THEORY of R A IN. 47 



THIS laft cafe properly applies to the phenomena of breath, 

 and fleam, which have been rendered vifible, in mixing with 

 air that is colder than themfelves ; and it explains the various 

 appearances that may occur, in mixing together feveral portions 

 of air, more or lefs faturated with humidity, and in different 

 temperatures of heat and cold : For 



IT is not every mixture of the atmofpheric fluid, in different 

 temperatures, that mould, according to the theory, form a vifi- 

 ble condenfation ; this effect requiring, in that atmofphere, a 

 fufficient degree of faturation with humidity. Neither is it ne- 

 cefTary, for this effect, that the two portions to be mixed fhould 

 each be faturated with humidity up to the temperature in which 

 it then is found ; it is fufficient, that the difference in the tem- 

 peratures of thofe portions to be mixed fhould more than com- 

 penfate the defect in point of faturation. But, if a mixture 

 fhall be made of two portions of the atmofphere, both fully fa- 

 turated with humidity, then, however fmall may be the differ- 

 ence of their temperatures, there is reafon to believe, that a con- 

 denfation, proportionate to this difference, will take place. 



HAVING thus explained the atmofpherical appearance of vifi- 

 ble mift, produced in the mixture of invifible fluids, we may 

 now apply this rule of condenfation as a principle for the theory 

 of rain. 



RAIN is the diflillation of water, which had been firfl dif- 

 folved in the atmofphere, and then condenfed from that flate of 

 vapour or folution. It is the explanation of this condenfation 

 that muft form the theory of rain. So far, therefore, as the 

 condenfation of aqueous vapour has been explained, and fo far 

 as the evaporation, of water from the furface of the globe is un- 

 derftood, we have a theory for the general appearance of rain. 



WATER, indeed, is condenfed in a cloud equally as in rain, 

 and yet clouds may fubfift without rain. But, it is evident, 

 that, without condenfation of aqueous vapour in the atmofpherej 

 no rain could be produced j and that, however different caufes 



may 



