On the Barometer. 143 



The barometer unquestionably bas been observed, inEng- 

 Jand, to rise above 31, and to sink heloir 28; viz. to ex- 

 ceed those limits, each way, by the teritli of an inch. 



The lowest point at which 1 have observed the barome- 

 ter is 28| inches, viz. on November gth, 1810; which 

 was accompanied by a remarkably b.igh wind, and torrent 

 of rain. 



The ordinary range, or variation, in the barometer, differs 

 according to the latitude of the place : — thus, at the equator 

 (except during the hurricane season) it is scarcely any 

 thing; — and at the poles, it is probably not less than four 

 inches ; varying in an increasing ratio, from the equator^ 

 to each pole *. 



The column of mercury in the tube of a barometer, as 

 most persons know, is sustained by the counter-pressure 

 of a coIun)n of air, of the san)e base, extending to the top 

 of the atmosphere; which, when at its mean state of gra- 

 vity, that is, when the barometer is at 29f inches, presses 

 on the base of the mercury, with a weight which is equal 

 to, or after the rate of, fifteen pounds upon each square inch 

 of surface. 



In a comparison of barometers placed in different situa- 

 tions, with respect to elevation ; an allowance must be 

 made, at a mean rate, of lOOdih of an inch on the baro- 

 meter for each complement of nine feet mea.sure in height ; 

 that quantity being added to the higher barometer, or siiO- 

 tructed from the lower one, in order to make the com- 

 parison equal f. (See I'alld I. t^c. page 279, of PhUosO" 

 pliical Magazine, for Odoher 1810.) 



It has been confidently asserted by respectable authors^ 

 that the barometer is affected by the atmospherical tides ; 

 it does not appear, however, that any observations have 

 actually proved the fact. 



With respect to the periodical rise and full of the baro- 

 meter noticed at Calcutta; in the first place, the variation 

 appears to h/Ue l)een very small ; and secondly, it does not 

 correspond in point of lime with the attractive influence 

 either of the sun or moon. 



That the atmosphere is influenced by ihe attractive power 

 «f the sun and moon, there can be no question ; but it seems 



• The barometer at Pr.RW, under the equator, varies ordinarily, only to 

 the citeiit ot two or three-tenihs of an inch i but at PeTEitiBCROH, to the 

 extent of three inches. 



f 'I'he difibrenrc in the heij^'ht of two barometers equally well con- 

 •tructcd, one bcinjj in the frioimd floor, and the other in the attic, will of 

 (ourie amouut to several UunUrtUihs of an inch. 



contrary 



