on the State of the Barometer, J57 



therefore, to fall towards noon, why does the latter fo oftcu 

 rife afterwards ? What then lellcns the quantity of thcftt 

 vapours when tlie heat, which by its nature promotes their 

 folution, Hill increafes, or at any rate does not become lefs ? 

 How docs it happen that the barometer falls in the night ? a* 

 about that time the vapours are, for the mofi; part, precipi- 

 tated from the atmofphore. Why do they exercife the 

 ffreateft force exaftlv at midnight? and, Bv vyhat law oi 

 nature are they more conneAed with that thaa v.ith any 

 other period ? 



It may here evidently be feen, that the coriftant falling of 

 the barometer about noon and midnioht can be occaiiotiecl 

 neither by heat nor bv the etfetl of vapours. 



Befides thefe two cauli.'s, and the winds, we are acquainted 

 with no others in our atmofphere which can make the baro- 

 meter to fall. But as the wind often becomes calm about 

 noon and midniehl, or, when it blows, is of fuch a nature 

 that it does not make the column of air which preffes on the 

 baromtter lighter, it is evident that the falling of the mer- 

 cury about thefe times cannot arife from the wind. 



Wliat then remains Lat to feck \^ ithout our atmofphert 

 for the caufe of this phaMiomcnon ? !t is evident that it is to 

 be found in the fun ; for, as by its attraftion it mo\ es the 

 water of the ocean, it miifl excrcife a liill fhongcr action on 

 the column of air ; and from what is knovvU refpecting the 

 flux and reflux of the fca, the whole phasnomenon may he 

 cafily explained. In the tJrft plate, as the atmofphtric flux 

 and relliix thcmfelvcs muR depend on the fan's paifage over 

 llie meridian, it mult alfo take place both at the diurnal and 

 the nocturnal paiiage of the hui : becaufe the flux and rcfluji 

 f)f ilic atmofphcre, hke thofe of the fea, muft happen at the 

 fume time on oi>polite lides of the tcrrcf^rlal hemilphere. In 

 the lait place, the atmoipheric flux which produces the falling 

 of our barometer is only an effcirl of the flux which takes 

 place between the tropic*, where the air during the pafl'age 

 of the fun afcends, and therefore proceeds thence from us, 

 as happen-! in all the tides of the lea which lie beyond thtfe 

 circljs. 7'hat thcl'c tides, which arife by communication, are 

 tranfmitted more fpeedily and by a longer duration in the 

 atmofphcre than in the ocean, may be concluded from this 

 circumltance, that tlie particies of the air have Icfi gravity 

 and Icfi mutual adhcI;o:>. 



XXVI. Letter 



