BAR 



BAR 



and its falling, foul weather ; as rain, fnow, higli winds, and winds,tI;ough not accompanied with rain; though, however, it 



^°™^; . , , .„ r L • ,• f^'lsmorefor wind and rain together, than for°ciiher of them 



2. In very hot weather, the Jail or the mercur)' indicates « ,^ ., .. 



thunder. 



3. In winter, the rifing prefages froft ; and in frofty wea- 

 ther, if the merc'jn,' falls three or four divifions, there will 

 certainly follow a tliaw : but in continued froll, if the mer- 

 cury rifes, it will certainly fnow. 



4. \\'hen foul weather happens foon after the falling of 

 the mercury, exptft but little of it : and, on the contrary, 

 cxpeil but little fair weather when it proves fair (lioitly 

 after the r^ercury has riicn. 



alone. Alfo, if, after rain, the wind change into any part of 

 the north, with a clear and dry iky, and the mercury rife, it 

 IS a certain fign of fair weather. 



2. After ver\- great ftorms of wind, when the mercury 

 has been low, it commonly rifes again very fail. In fettled 

 fair and diy weather, except the barometer fink much, ex- 

 pect but httle rain ; for its fir.ali finking then is only for a 

 little wind, or a few drops of rain ; and the mercury foon 

 rifcs again to its former ilation. In a wet feafon, fuppofe in 



hay-time and harveft, the fmalleft finking of the mercury 



5. In loul weather, when the mercun- nfcs much and mull be reg;ided ; for when the conllitution of the air is 

 high, and fo continues for two or three days before the foul much inchned to fiiowcrs, a little finking in the barometer 

 weather is quite over, then expeft a continuance of fair wea- then denotesmore rain, as it never at this time ftandsvery hi^h 

 ther to follow. And if, in fuch a feafon, it rife fuddenly. very faft, and high^ 



6. In lair weath.er, when the mercury- falls much and low, expeft not fair weather more than a day or two, but 

 and thus continues for two or three days before the rain rather that the mercurv will fall again ver)' foon, and raiii 

 tomes ; then cxpeft a great deal of wet, and probably high immediately follow. The ttow gradual rifing, and keep- 

 W'wds. ing on to do fo for two or three days, are moil to be de- 



7. The unfettled motion of the mercury, denotes uncer- pended upon for a week's fair weather; and the unfettled 

 tain and changeable weather. ftatc of the quicklilver always denotes uncertain and clisn ^e- 



8. You are not fo ftrnftly to obferve the words engraved able weather, efpecially when the mercury Hands any where 

 on the plates (though for the moil part it will agree with about the word channealL on the fcale. 



them), r.s the mercury's rifing and falling ; for if itftands at 3. The greateft heights of the mercury, in this country, 

 much rain, and then rifes up to changeable, it prefages fair are found upon eailerly and north-eafterly winds ; and it 

 weather, although not to continue fo long as it would have may often rain or fnow, the wind being in thcfe points, and 

 done, if the mercuiy were higher : and fo, on the contrary, the barometer may fink but little or not at all, or it may'even 

 if the mercury Hood at fiir, and falls to changeable, it pre- be in a rifing ftate, the effed of thofe winds counteracling. 

 fages foul v.-eathcr, though not fo much of it, as if it had But the mercury finks for wind, as well as for rain, in all 

 funk down lower. _ the other points of the compafs ; but rifes as the wind Ihifts 

 From thefe obfervations it appears, fays Mr. Rowning abodt to the north or call, or between thofe points : but if 

 (Nat. Philof. partii. difl". 4.), that it is not fo much the the barometer fiiould fink with the wind in that quarter, ex- 

 height of the mercury in the tube that indicates the wea- pecl it foon to change from thence ; or elfe if the fall of 

 ther, as the motion of it up and down : wherefore, in the mercury (hould be ccnfiderable, a heavy rain is likely to 

 order to pafs a right judgment of what weather is to be enfuc, as it fometimes happen?, 

 expefted, we ought to know whether the mercury is ex- Barometer, CauJ'c of the Phtmmtna of the. Thofe 



attly rifing or falling, to which end the following rules are 

 ofufe: 



1. If the furfaceofthe mercury is convex, Handing higher 

 in the middle of the tube than at the fides, it is generally a 

 fign that the mercury- is then rifing. 



2. If the furface of the mercury is concave, or hollow in 

 the middle, it is finking. And, 



3. If it is plain or level, or rather if it is a little convex, 

 the mercurj' is (lationary ; for mercury being put into 

 a glafs tube, efpecially a fmall one, will naturally have its 

 furface a httle convex ; becaufe the particles of mercury at- 

 traft each other more forcibly than they are attracted by 

 glafs. Farther, 



which have been enumerated, are the chief pli^enomena of the 

 barometer ; to account for which, the hypothefcs that have 

 been framed are almoil innumerable. It would far exceed 

 our limits to detail them all; we mull content ourfelves witt 

 briefly reciting /ome of the principal, and refer the reader 

 who is defirous of farther information to De Luc's " Re- 

 cherches," vol. i. ch. iii. 



Some, as Pafcal, Beal, V'allis, and Garcin, have accounted 

 for the change in the wciglit of the air by the augmentation 

 of the atmofphere in confcquence of the introduction of va- 

 pours, and its diminutioii by their fall ; others, as Perrier, 

 Garden, Le Cat, and De Mairan, have afcribed it to the 



variations of heat ; and others, as Garden, to the alterations 

 4. If the glafs be fmall, fhake the tube; and if the air be of the fpecific gravity of the air ; and Dr. Halley refers it 

 growing heavier, the mercrry will rile about halt the tenth to the accunnulation or difperfion of the air by contrary 

 of an inch higher than it ilond before ; if it is growing winds. Wallis, Halley, and De Mairan have fuppofed that 

 Eghter, it will link fo much. This proceeds from the m.er- there is a difference in the vertical prefTure of the air, when 

 cury {licking to the fides of the tube, which prevents the in motion and at reft. Wallis, and fome other philofophers 

 free motion of it, until it is difengaged by the (hock. There- have conceived that the height of the barometer depends 

 fore, when an obfervation is to be made by fuch a tube, it upon the variations that occur in the elafticitv of the air 

 ought always to be (haken firfl ; for fometimes the mercury and that it is direftly proportional to thefe variations. Some 

 will not vary of its own accord, until the weather it ought have alfo had recourfe to the contrattions and dilatations of 

 to have indicated be prefent. the mercurv' itfclf, as Wallis and Liller ; others, as Gei Hen, 

 To the preceding rules we may fubjoin the following, fuppofe vibrations produced in the particles of air by the 

 deduced from later and more accurate oblervation ot the winds. De la Hire and De Mairan imagine that air is re- 

 motions of the barometer, and the confequent changes in the moved from the fouth to the north, and from the north to 

 air of this country : the fouth : Mariotte fuppofes that the inchnation of the 

 1. In winter, fpring, and autumn, the fudden falling of winds to the furface of the earth is fometimes greater and 

 the mercury, through a large interval, denotes high winds fometimes lefs. Woodward and Hamberger conceive that 

 and lloi-ms ; but in fummer it denotes heavy Ihowers, and there is a Ihock of vapours againll the air, when they rife 

 often thunder; and it always finks loweft of all for great and that this ceafes when, they are at reft. Leibnitz fup- 



pofej 



