THEORY of R4IN. 65 



THE meeting together of different ftreams of air is not fuffi- 

 cient for the production of rain ; it is alfo required, that thofe 

 ftreams of air be in degrees of heat fufiiciently different to pro- 

 duce a proper condenfation ; but as every place, except the 

 poles themfelves, may be confidered as fituated between two 

 different regions, the one hotter and the other colder than it- 

 felf, any place in which circumftances are found favourable for 

 the commixtion of different winds, may be confidered as having 

 alfo this condition favourable for the condenfation of rain ; 

 and thus thefe two conditions, the mixture of winds, and the 

 proper temperatures of thofe winds, may be comprehended 

 under this firft principle. 



THE fecond principle to be now confidered, is the quantity of 

 humidity contained in the ftreams of air which are mixed for 

 the production of rain. Here is a diftinct principle, which is 

 perfectly different from the firft ', and, according to the theory, 

 the quantity of rain in any place, cateris paribus, will neceffa- 

 rily depend upon this principle. There being thus two different 

 principles for determining the quantity of rain, we muft al- 

 ways have an eye to each of thofe principles, in comparing na- 

 tural appearances with the theory. With this view, it will be 

 proper to confider feparately thefe two principles as the caufe of 

 rain ; and it will be eafy to underftand the compounded effect 

 of both, after afcertaining what mare of operation may belong 

 to each. 



WERE the furface of this globe uniform, or a perfect fphe- 

 rical form, whether of fea or land, it does not appear how 

 there fhould be produced any wind, or ftreams of air, but what 

 were regular, in being determined by the influence of the fun 

 and motion of the earth, by which the different parts of that 

 furface would come varioufly, but regularly, to be affected by 

 the influence of this luminary. But the furface of this globe 

 is compofed of fea and land ; and this diverfification is various 

 and irregular. Here is, therefore, a fource of variable winds, 



I or 



