318 OUTLINES OF NATURAL PHILOSOPHY". 



those inequalities of surface that promote the mix- 

 ture of air. 



5. In the midst of such deserts, where hills oc- 

 cur, moisture is precipitated, sometimes in the form 

 of rain, but most frequently of dew, so that springs 

 of fresh water arise, and great fertility is produ- 

 ced. 



6. There is in our climate hardly any instance of rain 

 without a change of wind, and very rarely a change 

 of wind without rain in a greater or smaller quan- 

 tity. 



7. The lowness of the mercury in the barometer is a 

 sign of rain. It is a certain indication of the sub- 

 version of the equilibrium of the atmosphere, and 

 makes it probable, that before the equilibrium is 

 restored, winds from different quarters, and of dif- 

 ferent temperatures, must come into collision with 

 one another. 



430. The Hygrometer is an instrument intend- 

 ed to measure the quantity of humidity contained 

 in the air at any time. The power of evaporation 

 to produce cold has been very happily applied by 

 Professor LESLIE, to the construction of an instru- 

 ment of this kind. 



431. The vapour raised up into the air, is of a 

 quantity sufficient to afford all the rain that falls, 



or 



