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Show BA BA accordingly the Mercury in a Tube, under various, and more oppofite to each other in the BA Which Contraction plainly appears from the concave Figure of both Surfaces, not only in that of the Quickfilver in the Tube, but alfo, if well obfervd, in that whichftag- nates in the Pot or Dihh itfelf. te The Difficulty feems to lie in reconciling the fame Effe@& of the Quickfilver 8 rifing in the Tube, from fuch feemingly different Canfes, as great Heat, and intenfe Froft : And thofe who fhall affent in one Particular, and grant that Warmth is a probable Caufe of its Reftitution to its Nature, will yet be at a Stand howto imagine, that great Froft fhould likewife bring the Quickfilver nearer to its own Nature too. pie To which he anfwers, that Salts liquified will coagulate orcryftalize, 7. e. will return to their own proper Natures, both in Cold, and in Heat; and therefore, tho’ moft Men practife the fetting them in a cool Cellar for that Purpofe, yet fome (as Zwelfer) advite, as the beft Means to have them {peedily and fairly cryftaliz’d, to keep them conitantly in Balneo. Thus alfo the Lympha of the Blood becomes a Jelly, if it be fet in a cool Place, and the fame is infpiffated in like Manner by Warmth. This Account of Dr. Lifer’s, however ingenious, is objected againft, as coming far fhort of accounting for the Pheuomena of the Barometer, and that in fome refpects it contradi¢ts them. The Changes of the Weight of the Atmofphere, muft be laid down as the Canfe of thofe in the Barometer ; but then whence thofe Alterations inthe Atmofphere, will be no eafy Matter to determine. It is probable the Winds which are driven this or that Way, mayhave a great Share; and the Vapours and Exhalations arifing from the Earth, alfo the Changes of the Air in the neighbouring Regions; and alfo the Flux and Reflux occafioned in the Air by the Moon, Dr = y is of Opinion, the Winds and Exhalations are fufficient, and ontheir footing, gives a very probable Rationale of the Baro ‘, the Subftance of which is to the Purpofe following. 1. ‘The ‘Winds muft neceffarily alter ‘the Weight of the Air in any particular Country, and that either by bringing together or accumulating a greater Quantity of Air, and fo loading the Atmofphere of any Place; which will be the Cafe, as often astwo Winds blow at the fame Time, from oppofite Points towards the 4ame Point ; or by {weeping away Part of the Air, and removing fome of the Load, give Roomfor the Atmofphere to expand itlelf, which will be the Cafe, when two Winds blow at the fame Time, and from the fame Point oppofite Ways; or in the laft Place bycutting of the perpendi- cular Preffure of the Atmofphere, which hap- ens as often as anyfingle Wind blows briskly any Way; it being found by Experiment, a trong Blaft of Wind, even made by which it paffes, as well as in another at a Diftance from it, fubfides confiderably. 2. The cold, nitrous Particles, and even Air itfelf condens’d in the Northern Parts, and driven elfewhere, muft load the Atmof{phere, and increafeits Preffure, 3. Dry, heavy Exhalations fromthe Earth, mutt increafe the Weight of the Atmofphere, and heighten its Elaftick Force, as the Speci~ fick Gravity of Menfiruums are found to be increas d, bydiffolv’d Salts and Metals 4. The Air being render’d heavier from thefe and the like Caufes, is thereby the more able to fupport the Vapours, which being like. wife intimately mix’d with it, and fwimming every where equally throughout it, make the Weather ferene andfair. Again, the Air being made lighter fromthe contrary Caufes, it becomes unable to fupport the Vapours, wherewith it is replete; thefe therefore precipitating; are gath Clouds, and thofe in their Progrefs coaleice into Drops of Rain. Thefe Things being obfe pretty evident that the increafe the Weight of the Air, and m more able to fupport the Mercury Barometer, do likewife make a ferene a dry Seafon ; and the fame Caufes whic der the Airlighter, and lefs able to fupport the Mercury, do likewife generate Clouds and Rain. Hence, Firft, whenthe Air is lighteft, and the Mercury in the Barometer. loweft, the Clouds are very low; and when after Rain the Clouds break, and a calm Sky gain fhines forth, being purg’d of its Vapouts, it appears exceedingly bright and traniparent, and affords an eafy Profpect of remote Objects. Secondly, Whenthe Air is heavier, and the Mercuryftands higher in the Tube, the Weather is calm, tho’ fomewhat lef clear, by Reafon that the Vapours are difpers ’d every where equally; 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 invelop’d in pretty thick former thanthe latter, Laftly, Hence is is, that between the Tro- : ations in the Height of the Mer- cury, wouldonly be relative or topical; there at feveral Places what a Tube at London or at Pifa, or at Zp the fame Time. being a conftant Welt Wind found in the Atiantick Ocean, in a Latitude that correfponds to ours. ‘To this it may be added, that the cold condens’d Airoft! Northern Parts is brought hither in a No Wind, Fourthly, Hence in