fHEORT of RAIN. 77 



motions of the atmofphere, which may naturally occafion the 

 mercury to fall, we fliall, in this manner, have explained the 

 indications of the barometer, fo far as connected with the caufe 

 of rain : At the fame time, it is not pretended to explain, upon 

 this principle, why the falling of the barometer fliould, in ge- 

 neral, be a neceflary indication of rain, any more than the fud- 

 den rifing of the mercury in that inflrument. 



IN the temperate regions of the earth, there are great 

 changes of the barometer, compared with thofe which happen 

 in the torrid region between the tropics, where they are but 

 fmall. This alfo fhould be the cafe, according to the nature 

 of things ; that is to fay, in confidering heat and cold, upon 

 the furface of the earth, as the caufes of motion in the portions 

 of the atmofphere which come more immediately within the 

 compafs of our obfervations. The torrid region, however 

 greatly affected by the diurnal motion of the earth, or influ- 

 ence of the fun, is but little affected by the annual progrefs, 

 which, to all the reft of the earth, from the 4oth degree of la- 

 titude, is fo extremely interefting. This region of perpetual 

 fummer, having, on each fide, a temperate region, can never 

 have the tranquillity of its atmofphere molefted with extreme 

 rarefaction and condemnation, like the temperate zone, bounded, 

 on the one fide, with this torrid region, and, on the other, 

 with a region that experiences fuch an extreme vicimtude in its 

 temperature. The changes from fummer to winter, and from 

 winter to fummer, neceffarily produce great motions in the at- 

 mofphere ; but it is only in the atmofphere of the temperate 

 regions that the effect of thofe motions is fek upon the ba^ 

 rometer. 



THE tranquillity of die barometer, in the equinoctial fitua- 

 tion, does not arife from the want of a moving caufe to actuate 

 the atmofphere in that region ; for there die fun, which may be 

 confidered as the prime mover of the machine, is at all times 

 powerful. But this tranquillity arifes from defect in die joint con- 



fpiring 



