50 THEORY of R A IK 



the globe would be parched with drought, and the winter he- 

 mifphere deluged with rain. 



To fee this, let us confider the fummer hemifphere of the 

 globe, warmed by- the influence of the afcending fun. From 

 the laws of hydroftatics, it will appear, that there fhould 

 be formed, on this occafion, two oppofite currents in the at- 

 mofphere above this half of the globe, the one moving along 

 the furface of the earth, from the polar region towards the equa- 

 tor, the other flowing above, in a contrary direction. This 

 circulation, therefore, being fuppofed, let us fee what follows, 

 according to the ac"lual conftitution of things. On the one 

 hand, the evaporation of the winter's moifture from the furface 

 of the continent, warmed by the fummer fun, mud tend to 

 faturate with humidity the polar atmofphere, as it acquires an 

 evaporating power from- its increaling heat ; on the other hand, 

 the progrefs of the upper current, from the tropic towards the 

 pole, in having its degree of heat diminifhed by the general 

 cooling caufe, will naturally bring the mafs to a point of fatu- 

 ration with the aqueous vapour which it had received. In this 

 ftate of things, the two oppofite currents in the atmofphere, 

 while feparate, might pafs on without condenfing humidity fuf- 

 ficient to produce rain ; but the moment that fufEcient portions 

 of thofe faturated flreams fhall mix, not only cloud, but fhowers 

 will be produced ; becaufe the fudden formation of a mean de- 

 gree of heat, in the mixture of two portions in different tem- 

 peratures, mufl condenfe a quantity of vapour fufEcient to form 

 rain. 



RAIN having fallen in a place, in confequence of the mix- 

 ture in the atmofphere above, this will naturally be followed by 

 clearnefs in the fky and funfhine, which is fo neceffary for 

 warming the furface of the earth, and for giving health and vi- 

 gour to growing plants. 



BUT, without the particular law now invefligated, refpec"ling 

 evaporation and condenfation of vapour, neither rain nor dew 



could 