the Northern Regi the Variation of the Mercury is m than will render the Atmofphere lighter, and foundin the Southern ones; the Win both more ftrong, more frequ anydeterminate Portion of the Globe 3 as for Inftance, over AB; if jthe upper Veficle s be i Laftly, Setting all Objections afide, thefe ts Phenomena, the Fall of the Mercury before, and the Rife after Rain, are realunexplicable on the Foot of this Hythefi and when the Wind blows from the ners, there . would gain at But the very contrary is Numberof water: Vehicles, v. g. a Millio n, floating in anypart cf the Atmoiphere , over the Rife and Fall of the Mercury. : Cafe there are two Winds blowing toward s us at the fame Time, and from oppofite Cor- en collectively. Thus loft, another at Paris, nd true in Faét; for fromall the Obferi 5 the Barometersrife and fall together in ¢everal Parts of the Globe ; fo. that it muft be fome Alteration in the abfolute Weight of che Atmofphere, that accounts for Thirdly, Hence it is, that with us the North, and North-Eaft Corners : Forin that In order to account for thefe Variat ions of the Phenomena of the Barometer, Mr. would be ftill the ame Quantity fupported bers advances what follows: SuppofeCham= any Mercury ftands higheft in the coldeft Sea- fons, Thus, he adds, that the watry Vapour s, while fuftain’d in the Air, increafe its Weicht : but when let fall, ceafe to we 3h along with it. "Thus the Weight of the Air is dimini fh’d, and thus the Mercuryfalls, and, Rain enfues, — But this Principle of M. Leibnitz is falfe, notwithftanding the Experiment he brings to confirm it, as Dr. De/aguliers has made appear by the Counter-Experiment, For while the Variation of the Mercury is fcarce Je; the Winds there being extremely gentle; and ufually blowing the fame Way. ”" Nowthis Account, however well adapted to many of the particular Cafes of the Barometer, yet comes fhort of fome of the moft principal, and the moft obvious ones; and is a Bodyis immers’d in a Fluid, whether it be befic able tofeveral Objections. equal with it, or heavier, or lighter than the Forfirft, If the Wind were the fole Agent Fluid, whether it be at Reft or in Motio n, in. effecting thefe Alterations, we fhould have adds to the Fluid a Weight of an equal Bulk no Alterations without a fenfible Wind, nor of the Fluid, as follows from that Lawi n any Wind without fome Alteration of the Hydroftaticks, that Fluids gravitate acco: Mercury ; both which are contrary to Ex- to their r Altitudes, a perience. : ; But however, if this Principle of M. LejlSecondly, if two Winds be fuppos’d blow- nitZ were true, yet it is defe ctive, and that ing from the fame Place, wz. London, oppofite in the fame refpe@t with Dr. Flalley’s ; nor Ways, viz. N. E. and S. W. there will be two would it account for the Phezomena more than others blowing from oppofite Points, viz, N. the other, For fuppofing the Vapours, bybeing conW. and S. W- to the fame Place; which twolaft will balance the firft, and bring as much dens’d, to be put ina Motion downward, and Air towards the Point, as the others {wept fo ceafing to gravitate with the Atmot! phere, from it. Or thus, in Proportion as the Air is they will therefore fall till they reach a Part of carried off N. E. and S. W. the adjacent Air the Atmofphere of the fame {pecifick Gravit y will crowd in from the other Points, and form with themfelves, and there they will hang as a couple of new Currents in the Direction before: If the Mercuryfalls, it will be only N. W. and S. E. to fill up the Vacancy, and during the Time ofthat Defcent ; for thefe reftore the Equilibrium; this is a neceffary once fix’d, the former Gravity is retrie v'd; ce from the Laws of Fluids. or were it not retriev’d, yet no Rain would if the Wind were the fole Agent, enfue the Fall of Mercury. Clouds, which appear to be formed out of the groffer Exhalations, and which the Air is then able to fuftain, tho’ a lighter Atmo- {phere could not, : : for {uppofe two contrary Winds the Air from over London, it is ut few Winds, if any, reach above ; all therefore they can do, will a certain Part of the Column of a: If the Confequence of the Fall of the Mercury, yer there is rent Reafon for the Rains following ipours indeed may be let lower, but will only be till they come into-an Air of e ipecifick Gravity with themfelves, theywill {tick as before. buitZ has endeavour’d to fupply the s of this Hypothefis with a new one of e afferts, That a Body immers’d , only weighs with that Fluid, while uftain’d thereby ; fo that whenit ceafes to bef ain'd, 7. e. to fall, its Weight ceafes to make a Part of the Fluid, which bythis Means becomes lighter. condens’d by the Cold of the upper Regions, their {pecifick Gravity will be increas’ » and they will defcend ; the horizontal Clafg1 v. g- to 2, 2 to 3, &c. where meeting with other Veficles, not yet precipitated, theywill coale{ce or run into larger Veficles, by the known Laws of Attraction. Or, if we rather chufe to have the Wind act, let it drive horizontally or obliquely: In the former Cafe, the Veficles Clafs 8 will be Z driven |
