FARO 
Ft the furface of the-mercury is convex, (landing higher 
in the middle of the tube titan at the (ides, it is a fign that 
the mercury'is then riling'. 2. But if the furface be con¬ 
cave, or hollow in the middle, it is then linking. And, 
3. If it be plain, or rather a very little convex, the mer¬ 
cury is'lfationary.: for mercury being put into a glafs tube, 
efpecially a fmall one, naturally has its furface a little con¬ 
vex, beeaufe the particles of mercury attract one another 
more forcibly than they are attracted by glafs. Farther, 
4. If the glafs be fmall, lliake the tube; then, if the air 
be grown.heavier, the mercury will rife about half a 10th 
of an inch higher than it (food before; but, if it be grown 
lighter, it will link as much. And, in the wheel or cir¬ 
cular-barometer, tap the indritment gently with the finger, 
and the index will vilibly (fart forwards or backwards ac¬ 
cording to the tendency to rife or fall at that time. This 
proceeds front the mercury’s flicking to the (ides of the 
tube, which prevents the free motion of it till it be dif- 
engaged by the fitock : and therefore when an obfervation 
is to be made with fuch a tube, it ought to be firft fitaken ; 
for fometimes the mercury will not vary of its own ac¬ 
cord, till the weather is prefent which it ought to have 
indicated. 
To the foregoing may be added the following additional 
rules, more accurately drawn from later and more dole 
obfervation of the motions of the barometer, and the con- 
fiquent changes in the air in this country. 1. In winter, 
fpring, and autumn, the bidden falling of the mercury, 
.and that for a large fpace, denotes high winds and (forms ; 
but in Cummer it denotes heavy fliowers, and often thun¬ 
der : and it always finks lowed of all for great winds, 
though not accompanied with rain ; though it falls more 
for wind and rain together than, for either of them alone. 
Alfo, if after rain the wind change into any part of; the 
north, with a clear and dry (ky, and the mercury rife, it 
is a certain fign of fair weather. 2. After very great (forms 
of wind, when the mercury has been low, it commonly 
rifes again very fall. In fettled fair and dry weather, ex¬ 
cept the barometer fink much, expect but little rain; for 
its fmall linking then, is only for a little wind, ora few 
drops of rain; and the mercury foon rifes again to its for¬ 
mer ftation. In. a wet feafon, fuppofe in hay-time and 
harvefl, the fmallefl finking of the mercury mutirbe mind¬ 
ed ; for, when the conflitution of the air is much inclined 
to fhowers, a little finking in the barometer then denotes 
more rain, as it never then (lands very high. And if, at 
fuch a feafon, it rife fuddenly, very faff,.and high, expedt 
not fair weather more than a day or two, but rather think 
the mercury will fall again very foon, and rain imme¬ 
diately to follow : the flow gradual riling, and keeping on 
for two or three days, being molt to be depended on for a 
week’s fair weather. And the unfettled (fate of the quick- 
filver always denoting uncertain and changeable weather, 
efpecially when the .mercury (lands any where about the 
word changeable on the fcale. The greateft heights of the 
mercury in this country, arifing from eafterly and nortli- 
eafterly winds, it may often rain or blow, the wind being 
in thefe points, though the barometer fink little or none, 
or perhaps may be in a rifing (fate. But the mercury links 
for wind, as as well as rain, in all the other points of the 
compafs ; but rifes as the wind fiiifts about to the north 
or eaft, or between, thofe points : but, if the barometer 
fliould fink with the wind in that quarter, expect it foon 
to change front thence ; or ell'e, fliould the fall of the mere 
cury be much, a heavy rain is then likely to enfue, as 
fometimes happens. 
To account for the foregoing phenomena of the baro¬ 
meter, many hypothefes have been framed, which may be 
reduced to two general heads, viz. mechanical, and chemical. 
The chief writers upon thefe caufes, are Pafcal, Beal, 
Wallis, Garcian, Garden, Lifter, Halley, Garden, De la 
Hire, Mariotle, Le Cat, Woodward, Leibnitz, De Mai- 
ran, Hamberger, D. Bernoulli, Mufchenbroek, Cham¬ 
bers, De I.uc, Black, See. and an account of mod of their 
hypothefes may be feen at large in M. de Luc’s Rcc/icrc/ies 
METE R. 739 , 
Juries Modifications de l'Atmofphere, vol. r. c. 3. alfo the 
Philofop.hical TranfaCtions. We (hall here notice the prin¬ 
cipal of them. 
