78 rHEO RY of RAIN. 



fpiring together of all thofe caufes, which, in the temperate zone, 

 excite to fo great changes in the preffure of the impending at- 

 mofphere. This moving caufe in the equinoctial region is more 

 equable than it is any where elfe, except perhaps at the poles ; 

 and it is always exerted nearly in the fame direction, in forming 

 a motion from eaft to weft. When it is otherwife, that is, 

 when fome cafual change interrupts the equable progreflion of 

 the atmofphere, the moil violent effects may appear, in particu- 

 lar places, for a fhort fpace of time, without much change in 

 the general quantity of the atmofphere in this region, by which 

 the ftation of the mercury in the barometer is determined ; 

 confequently, the barometer, which is much . affected by the 

 motions of the atmofphere in the temperate regions, mould 

 be but little affected with thofe motions in the middle region. 



To conclude, the barometer is an inftrument neceffarily con- 

 nected with motions in the atmofphere ; but it is not equally 

 affected with every motion in that fluid body. The barometer 

 is chiefly affected by thofe motions by which there are produced 

 accumulations and abftractions of this fluid, in places or regions 

 of fufEcient extent to affect the preffure of the atmofphere upon 

 the furface of the earth. But as every commotion in the atmo- 

 fphere may, tinder proper conditions, be a caufe for rain, and 

 as the want of commotion in the atmofphere is naturally a 

 caufe of fair weather, this inftrument may be made of great 

 importance for the purpofe of meteorological obfervations, al- 

 though not in the certain and more fimple manner in which it 

 has been, with the increafe of fcience, fo fuccefsfully applied 

 to the meafuring of heights. 



HAVING thus eftabliflied this principle, that the barometer 

 is but little affected with the fall of rain, we may now employ 

 fome general obfervations of this inftrument, to difcover or illu- 

 ftrate certain operations in the atmofphere, which are more im- 

 mediately connected with the caufe of rain in the region now 



considered. 



IT 



