316 OUTLINES OF NATURAL PHILOSOPHY. 



nearly saturated, with humidity, be driven against 

 one another hy contrary winds, the consequence 

 must be a precipitation of humidity, or the forma-, 

 tion of clouds. 



This simple and ingenious theory of the formation 

 of clouds, was a discovery of the late Dr HUT- 

 TON. See Edinburgh Transactions, vol. i. p. 41. 

 It is not necessary to the theory, that the curve 

 which terminates the perpendiculars BC, B'C', 

 &c. should be a logarithmic curve ; it is sufficient 

 that it be a curve convex towards the axis AB. Of 

 this Dr HUTTON had satisfied himself by the obser- 

 vation of natural phenomena. If the line termi- 

 nating the perpendiculars was straight, and much 

 more were it concave, no precipitation could ever 

 take place. 



Professor LESLIE has shewn, that the collision of op- 

 posite currents of air of different temperatures, may 

 furnish a supply sufficient for the production of the 

 heaviest rain. Experiments on Heat and Moisture^ 

 p. 130. 



428. The clouds thus formed, are not disposed 

 equally over the whole atmosphere, but occupy a 

 peculiar region, elevated at an average between two 

 and three miles above the earth. 



The mixture of different portions of air is likely to 



take place most frequently when the two opposite 



3 currents 



