of the batometer.

BAROCHE, a town of Cambaya, in the dominions of the Great Mogul; it is walled round, and was formerly a place of great trade. It is now inhabited by weavers and fuch mechanics as manufacture cotton cloth. Here they have the beft cotton in the world, and of confequence the beft baftas are manufactured in this place. The English and Dutch had formerly factories here, which are now abandoned. E. Long. 72. 5. N. Lat. 22. 15.

BAROCO, in logic, a term given to the fourth mode of the fecond figure of fyllogifms. A fyllogifm in baroco has the firft propofition univerfal and affirmative, but the fecond and third particular and negative, and the middle term is the predicate in the two first propofitions. For example,

Nullus homo non eft bipes: Non omne animal eft bipes : Non omne animal eft homo. BAROMETER (from Bag weight, and r measure), an inftrument for measuring the weight of the atmosphere, and of use in fortelling the changes of the weather, and alfo for measuring the height of mountains, &c.

The common barometer confifts of a glafs tube herPrinciples metically fealed at one end, and filled with quickfilver well defecated and purged of its air. The finger being then placed on the open end, in immediate contact with the mercury, fo as not to admit the leaft particle of air, the tube is inverted, and the lower end plunged into a bason of the fame prepared mercury; then upon removing the finger, the mercury in the tube will join that in the bafon, and the mercurial column in the tube will fubfide to the height of 29 or 30 inches, according to the ftate of the atmosphere at that time. This is the principle on which all barometers are conftructed. Of their invention, the different kinds of them, and the theories by which their phenomena are folved, we fhall proceed to give an hiftorical account.

and im

In the beginning of the laft century, when the docDifcovered trine of a plenum was in vogue, philofophers were of by Galileo, opinion, that the afcent of water in pumps was owing to the abhorrence of a vacuum; and that by means of proved by Torricelli fuction, fluids might be raised to any height whatever. But Galileo, who flourished about that time, difcovered that water could not afcend in a pump unless the fucker reached within 33 feet of its furface in the well. From hence he concluded, that not the power of fuction, but the preffure of the atmosphere, was the caufe of the afcent of water in pumps; that a column of water 33 feet high was a counterpoife to one of air of an equal bafe, whofe height extended to the top of the atmofphere; and that for this reafon the water would not follow the fucker any farther. From this Torricelli, Galileo's difciple took the hint; and confidered, that if a column of water of about 33 feet in height was equal in weight to one of air having the fame bafe, a column of mercury no longer than about 29 inches would be fo too, because mercury being about 14 times heavier than water, a column of mercury muft be 14 times fhorter than one of water equally heavy. Accordingly, having filled a glafs tube with mercury, and

5

inverted it into a bafon of the fame, he found the mer- Barometer, cury in the tube to defcend till it ftood about 29% inches above the furface of that in the bafon.

3

Strange hv

pothefis of

Notwithstanding this clear proof of the preffure of the atmosphere, however, the affertors of a plenum left no means untried to folve the phenomena of the Tor- Linus. ricellian experiment by fome other hypothefis. The molt ridiculous folution, and which at the fame time gave the adverse party the greatest difficulty to overthrow it, was that of Linus. He contended, that in the upper part of the tube, there is a film, or rope of mercury, extended through the feeming vacuity; and that, by this rope, the reft of the mercury was fufpended, and kept from falling into the bafon. Even this fo abfurd hy: Experi pothefis he pretended to confirm by the following ex-ments in periments. Take, fays he, a fmall tube, open at both confiimaends, suppose about 20 inches long; fill this tube with tion of it. mercury, ftopping the lower orifice with your thumb: Then clofing the upper end with your finger, and immerging the lower in flagnant mercury, you shall per ceive, upon the removal of your thumb, a manifeft fuction of your finger into the tube; and the tube and mercury will both flick fo close to it, that you may carry them about the room. Therefore, fays he, the internal cylinder of mercury in the tube is not held up by the preponderate air without; for if fo, whence comes fo ftrong a fuction, and fo firm an adhesion of the tube to the finger?-The fame effect follows, though the tube be not quite filled with mercury; for if a little space of air is left at the top, after the tube is immerged in the stagnant mercury, there will be a confiderable fuction as before.

gens.

