\ 



.' -*^ 



-J 



<* 



in tjiat wkicii feems to hold mofl univerfally, viz. 

 that when the 'high winds blow, the mercury is the 

 'lowei-, they fomctimes fail, yet the following obfer- 

 vations have been made by feveral authors. 

 Dr. Halley obferves, that in calm weatHer,^ when 

 the air is inclined to rain, the mercury is continually 

 lovV-j in ferene good fettled weather, high. ^ . 

 That on great winds, though unaccompanied with 

 rain, the merCury is loweft of. all, with regard to the 

 point of the compafs the wind blows on ; that, ce- 

 teris parihus, the greatelV heights of the mercury are 

 on eafterly and north-cafterly winds i that after great 

 ftorms of wind, v/hefi the mercury has been low, it 



■ 



rifes again very faft. 



That in calm frofty \Veather itftands high, : - 

 That the more northerly places find greater altera- 

 tions than the more fouthera-, and that within the 



. tropics, arid near them, there is little or no variation 

 of the mercury at all. - /'*'■ 



Dr. Beal obferves, that, ceteris paribus^' i\\Q mftroirf 

 is higher in cold weather than in warm, and ufually 

 higher in morning and evening than at rilid-day, 

 Tliat the mercury is higher in fettled and fair weather, 

 than either a litde before, or after, or in the rain ; 

 and that it generally delcends lower after rain, than 

 it was before it ; if it cliance to rife higher after rain, 



■ It is generally foUo^^-ed by a fettled ferenity. 

 That there arc frequendy great changes in the air, 

 without any perceptible alterations in the barometer. 

 As to the predidiohs from the barometer. Dr. Hal- 

 ley has. fpund,, '^ - " '■;,;, .' •/ '' . ' ." , ' ;■ 

 That the rifing of the mercury forebodes fair weather 



after fouT^ and an eafterly or nprth-eafterly wind. 

 That the falling of the mercury portends foutherly 

 or wefterly winds, with rains, or ftormy winds, or 



Both:- :f; - ■ . . : .■.„ . ; 



That in a ftorm the mercury beginning to rife, is a 

 pretty fure fign that it begins to abate. . 

 Mr. Patrick obferves, that the falling of the merciiiy 

 in hot weather prefages thunder \ that when foul wea- 

 ther happens after the fall of the mercury, it feldom 

 holds long-, aiid the fame is obferved, if fair weather 

 fucceeds pitfcntly after its rife., '^ , ^ 

 Hence Mr. Pointer conceives, that the principal 

 caufe of the rife and fall of the mercury, is from the 

 variable winds which are found in the temperate 

 zones, and whofe great inconftancy here in England 

 is moft notorious. • ' 



A fccond caufe he takes to be, the uncertain exha- 

 lation and perlpiration of the vapours lodging in the 

 air, whereby it comes to be at one time much more 

 <;rdwded than at another, and confequently heavier ; 

 but this latter, in a great meafure, depends upon the 

 ormer. ; .. . - . 



And frorri' thefe principles, he endeavours to explain 

 the feveral pjio^nomena of the barometer. 





