BAR 



■p'ofes that there is a diminution in the weight of the air when 



rain falls ; and De Mairan apprehends, that an agitation of 

 the air is occafioned by vapours ; and Bcrnouiili is of opi- 

 nion, that an augmentation of th' almofphere is prodnced 

 by a dilatation and difcharge of tht uir inclofed within the 

 bowels of the earth, and that there is a diminution of it when 

 thecontraryhappens. To thefc feveral caiifes aftingfeparately 

 or cor.jointly, and to fvjvcral circumtlanccs attending their 

 different operation, the variations of the barometer have 

 been attributed. But thefe caufes may ail be reduced to 

 three general claiTes : viz. variations of temperature ; the 

 velocity and other qualities of different winds ; and the 

 agency of vapour. 



Dr. Lifter accounts for the chanpfcs in the barometer from 

 the alterations of heat and cold. This, he fays, he has often 

 obferved, th^t in llorms, &c. when the mercurj' is at the 

 loweft, it breaks, and emits fmall particles, which he calls a 

 kind of fretting ; and argues, that in all limes of its defcent, 

 it is more or lefs on the fret. In this diforder, he thinks, 

 its parts are contrafted, and bro\ighl clofer together;- and, 

 for that reafon, deicend : befides, in the fretting they let go 

 little particles of air, before inclofed in them, and ihele riling 

 into the top of the tube, the mercury muft. fink, both from 

 the column's being fiiortened by their efcape, and by their 

 lying upon it. Mercury therefore, he adds, rifes either in 

 very hot or very cold weather, between the tropics, ice. as 

 being then in its natural ftate ; and again, in the iv.tennediate 

 degrees of heat and cold it falls, as being contracted, and 

 as it were convulfed, and drawn together. Phil. Tranf. 

 N^ 165. But his account, however ingenious, yet comes 

 far Ihort of accounting for the phenomena ; nay, in fome 

 refpeels it contradifts them. 



The changes in the weight of the atmofphere, therefore, 

 mull be laid down as the caufe of thofe in the barometer ; 

 tut then, the caufe of that caufe, or whence thofe altera- 

 tions arife in the atmofphere, it will be no eafy matter to 

 determine ; there being, perhaps, no one principle in nature 

 that will account for fuch a variety of appearances, and thofe; 

 too fo irregular. It is probable the winds, as driven this or 

 that way, have a great (hare in them ; fome (hare too, va- 

 pours and exhalations, rifing from the earth, may have ; 

 fome, the changes in the air of the neighbouring regions ; 

 and fome, the flux and reflux occafioned in the air by the 

 moon ; and alfo fome chemical caufes operating between 

 the different particles of matter. 



Dr. Halley thinks the winds and exhalations fufficient ; 

 and, on this ground, gives us a rationale of the baro- 

 meter. The fubftance of what may be faid on that head, is 

 as follows : 



I ft, then. The winds muft neceflarily alter the weight of 

 the air in any particular country ; and that, either by 

 bringing together and accumulating a greater quantity of 

 air, and fo loading the atmofphere of any place ; which 

 will be the cafe, as often as two winds blow at the fame 

 time, from oppofite points towards the fame point : or 

 by fweeping away a part of the air, and removing fome 

 of the load, and thus giving room for the atmofphere to 

 expand itfelf; which will be the cafe when two winds 

 blow at the fame time, and from the fame point, oppofite 

 ways : or laftly, by cutting off the perpendicular prefTure 

 of the atmofphere ; which happens as often as any fingle 

 wind blows briflily any way ; it being found, by experi- 

 ment, that a ftrong blaft of wind, even made by art, will 

 render the atmofphere hghter ; and accordingly, the mer- 

 cury, in a tube under which it paffes, as well as in another 

 at a diftancc from it, will fubfide confideiably. See Phil. 

 TranL N^ 292. 



BAR 



2dly, The cold nitrous particles, and even air itfelf co» 

 dcnfed in the northern parts, and driven elfewhere, mufl 

 load the atmofphere, and increafe its preffure. 



3dly, Heavy dry exhalations from the earth muft increafe 

 the weight of the atmofphere, and heighten its elaftic force, 

 as we find the fpecilic gravity of mcnllruums increafed by 

 diffolved falts and metals. 



4thly, The air being rendered heavier from thtfe and the 

 like caufes, is thereby the more able to fupport the vapours ; 

 which being likewife intimately mixed with it, and fvvim- 

 rr:ing every where equably through it, make the weatlier 

 ferene and fair ; again, the air being made lighter, from the 

 contrary caufes, it becomes unable to fuppoit the vapours 

 wherewith it is replete ; thefe, therefore, precipitating, are 

 gathered into clouds ; and thofe, in their progrefs, coalefce 

 into drops of rain. 



Thefe things ohferved, it appears pretty evident, that the 

 fame caufes wiiich increafe the weight of the air, and make 

 it more able to fupport the mercury in the barometer, do 

 likewife occafion a ferene Hey, and a dry feafon ; and the 

 fame caufes which i-ender the air lighter, and lefs able to 

 fupport the mercurj', do likewife generate clouds and rain. 

 Hence, ift. When the air is lighteft, and the merciry in 

 the barometer is loweft, the clouds are ver)' low, and iviovo 

 fu'iftly ; and when, after rain, the clouds break, and a calm 

 fey again fiiines forth, being purged of the vapours, it ap- 

 pears exceedingly bright and ti'anfpartnt, and affords an ealy 

 profpect of remote objccls. 



2dly, When the air is heavier, and the mercury ftands 

 higher in tlie tube, the weather is calm, though fomewhat 

 lefs clear, becaufe the vapours are difperfed every where 

 equally ; if any clouds now appear, they are very high, and 

 move flowly ; and when the air is heavieft of all, the earth 

 is frequently found enveloped in pretty thick clouds, which 

 appear to be formed out of the grofler exhalations, and 

 which the air is then able to fuftain, though a lighter atmo- 

 fphere could not. 



3dly, Hence it is, that with us the mercury ftands higheft: 

 in the coldeft feafons, and when the wind blows from the 

 north or north-eaft corner : for, in that cafe, there are two 

 winds blowing towards us at the fame time, and from op- 

 polite corners ; there being a conftant weft wind found in 

 the Atlantic ocean, at the latitude correfpondiug to our's. 

 To which we may add, that in a north wind, the cold con- 

 denfed air of the northem parts is brought hither. 



4thly, Hence, in the northern regions, the variation of the 

 mercuiy is more fenfible than in the fouthern ones ; the 

 winds being found more ftrong, more frequent, more 

 various, and more oppofite to each other in the former, than 

 in the latter. 



Laftly, Hence it is, that between the tropics, the variation 

 of the mercury is fcarcely fenfible ; the winds there being 

 extremely gentle, and ufually blowing the fame way. 



But this account, however well adapted to many of the 

 particular cafes of the barometer, feems to come fhort of 

 lome of the principal and moft obvious ones : and is, befides, 

 liable to feveral objections. 



For, I ft. If the wind were the fole agent in effeiSing thefc 

 alteratiouo, we fhould have no alterations without a fenfible 

 wind, nor any wind without fome alteration of the mercury ; 

 both which are contrary to experience. 



2dly, If two winds be fuppofed blowing from the fame 

 place, viz. London, oppofite v.ays, viz. N.E. and S.W. there 

 will be two others, blowing from oppofite points, viz. N.W. 

 and S.E. to the fame place ; which t«o laft will balance 

 the firft, and bring as much air towards the point, as the 

 others fwept from it. Or thus, in proportion as the air 



h 