5

Thefe experiments, which are themselves clear proofs Refuted. of the preffure of the air, fupported for fome time the funicular hypothefis, as it was called, of Linus. But when it wasdifcovered, that if the tube was carried to the top of an high mountain the mercury flood lower than on the plain, and that if removed into the vacuum of an air-pump it fell out altogether, the hypothefis of 6 Linus was rejected by every body.-There are, how- Remarkever, two experiments which create a confiderable dif- able experificulty. One is mentioned by Mr Huygens, viz. that ments by Mr Huyif a glafs tube 75 inches long, or perhaps longer, is filled with mercury well purged of its air, and then inverted, the whole will remain fufpended; whereas, according to the Torricellian experiment, it ought to fubfide immediately to the height of 29 or 30 inches. It is true indeed, that, upon fhaking the tube, the mercury prefently fubfides to that height; but why it fhould remain fufpended at all, more than twice the height to which it can be raised by the preffure of the most dense atmosphere, feems not eafily accounted for; and accordingly, in the Philofophical Tranfactions, we find attempts to account for it by the preffure of a medium torily ac more fubtile than the common air, and capable of per- counted for vading both the mercury and glafs. We find there in the Phialfo another very furprising fact of the fame kind men- lofophical tioned; viz. that a pretty large tube under 29 inches. in length, filled with mercury, and inverted into a bafon of the fame, will remain full, though there be a fmall hole in the top. This, too, is there accounted for by the preffure of a medium more fubtile than common air; but by no means in a fatisfactory manner. Mr. Rowning, who mentions the phenomenon of the 75 inch Mr Row tube, accounts for it in the following manner. "The ning's folu| tion. C 2 caufe

Unfatisfac

Tranfac

tions.

8

Infufficient.

Barometer. caufe of this phenomenon feems to be, that by the great weight of fo long a column of mercury, it was preffed into fo clofe contact with the glass in pouring in, that, by the mutual attraction of cohefion between the mercury and the glafs, the whole column was sustained af ter the tube was inverted."-Here, however, we must obferve, that this folution feems equally unfatisfactory with that of the fubtile medium already mentioned; because it is only one end of the column which fuftains fo great a preffure from the weight of the mercury; and therefore, though five or fix inches of the upper part of the tube, where the preffure had been strongeft, might thus remain full of mercury, yet the reft ought to fall down. Befides, it is only the outfide of the mercurial column that is in contact with the glafs, and confequently these parts only ought to be attracted. Therefore, even granting the preffure to be equally violent, on the invertion of the tube, all the way from 29 to 75 inches, yet the glafs ought to be only as it were filvered over by a very thin film of mercury, while the middle parts of the column ought to fall out by reason of their fluidity.

10

Another experiment with fiphons.

bers.

12

Infufficient.

