On the Evening and Morning Dew, 1 15 



has explained, in fo ingejiious a manner, the greater part of 

 the phenomena of meteorology ; and 1 foon conceived that 

 they were fiifficient for the objeA in qiieition. 



The principles from which we mull fet out, and which it 

 is here proper to call to mind, are the three following : 



ift. The air, every thing elfc being equal, dilfolves more 

 water in proportion as it is denfer, that is to fay, more com- 

 prefled. 



2d, It diflblves a greater quantity alfo in proportion as its 

 temperature is more elevated. 



3d, The denlity or fpccific gravity of air which holds water 

 in iolution, is lefs than the fpecific gravity of pure air, the 

 prelfure and temperature being equal ; and this fpecific gra- 

 vity is lefs according as the quantity of water diflblved in the 

 air is greater. 



It muft, in the next place, be admitted, that the changes 

 of preflure and weight which may take place in fonie co- 

 Jumns of the atmofphere, muft neceflarily difturb its equili- 

 brium, and produce in it movements or currents. 



Thefe bafes being laid Jown, let us fuppofe for a moment, 

 to render our examination eafier, that the earth, deprived of 

 its rotary motion, is in an immoveable ftate before the fun ; 

 and let us fet afide alfo the confideration of all local influ- 

 ence, or in general all caufes that tend to perturb the regU" 

 larity of eft'e6ls. In this ftate of things what will take place 

 in our atmofphere ? The air cxpofed to the rays of the fun 

 will be heated, and particularly that part of it near the earth, 

 on account of its greater denfity and the reverberation of the 

 planet. The air thus heated will acquire a greater difl^olving 

 power, and will indeed diflblve a great deal of water if there 

 be any in contaA with it. It will take up water therefore 

 from the fea, from lakes, rivers, and ponds, and from re- 

 fervoirs, the furface of which is expofedto its aftion : it will 

 even abforb part of the nioifture with which the earth may 

 be impregnated. 



Let us now confider what will take place in a vertical co- 

 lumn affumod in the atmofphere, and fird in one lying di- 

 jrcctly imder the fun. The air being heated at its lower part, 

 Q 2 and 



