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in that which feems to hold moft univerfally, viz. 
that when the high winds blow, the mercury is the 
lower, they iometimes 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 
love •, in ferene good fettled weather, high. 
That on great winds, though unaccompanied with 
rain, the mercury is lowed of all, with regard to the 
point of the compafs the wind blows on •, that, ce- 
teris paribus , the greateft heights of the mercury are 
on eafterly and north-eafterly winds •, that after great 
ftorms of wind, when the mercury has been low, it 
rifes again very faft. 
That in calm frofty weather it hands high. 
That the more northerly places find greater altera- 
tions than the more fouthern ; and that within the 
tropics, and near them, there is little or no variation 
of the mercury at all. 
Dr. B^al obferves, that, cateris paribus , the mercury 
is higher in cold weather than in warm, and ufually 
higher in morning and evening than at mid-day. 
That the mercury is higher in fettled and fair weather, 
than either a little before, or after, or in the rain ; 
and that it generally defeends lower after rain, than 
it was before it-, if it chance to rife higher after rain, 
it is generally followed by a fettled ferenity. 
That there are frequently great changes in the air, 
without any perceptible alterations in the barometer. 
As to the predidtions from the barometer. Dr. Hal- 
ley lias found, 
That the riling of the mercury forebodes fair weather 
after foul, and an eafterly or north-eafterly wind. 
That the falling of the mercury portends foutherly 
or wefterly winds, with rains, or ftormy winds, or 
both. 
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 mercury 
in hot weather prefages thunder ; that when foul wea- 
ther happens after the fall of the mercury, it leldom 
holds long ; and the fame is obferved, if .fair weather 
fucceeds prefently 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 fecond caufe he takes to be, the uncertain exha- 
lation and perfpiration of the vapours lodging in the 
air, whereby it comes to be at one time much more 
crowded than at another, and confequently heavier ; 
but this latter, in a great meafure, depends upon the 
former. 
And from thefe principles, he endeavours to explain 
the feveral phrenomena of the barometer, 
i. The mercury’s being low, inclines it to rain; be- 
caufe the dir being light, the vapours are no longer 
fupported thereby, being become fpecifically heavier 
than the medium wherein they are floated; lo that 
they defeend towards the earth, and in their fall, meet- 
ing with other aqueous particles, they incorporate 
together, and form little drops or rain -, but the mer- 
cury’s being at one time lower than another, is the 
effedt of two contrary winds blowing from the place 
where the barometer (lands, whereby the air of that 
place is carried 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 ftiould blow a gale of wefterly 
wind, and at the fame time an eafterly wind in the 
Irifti fea ; or if in France it ftiould blow a northerly 
wind, and in Scotland a foutherly, it muft be grant- 
ed, that that part of the atmofphere impendent over 
England, would thereby be exhaufted and atte- 
nuated, and the mercury would fubfide and the 
vapours which before floated in thofe parts of the air, ‘ 
-of equal gravity with tnemfelves, would fink to the 
earth. 
,a. The greater height of the barometer is occafioned 
BAR 
by two contrary winds blowing towards the place of 
obfervation, whereby the air of other places is brought 
thither and accumulated ; fo that the incumbent cy- 
linder of air being increafed both in height and 
weight, the mercury preffed thereby muft needs rife 
and (land high, as long as the winds continue fo to 
blow ; and then the , air being fpecifically heavier, the 
vapours are better kept fufpended, fo that they hav® 
no inclination to precipitate and fall clown in drops, 
which is the reafon of the ferene good weather, which 
attends the greater heights of the mercury. 
3. ' The mercury finks the lowed of all by the very 
rapid motion of the air in ftorms of winds. 
For the trad of the region of the earth’s furface, 
wherein thefe winds rage, not extending all round 
the globe, that ftagnant air which is left behind, as 
likewife that on the hides, cannot come in fo faft as to 
fupply the evacuation made by fo fwift a current ; fo 
that the air muft neceffarily be attenuated when and 
where the faid winds continue to blow, and that more 
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 ; and the great agi- 
tation of its particles is the reafon why the vapours are 
diffipated, and do not condenfe into drops, fo as to 
form rain, otherwife the natural confequence of the 
air’s rarefadtion. 
4. The mercury Hands the higheft upon an eafterly 
or north-eafterly wind ; becaufe ? in the great Atlan- 
tic ocean, oh this fide the thirty-fifth degree of north 
latitude, the wefterly and fouth-weflerly winds blow 
almoft always trade : fo that whenever here the winds 
come up at eaft and north-eaft, it is Eire 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 (land 
high, as often as thefe winds blow. 
5. In calm frofty weather, the mercury generally 
(lands high, becaufe, as he conceives, it feldom freezes 
but when the winds come out of the northern or 
north-eaftern quarters, or at lead, unlefs thofe winds 
blow at no great difcance off. 
For the northern parts of Germany, Denmark, Swe- 
den, Norway, and all that trad, from whence north- 
eaftern winds come, are fubjedt to almoft continual 
froft all the winter, and thereby the lower air is very 
much condenfed, and in that (late 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 preffed to a more 
ordinary height; and, as a concurring caufe, the 
(hrinking of the lower parts of the air into leffer room 
by cold, muft needs caufe a defeent of the upper parts 
of the atmofphere, to reduce the cavity made by this 
contradlion to an equilibrium. 
6. After great ftorms of winds, when the mercury 
has been very low, it generally rifes again very faft : 
he fays, he once obferved it to rife an inch and a half 
in lefs than fix hours, after a long continued dorm of 
fouth-weft wind. 
The reafon is, becaufe the air being very much rare- 
fied by the great evacuations that fuch continued 
' ftorms make thereof, the neighbouring air runs in 
more fwiftly, to bring it to an equilibrium, as we fee 
water runs the fader 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 mercury fhould fink lower in 
that extreme ; and then the northerly winds bringing 
the 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, muft of neceffity, make the mercury in fuch cafe 
(land higher in the other extreme. 
8. This remark, that, there is little or no variation 
near the equinoctial, does, above all others, confirm „ 
the 
