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That the Falling thereof portends Southerly or Wcflerly Winds with Rain, or ftormy Winds, or both.
That in a Storm, the Mercury beginning to rife, is a pretty fure Sign that it begins to abate.
Mr. 'Patrick obferves, that in hot Weather the Falling of the Mercury prefages Thunder ; that when foul Weather- happens foon after the Fall of the Mercury, it feldora holds long ; and the fame is obferv'd, if fair Weather fucceed presently after its Rife.
Laftly, Tfir. 2)erbam, from a long Series of 'Barometrical Obfervations made by Dr. Schcucher, at Zurich, compar'd with others made at the fame times by himfelf at Upminfter, obferves that throughout the whole Year the Mercury was lower at the former Place than at the latter, by fome times one and fometimes above two Inches ; tho the Difference at a Medium he computes to be about half an Inch ; and thence concludes the Situation of Zurich to be near ■*■ of an Englifb Mile higher than that otUjpmmfter. He found, however, a confiderable Harmony between the two ; the one ufually rifing and falling, and that much or little, as the other did ; tho this Harmony is not fo compleat, as has been obferv'd m Barometers nearer home, as at London, "Paris, in Lancajhire, &c.
Caufe of the Phenomena of the Barometer.
Thefe are the Phenomena of the Barometer $ to ac- count for which, the Hypothcfes that have been framed, are almoft infinite. Indeed, as the Weight of fSnsJ&mQ- , ffhere is generally allow'd to be the Foundation of the Ba- rometer, fo 'tis generally granted, that the Alterations in the Weight of the Air, are the Occafions of thofe in the Barometer ; and yet even this does not obtain univerfally. Dr. Lifter, for inftance, accounts for the Changes of the Barometer from the Alterations of Heat and Cold. This, he fays, he has often obferv'd, that in Storms, ggta when the Mercury is at the loweft, it breaks and emits fmall Par- ticles, which he calls a kind of fretting ; and argues, that in all times of its Defcent, it is more or lefs on the Fret. In this Diforder, he thinks, its Parts are contracted and brought clofer together, and for that reafon defcend : Belides, in the fretting they let go little Particles of Air, before in- clofed in 'em ; and thcie rifing into the Top of the Tube, the Mercury muft fink, both from the Column's being fhor- ten'd by their Efcape, and by their lying upon it. Mer- cury therefore, he adds, rifes either in very hot or very cold Weather, between the Tropicks, £5>c, as being then in its natural State ; and again, in the intermediate Degrees of Heat and Cold it falls, as being contracted, and as it were convuls'd and drawn together. But this Account, how- ever ingenious, comes far fhort of accounting for the Phamo- mena ; nay, in fome refpects, contradicts them.
The Changes in the Weight of the Atmofphere, there- fore, muft be laid down as the Caufe of thoie in the Ba- rometer ; but then, for the Caufe of that Caufe, or whence thofe Alterations arife in the Atmofphere, will be no eafy Matter to determine ; there being, perhaps, no one Princi- ple in Nature, that will account for fuch a Variety of Ap- pearances, and thofe too fo irregular. 'Tis probable, the Winds, as driven this or that way, have a great Share in 'em ; fome Share too, Vapours and Exhalations rifing from the Earth may have ; fome, the Changes in the Air of the neighbouring Regions; and fome, the Flux and Reflux oc- cafion'd in the Air by the Moon.
Dr. Hallcy thinks the Winds and Exhalations fuffkient ; and on their footing gives us a very probable Rationale of the Barometer : the Subrtance of what may be faid on that Head is as follows.
ift, Then, the Winds muft ncceffarily 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 ot any Place; which will be the Cafe, as often as two Winds blow at the fame time from oppofite Points towards the lame Point ; or, by fweeping away part of the Air, and removing fome of the Load, give 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 cut- ting off the perpendicular Preffure of the Atmofphere, which happens, as often as any fingle Wind blows briskly any way ; it being found by Experiment, that a ftrong Blaft of Wind, even made by Art, will render the Atmofphere lighter ; and accordingly, the Mercury, in a Tube under which it paffes, as well as in another at a Diftance from it, fubfides confiderably. Sec Philof Iranfatl. N°. 292.
zdly, The cold nitrous Particles, and even Air itfelf con- dens'd in the Northern Parts, and driven elfewhere, mull load the Atmofphere, and increafc its PrefTure.
