Il6 Obfervations on the 



and becoming fpecifically lighter by diffblving water, will 

 rife and be replaced by the adjacent air. A current, there-p 

 fore, will be eftablifhed in the column from the bottom up- 

 wards. Were this column infulated from the neighbouring 

 columns, as it would be if contained in a vertical tube, in pro- 

 portion as the inferior air charged with water rofe, the upper 

 air would defcend; would become charged with water in its 

 turn ; and would afcend, and be continually replaced without 

 interruption. Befides, the air holding water in folution, when 

 it attained to a certain height, and experiencing there a colder 

 temperature with a lefs degree of preffiire, would become 

 fuper-faturated, and would abandon its excefs of water. There 

 would thus be formed a fog or cloud, which might continue 

 to afcend in confequence of the movement it had acquired, 

 but which, when accumulated to a certain point, would fall 

 down in rain. It may readily be conceived that this tranf- 

 lation of air from a lower to a higher, and from a higher to 

 a lower fituation, would be performed by reciprbcal infiltra- 

 tion, or by a current in both direftions regularly maintained. 

 But things are not altogether fo in fuch a vertical column, 

 becaufe in reality it is not feparated from the reft. The latter 

 J3 fubjeft to the fame effefts, only that they are lefs in pro- 

 portion as thefe columns are removed from the dirc6lion of 

 the fun : the heat, folution of water, and afcenfional force, 

 go on decreafing the further they are diftant from that direc- 

 tion. If the furface of the earth, therefore, be fuppofed a 

 plane, it will be neceflfary to reprefent the air which rifes as 

 a fort of cone, with its fummit direfted towards the heating 

 luminary ; and on the other hand, as the abfolute gravity of 

 each column is increafed by the whole of the water it dif- 

 folves, the equilibrium requires that there fliould be a dif- 

 charge from each, into the lateral ones ; a circumftance 

 which muft evidently be gffefted where there is the lead 

 preflure. "Thus, on the fame fuppofition of the earth being 

 a plane, we fhould fee the fuperior air defcend and force it- 

 iclf along the fides of the cone above mentioned, producing, 

 by this oblique motion at the furface of the earth, a current 

 proceeding in every diredioi^ fron^ the quarter of the fun ; 



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