affured that there are certain fubftances, of which glafs Barometer. is one, through which the electric matter cannot pafs but with difficulty. We are likewise certain, that tho' the electric matter paffes through the pores of water, metals, &c. with very great facility, yet it ftill must meet with some refiftance from their folid and impenetrable parts, which cannnot be pervaded by any material fubftance. We know alfo, that all fubftances do na turally contain a certain quantity of this electric mat. ter, which they are not always ready to part with; and. when by any means the fluid they contain is fet in mo tion, they are then said to be electrified. Now, though. we are certain, that the friction of glafs by mercury does fet in motion the electric fluid contained in the mercury or in the glafs; yet when the tube is filled with the metallic fluid, whatever quantity has been extricated either from the glafs or mercury during the time of filling, will be reabforbed by the metal and conveyed to the earth during the time of inverfion; and confequently the mercurial tube, when inverted, will not be electrified, but both glass and mercury will be in their natural flate. Here, then, the preffure of the electrical fluid is kept off in fome meafure from the upper part of the mercury by the glafs, which it cannot penetrate eafily at leaft. To the mercury in the bafon it has free accefs, and therefore preffes more upon the lower than the upper part; the confequence of which is a fufpenfion of the mercury. It is true, this fluid very eafily penetrates the metallic matter; but it must be confidered, that the electric fluid. itself is in fome measure entangled in the particles of the quickfilver, and cannot be extricated without motion. As foon therefore as the tube is fhaken, some part of the electricity is extricated, and the mercury begins to defcend. The fubtilty of the medium is fuch, that no fooner has it begun to extricate itself, than, by the motion of the metal downwards, it iffues forth in great quantities, fo as to become vifible, like a blue flame, in the dark. The equilibrium is therefore de.. ftroyed in an inftant, as it would be were we to admit air to the top of the barometer; nay, in a more effectual manner. For if a fmall quantity of air was admitted to the top of a barometer, the mercury would. only defcend in proportion to the quantity of air admitted; bat here, no fooner is a quantity of electric matter admitted, than it procures admiffion for a vaft deal more, and confequently the mercury descends with accelerated velocity.-On this principle the afcent of water in the fiphon while in vacuo is fo eafily accounted for, that we need not take up time in explaining it farther.-But why an inverted glafs tube fhould remain full of mercury when it has a hole either great or small in the top, is more difficult to be accounted for, and requires this farther circumstance to be taken into confideration, viz. that though all folid bodies will, by the action of gravity, or by any other impulfe, easily approach very near to one another, yet they cannot be brought into abfolute contact without a very confiderable force, much greater than is fufficient to overcome their gravity; and thus it appears from fome experiments, that the links of a chain are by no means in contact with one another, till the chain has a confiderable weight appended to it. This may be the cafe with the tube in queftion. The air by its gravity de-. fcends upon it, and is ready to enter the fmall hole in the

The other experiment hinted at, is with regard to fiphons; which though it belongs more properly to the article HYDROSTATICS, yet feems neceflary to be mentioned here. It is this: That a fiphon, once fet a running, will continue to do fo though fet under the receiver of an air-pump and the air exhaufted in the most perfect manner; or if a fiphon is filled, and then fet under a receiver and the air exhausted, if by any contrivance the end of the lower leg is opened, it will immediately begin to run, and difcharge the water of any veffel in which the other leg is placed, as though it was in the open air. The caufe of this phenomenon, as well as the former, feems very difficult to be inveftiSolation by gated. In Chambers's Dictionary, under the word Mr Cham- Siphon, we have a folution fomething fimilar to the funicular hypothefis of Linus abovementioned; namely, that "fluids in fiphons feem as it were to form one continued body; fo that the heavier part, defcending, like a chain pulls the lighter after it." This might be deemed a fufficient explication, if the fiphon was only to empty the water it at firft contains in itself: but when we confider that the water in the veffel, which much exceeds the quantity contained in the fiphon, is likewife evacuated, Mr Chambers's hypothefis can by no means be admitted; because this would be like the lighter part of a chain pulling the heavier after it. Concerning the caufe of thefe fingular phenomena, we can only offer the following conjecture. The ex-, istence of a medium much more fubtile than air, and which pervades the vacuum of an air-pump with the utmoft facility, is now fufficiently afcertained in the phenomena of electricity. It is alfo well known, that this fluid furrounds the whole earth to an indeterminate height. If therefore this fluid either is the power of gravity itfelf, or is acted upon by that power, it muft neceffarily prefs upon all terreftrial bodies in a manner fimilar to the preffure of the atmosphere. If then we could from any veffel entirely exclude this fubtile fluid, and form an electrical vacuum, as well as we can do an aerial one by means of the air-pump, we would in that cafe fee fluids as evidently raised by the preffure of the electric matter, as we now fee them raised by that of the air. But tho' this cannot be done, we are

13 Another folution from the action of electricity,

Barometer ufed for

weather.

Barometer. top; but, by a repulfive power from the glafs, its action is prevented, fo that the mercury cannot fall. 14 It was, however, fome time after the Torricellian experiment had been made, and even after it had been prognofti- univerfally agreed that the fufpenfion of the mercury cating the was owing to the weight of the atmosphere, before it was difcovered that this preffure of the air was different at different times though the tube was kept in the fame place. But the variations of altitude in the mercurial column were too obvious to remain long unobferved; and accordingly philofophers foon became careful enough to mark them. When this was done, it was impoffible to avoid obferving alfo, that the changes in the height of the mercury were accompanied, or very quickly fucceeded, by changes in the weather. Hence the inftrument obtained the name of the ea ther-glafs, and was generally made ufe of with a view to the foreknowledge of the weather. In this character, its principal phenomena are as follow.