Dr. Lifter accounts for the changes of the barometer 
from the alterations by heat and cold in the mercury itlelf; 
contracting by cold, and expanding. by heat. But this,, 
it is now well known, is quite infutficient to account for 
the whole of the effect. The changes in the weight or 
prelfure of the atmofphere muff therefore be regarded as. 
the principal caufe of thofe in the barometer. But then, 
the difficulty will be to affign the caufe of that caufe, or- 
whence arife thofe alterations that take place in the atmo¬ 
fphere, which are fometimes fo great as to alter its pref- 
fure by the tenth part of the whole quantity. It is pro¬ 
bable that the winds, as driven about in different direc¬ 
tions, have a great (hare in them ; vapours and exhala¬ 
tions, rifing from the earth, may alfo have foine (hare;; 
and fome perhaps arife from the flux and reflux occalioncd 
in the air by the moon ; as well as foine chemical caufes- 
operating between the different particles of matter. 
Dr. Halley thinks the winds and exhalations diffident ; 
and on this principle gives a theory, the f'ubflance of wiiich. 
may be camprifed in what follows :* iff, That the winds 
muff alter the weight of the air in any particular country ; 
and this, either by bringing together a greater quantity of 
air, and fo loading the atmofphere of any place ; which., 
will be the cafe as often as two winds blow from oppofite 
parts, at the fame time, towards the fame point: or by 
Sweeping away fome part of the air, and giving room for.' 
the atmofphere to expand itfelf; which will happen when; 
two winds blow oppolite ways from the fame point at the 
fame time : or laftly by cutting off the perpendicular pref— 
dire of the air; which is the cafewhen a (ingle wind blows- 
brilkly any way.; it being found by experience,, that a 
ftroug blaft of wind, even made by art,, will-render the. 
atmofphere lighter ; and hence the mercury in a tube be¬ 
low it, as well as in others-more diffant,.will confiderubly 
dibfide. 2dly, That the cold nitrous particles, and even* 
the air itlelf condenfad in the northern regions, and driven, 
elfewhcre, mull load the atmofphere, and increafe its pref- 
dire. 3d 1 y, That heavy dry exhalations from the earth- 
muff increafe the weight of the atmofphere, as well as its 
elaftic force ; as ue find the fpecific gravity of menlfruums 
increafed by diflblved falts and metals. 4thly, That the- 
air.being rendered heavier by thefe and the like caufes, is- 
thence better able to fnpport the vapours; which being, 
likewife intimately mixed with it,, make the weather ferene 
and fair. Again; the air being made lighter from the con¬ 
trary caufes, it becomes unable to fnpport the vapours 
with which it is replete ; thefe therefore precipitating, are 
collected into clouds, the particles of which in their pro- 
grefs unite into drops of rain. Hence he infers, it is evi¬ 
dent that the fame caufes which.increafe the weight of the 
air, and render it more able to fnpport the mercury in the 
barometer, do likewife produce a.ferene (ky, and a dry 
feafon ; and that the fame caufes which render the air- 
lighter, and lefs able to fnpport the mercury, do likewife 
generate clouds and rain.. But thefe principles, though 
well adapted to many of the particular.cafes of the baro¬ 
meter, feem however to fall fhort of fome of the principal; 
and mod obvious ones, belides being liable to feveral ob¬ 
jections. 
Leibnitz accounted for the fall of the mercury before- 
rain by another principle, viz. That as a body lpecifically 
lighter than a fluid, while it is fuftained by it,.adds more: 
weight to that fluid than when, by being reduced in bulk,, 
it becomes fpecifically heavier, and defeends ; fo the va¬ 
pour, after it is reduced into the form of.clouds, and de¬ 
feends, adds lefs weight to the air than it did before ; and 
lienee the mercury finks in the tube. But here, granting, 
that the drops of rain formed from the vapours always in- 
creafing in fize as they fall lower, were continually acce¬ 
lerated alfo in their motion, and -fo the air fuffer a conti¬ 
nued lofs of their weight as they defeend ; it may howe¬ 
ver be objected, that by the. defeent of the mercury the 
rain 
