64 HISTORY of the SOCIETY. 



or winds, of different temperatures, when they meet, muft produce 

 fuch mixtures as have been defcribed, and give rife confequently 

 to the condenfation of aqueous vapour. When the fupply of the 

 humid air, entering into the mixture, is continued, the quanti- 

 ty of cloud formed will continually increafe, and the fmall glo- 

 bules of condenfed moifture, uniting into drops, mufl defcend 

 in rain. 



But though we are thus in poffeflion of a principle by which 

 rain may be certainly produced, yet whether it be the only one 

 by which rain is produced may require fome farther investigation. 

 Dr Hutton accordingly, in order to determine this point, has 

 entered into a very ample detail concerning the rain under dif- 

 ferent climates, and in different regions of the earth. The re- 

 fult is, that the quantity of rain is, as nearly as can be eftima- 

 ted, every where proportional to the humidity contained in the 

 air, and the caufes which promote the mixture of different por- 

 tions of air, in the upper regions of the atmofphere. Between 

 the tropics, for inftance, the dry feafon is that in which the 

 uniform current of the trade-wind meets with no obftruction 

 in its circuit round the globe ; and the rainy feafon happens 

 when the fun approaches to the zenith, and when the fteadinefs 

 of the trade-wind either yields to irregular variations, or to the 

 ftated changes of the monfoons. 



Thus too, (to mention another extreme cafe), in certain coun- 

 tries diflant from the fea, having little inequality of furface, and 

 expofed to great heat, no rain whatever falls, and the fands of the 

 defert are condemned to perpetual fterility. Even there, however, 

 where a mountainous trad occurs, the mixture of different por- 

 tions of air produces a depofition of humidity; perennial fprings 

 are found ; and the fertile vales of Fezzan or Palmyra are ex- 

 empted from the defolation of the furrounding wildernefs. 



This ingenious theory attracted immediate attention, and 

 was valued for affording a diftinct notion of the manner in 



which 