i. The mercury's being low, inclines it to rain-, be- 



caufe the air being light, the vapours are no longer 

 Supported thereby, being become fpecijically licavier 

 than the medium wherein they are floated ; fo that 

 they defcend towards the earth, and in their fall, meet- 

 ing with^ other aqueous particles, they incorporate 

 together, and form little drops of 'rain ; but the mer- 

 cury's being at one time lower than another, is the 

 effe£l of two contrary winds blowing from the place 

 . ^ where the barometer ftands, whereby the air of that 

 ■place is caj-ricd both ways" from it, and confequently 

 ' the incumbent cylinder of air is diminilhed, and ac- 

 " ' cordingly the mercury finks. As for inftance, if in 

 the German ocean it fhould blow a gale of wefterly 

 '/ *wind, and at tlie fame time an eafterly wind in the 

 * ' Irifli fea •, or if in France it fhould blow a northerly 

 wind, swld in Scotland a foutherly, it muft be grant- 

 ed, that that part of the atmofphere impendent over 

 England, Would thereby be exhaufted and atte- 

 jiuated, "aiiid the mercury would fubfide-, and the 

 vapours whicjh before floated in thofe parts of the air, 

 jof equal gravity with themfelves, would fink to the 

 . earth.' ■ ' ■ 



t,k The greater height of the barometer is ocpafioned 



B: A R 



\yf two contrary winds blowing towards the place of 

 obfervatipn, whereby the air of other places is brought 

 thither and accumulated ; fo that the incumbent cy- 



linder of air 



fed 



weight, the mercury preffed thereby muft needs rilb 

 and ftand high, as long as the winds continue fo to 

 blow i and then the air being fpeciflcally heavier, the 

 vapours are better kept fufpended, fo that they have 

 no inchnation to precipitate and fall down in drops, 

 which is the reafon of the ferene good weather, which 

 attends the greater heights of the mercur}^ 



3. The mercury finks the loweft of all by the very 

 rapid motion of the air in ftorms of winds. 



For the traft of the region of the earth's' furface, 

 wherein thefe winds rage, not extending all round 

 the globe, that ftagriant air which is left behind, as 

 likewife that on the fides, cannot come in fo faft as to 

 fupply the evacuation made by fo fwift a current > fo 

 that the air muft neceflarily be attenuated when and 

 where the faid winds continue to blow, and that moi-e 

 or lefs, according to their violence : add to which, 

 that the horizontal motion of the air being fo quick as 

 it is, may, in all probability, take off fome part of 

 the perpendicular preffure thereof j and the great agi- 

 tation of its particles is the reafon why the vapours are 

 diffipated, and do not cbndenfe into drops, fo as to 

 form rain, otherwife the natural confequence of the 

 air's rarefaftion. . . " ' 



4. The mercury ftands the higheft upon an eafterly 

 or north-eafterly wind ; becaule, in the great Atlan- 

 tic ocean, on this fide the thirty-fifth degree of north 

 latitude, the wefterly and fouth-wefterly winds blow 

 almoft always trade : fo that whenever h^re the winds 

 come up at eaft and north-eaft, it is fure to be checked 

 by a contrary gale as foon as it reaches the ocean ; 

 wherefore, according to what is made out in the fe- 

 cond remark, the air muft. needs be heaped over this 

 ifland, and confequently, the mercury muft fta^d 

 high, as often as thefe winds blow, ^ 



5. In calm frofty weather, the mercury generally 

 ftands high, becaufe, as he conceives, it feldom freezes 

 but when the winds come out of the northern or 

 north-eaftern quarters, or at leaft, unlefs tliofe winds 

 blow at no great diftance off, . - • 



For the northern parts of Germany, Denmark, Swe- 

 den, Nonvay, and all that trad, from whence north- 

 eaftern winds come, are fubjefl; to almoft continual 

 froft all the winter, and thereby the lower air is very 

 much condenfed, and in that ftate is brought hither- 

 ward by thofe winds ; and, being accumulated by 

 the oppofition of the wefterly wind blowing in the 

 ocean, the mercury muft needs be prefled to a more 

 ordinary height; and, as a concurring caufe, the 

 Ihrinking of the lower parts of the air into leficr room 

 by cold, muft needs caufe a defccnt of the upper parts 

 of the atmofphere, to reduce the cavy:y made by this 

 contraftion to an equihbrium. 



6. After great ftorms of winds, when the mercury 

 has been very low, it generally rifes again very fiift : 

 he fays, he once obferved it to rife an inch and a half 

 in lefs than fix hours, .after a long condnued ftorm of 

 fouth-weft wind. 



■ The reafon is, becaufe the air being very much rare- 

 fied by the great evacuations that fiich continued 

 ftorms make thereof, the neighbouring air runs irj 

 more fwiftly, to bring it to an equilibrium, as wc fee 

 water runs the fafter for having a greater declivity. 



7. The variations are greater in the more northerly 

 places, as at Stockholm greater than at Paris [com- 

 pared by Mr. Pafchal •,] becaufe the more northerly 

 parts have ufually greater ftorms of wind than the more 

 foutherly, whereby the metxury fliould fink lower in 

 that <:xtreme ; and then the northerly v/inds bringing 

 die condenfed and ponderous air from the neighbour- 

 hood of the pole, and that again being checked by a 

 foutherly wind, at no great. diftance, and fo heaped 

 up, m.yft of necefllty, make the mercury in fuch caf^ 

 ftand higher in the .other extreme. 



8. This remark, that there is little or no variation 

 near the equinoftial, does^ above all others, confirm 



tlis 



