On ike Phenomena of the Morning and Evening Dew. 87 



3. Under equal preflure, and at like temperatures, air holding water in folution has a 

 lefs fpecific gravity than air alone ; and this fpecific gravity is lefs the greater the propor- 

 tion of water fo diflblved. 



It muft alfo be recolledled, that the changes of preflure and weight which may take 

 place in certain columns of the atmofphere, niufb neceflarily difturb the equilibrium, and 

 produce motion or currents in the fame. 



Thefe principles being eftabliftied, let us fuppofe for a moment, to render our inquiry 



more fimple and eafy, that the earth deprived of its rotatory motion, Ihall remain motion- 



lefs in the prefence of the fun, and let us befides difmifs from our confideration all local 



influence, and in general all thofe caufes which might difturb the regularity of the 



• effe£i:s. ' ' 



In this (late of things, what will be the confequence with regard to our atmofphere ? 



The air expofed to the rays of the fun will be heated, and principally in the part conti- 

 guous to the earth, on account of its greater denfity, and the reverberation of that planet. 

 This heated air will acquire a greater diflblving power, and will, in fa£l, dllTolve much 

 water if that fluid be prefent. It will take it from the feas, lakes, rivers, ponds, and other 

 refervoirs, whofe furface is expofed to its adlion, and will even abforb a portion of the 

 humidity of the ground. 



Let us in the next place confider, what happens in any aflumed vertical column in the 

 atmofphere, and firft in that placed immediately beneath the fun. The air heated at the 

 lower part diflblving water, and thus becoming fpecifically- lighter, will rife, and be re- 

 placed by other contiguous air. An afcending current will therefore be eftabliftied in the 

 columm If this column were feparated from the others, as if it were contained in a ver- 

 tical tube, in proportion as the lower air rofe loaded with water the upper air would de- 

 fcend, become charged in its turn, rife and be replaced ; and that perpetually. And the 

 air holding water in folution, would in confequence of under faturation from cold and 

 diminilhed preflure, when it had arrived at a certain height, let fall the excefs of water, 

 and form a mift or cloud, which might continue to rife by virtue of its acquired motion, 

 but which, after a certain accumulation, would fall again in rain. It may alfo be con- 

 ceived, that this tranflation of the air upwards and downwards, would be made either by a 

 mutual infiltration, or currents in both dire<Slions, which would be conftantly and regu- 

 larly kept up. 



But this is not entirely the cafe with our vertical column, becaufe it is not in facl fepa- 

 rated from the others in Its vicinity. Thefe alfo are fubje£ted to the fime operations, with 

 the exception only, that the effcfts are lefs the more remote they are from the column 

 immediately beneath the fun. The heat, the folution of water, and the force of afcenfion, 

 gradually diminifti as the diftance from this central column increafes. If therefore the 

 furface of the earth were a plane, the proper reprefentation'of the rifing air would be a cone 

 with its fummit directed to the fun 5 and on the other hand, as the abfolute weight of each 

 column is incrcafcd by the whole of the water it has diflblved, the equilibrium requires 



that 



