58 THEORY of RAIN. 



it can be explained by no other theory but this, in which 

 we find a diurnal caufe of mixing together the different parts 

 of the atmofphere, by means of heating the mountains, and 

 furface of the earth, by the great illumination of the fun, thus 

 rarefying fome parts of the atmofphere, and producing com- 

 motions in the fluid mafs of air which furrounds the ifland. 



THESE periodical commotions in the atmofphere of iilands in 

 tropical fituations muft not be confidered as a thing doubtful in 

 its nature, or a matter merely conjectural. The fac"l is well af- 

 certained in the fea and land breezes that are felt regularly 

 blowing every day in thofe oppofite directions. This is the 

 fact ; and that rain mould accompany thefe commotions follows 

 from the theory provifionally ; that is, in cafe of the proper 

 conditions for the condenfation of vapour being found in the 

 atmofphere thus mixed. Now, thefe conditions are not always 

 found where we find the breezes. But, in the iflands now un- 

 der confideration, fituated under the line, and in a fea that muft 

 be hotter than any other upon the globe ; a fea, either confined 

 conftantly between the tropics, or continually fupplied from the 

 tropical region of the Pacific ocean, it is not unreafonable to 

 fuppofe that fufEcient faturation with aqueous vapour may be 

 found in the atmofphere ; nor that, in the diurnal commotions 

 of this fluid, there may be portions of it mixed in different de- 

 grees of heat. 



IT is, in like manner, that the periodical rains which happen 

 regularly upon the different coafts of the peninfula of India 

 muft be explained. The regular monfoons in thofe feas occafion 

 the tranfportation of air, faturated with aqueous vapour, which 

 has flowed along the furface of the fea, here to be elevated and 

 mixed with thofe portions which, having loft their heat to a 

 fufEcient degree, are in a condition to produce, by their com- 

 mixture, a condenfation of water upon the land. Nothing can 

 better illuftrate this explanation than the great annual and periodi- 

 cal rains which happen upon this continent, and which are ex- 

 hibited 