%dly. Heavy, dry Exhalations from the Earth, muft in- creafc the Weight of the Atmofphere, and heighten its E- laftick Force, as we find the fpecifick Gravity of Menftru- ums increas'd by diflolv'd Salts and Metals,
qth/y, The Air being render'd heavier from thefe and the like Caufcs, i« thereby the more able to fupport the
Vapours; which being likewife intimately mix'd with it } and fwimming every where equably throughout it, make the Weather ferene and fair : Again, the Air being made lighter from the contrary Caufcs, it becomes unable to fup- port the Vapours wherewith it is replete ; thefe therefore precipitating are gathered into Clouds, and thofe in their rrogrels cdalefce into Drops of Rain.
Thefe things obferv'd, it appears pretty evident, that the fame Caufcs which increafc the Weight of the Air, and make it more able to fupport the Mercury in the Barome- ter, do likewife make a ferene Sky, and a dry Seafon; and the fame Caufcs which render the Air lighter, and lefs able to fupport the Mercury, do likewife generate Clouds and Rain.
Hence, 1/?, When the Air is lighteft, and the Mercury in the Baro?neter loweft, the Clouds arc very low, and move fwiftly; and when after Rain the Clouds break, and a calm Sky again mines forth, being purg'd of its Vapours, it appears exceedingly bright and tranfparent, and affords an eafy Profpect of remote Objects.
^ zdly, When the Air is heavier, and the Mercury ftands higher in the Tube, the Weather is calm, tho fomewhat lefs clear, by reafon the Vapours are differs 'd every where equally j 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 envelop'd in pretty thick Clouds., which appear to be form'd out of the groffer Exhalations, and which the Air is then able to fuftain, tho a lighter At- mofphere could not.
idly, Hence it is, that with us the Mercury ftands high- eft in the coideft Sea lbns, and when the Wind blows from the North or Ndrth-Eaft Corner: for in that Cafe there are two Winds blowing towards us at the fame time, and from oppofite Corners ; there being a conftant Welt Wind found in the Atlantick Ocean, at the Latitude correfponding to ours. To which we may add, that in a North Wind, the cold condens'd Air of the Northern Parts is brought hither.
4-thly, Hence, in the Northern Regions, the Variation of the Mercury is more fenfible than in the Southern ones ; the Winds being found both more ftrong, more frequent, more various, and more oppofite to each other in the for- mer than the latter.
Laftly, Hence it is, that between the Tropicks, the Va- riation of the Mercury Is fcarce fenfible ; the Winds there being extremely gentle, and ufually blowing the fame way.
Now, this Account, however well adapted to many of the particular Cafes of the Barometer, yet comes fhort of fome of the principal and moft obvious ones; and is, be- sides, liable to feveral Objections.
For, jft, If the Wind were the fole Agent in effecting thefe Alterations, we fhould have no Alterations without a fenfible Wind, nor any Wind without fome Alteration of the Mercury ; both which are contrary to Experience.
zdly, It twoWinds be fuppos'd blowing from the fame Place, viz. Lend 12, oppofite ways, 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 two 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 is carried off N.E. and S. W. the adjacent Air will croud in from the other Points, and form a couple of new Currents in the Direction N, W. and S. E. to fill up' the Vacancy, and reflore the ^Equilibrium. This is a ne- ceffary Confequencc from the Laws of Fluids.
z,dly, If the Wind were the fole Agent, the Alterations in the Height of the Mercury wou'd only be relative or topical ; there would be ftill the fame Quantity fupportcd at feveral Places taken collectively : Thus what a Tube at London loft, another at Paris, or at Pifa, or at Zuri&hi &c. wou'd at the fame time gain. But we find the very contrary true in Fact; for from all theObfervations hither- to made, the Barometers in feveral Parts of the Globe rife and fall together ; fo that it muft be fome Alteration iri the abfolute Weight of the Atmofphere, that accounts for the Rile and Fall of the Mercury.
Laftly, Setting afide all Objections, thefe popular Phe- nomena, the Mercury's Fall before, and Rife after Rain, are really inexplicable on the foot of this Hypothefis : for, fup- pofe two contrary Winds fweeping the Air from over Lon- don : wc know that few, if any, of the Winds reach above a Mile high ; all therefore they can do, will be to cut off a certain Part of the Column of Air over London : If the Confequence of this be the Fall of the Mercury, yet there is no apparent Reafon for the Rains following it. The Va- pours indeed, may be let lower, but 'twill only be till they come into an Air of the fame fpecifick Gravity with them- felves ; and there they will ftick as before.
M. Leibnitz endeavours to fupply the Defects of this Hy- pothefis with a new one of his own. He afferts, that a Bo- dy immers'd in a Fluid, only weighs with that Fluid while it is fuftained thereby; fo that when it ceafes to be fuf- Z tain\L
