sea and land, in no regular shape or 

 mixture, while the surface of the land is also irregular 

 with respect to its elevations and depressions, and 

 various with regard to the humidity and dryness of 

 that part which is exposed to heat as the cause of 

 evaporation. Hence a source of the most valuable 

 motions in the fluid atmosphere with aqueous vapor, 

 more or less, so far as other natural operations 

 will admit; and hence a source of the most irreg- 

 ular commixture of the several parts of this elas- 



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