1. The rifing of the mercury prefages, in generaf, fair weather; and its falling, foul weather, as rain, fnow, high winds, and storms.

2. In very hot weather, the falling of the mercury Mr Patrick. foreshows thunder.

16

by M: RowBing.

3. In winter, the rifing prefages froft; and in frofty weather, if the mercury falls three or four divifions, there will certainly follow a thaw. But in a continued froft, if the mercury rifès, it will certainly fnow.

4. When foul weather happens foon after the falling of the mercury, expect but little of it; and, on the contrary, expect but little fair weather when it proves fair fhortly after the mercury has risen.

5. In foul weather, when the mercury rises much and high, and fo continues for two or three days before the foul weather is quite over, then expect a continuance of fair weather to follow.

6. In fair weather, when the mercury falls much and low, and thus continues for two or three days before the rain comes; then expect a great deal of wet, and probably high winds.

7. The unfettled motion of the mercury denotes uncertain and changeable weather.

8. You are not fo ftrictly to observe the words engraved on the plates (though in general it will agree with them), as the mercury's rifing and falling. For if it stands at much rain, and then rifes up to changeable, it prefages fair weather; though not to continue fo long as if the mercury had rifen higher: and fo, on the contrary, if the mercury flood at fair, and falls to changeable, it prefages foul weather; though not so much of it as if it had funk lower..

"From

Thefe are the observations of Mr Patrick, on which Mr Rowning makes the following remarks. thefe obfervations it appears, That it is not fo much the height of the mercury in the tube that indicates the weather, as the motion of it up and down: wherefore, in order to pass a right judgment of what weather is to be expected, we ought to know whether the mercury is actually rifing or falling; to which end the following rules are of ufe.

"1. If the furface of the mercury is convex, ftanding higher in the middle of the tube than at the fides, it is generally a fign that the mercury is then rifing. "2. If the furface is concave, it is then finking: and,

"3. If it is plain, the mercury is flationary; or ra- Barometer. ther, if it is a little convex: for mercury being put into a glafs tube, especially a small one, will naturally have its furface a little convex, because the particles of mercury attract one another more forcibly than they are attracted by glafs. Further,

4. If the glafs is fmall, fhake the tube; and if the air is grown heavier, the mercury will rife about half the tenth of an inch higher than it ftood before; if it is grown lighter, it will fink as much. This proceeds from the mercury's fticking to the fides of the tube, which prevents the free motion of it till it is difengaged by the fhock: and therefore, when an ob fervation is to be made with fuch a tube, it ought always to be fhaken firft; for fometimes the mercury will not vary of its own accord, till the weather it ought to have indicated is prefent."

nomena pe

zones.

Here we must observe, that the abovementioned phe- Thefe phenomena are peculiar to places lying at a confiderable diftance from the equator; for, in the torrid zone, the mer- culiar to the cury in the barometer feldom either rifes or falls much. temperate In Jamaica, it is obferved by Sir William Beefton*, that and frigid the mercury in the morning constantly stood at one de- Pbilof. gree below changeable, and at noon funk to one degree Tranfact. above rain; fo that the whole fcale of variation there N° 220.. was only of an inch. At St Helena, too, where Dr Halley made his obfervations, he found the mercury to remain wholly ftationary whatever weather happened. Of these phenomena, their caufes, and why the barometer indicates an approaching change of weather, the Doctor gives us the following account.

66

1. In calm weather, when the air is inclined to rain, the mercury is commonly low.

"2. In ferene, good, and fettled weather, the mercury is generally high.

66

18 Phenomena of the barometer folved by De

3. Upon very great winds, though they be not Halley. accompanied with rain, the mercury finks lowest of all, with relation to the point of the compass the wind blows. upon.

"4. Cæterus paribus, the greatest heights of the mercury are found upon eafterly, or north-easterly, winds.

"5. In calm frofty weather; the mercury generally stands high.

"6. After very great ftorms of wind, when the mercury has been very low, it generally rifes again very fast. "7. The more northerly places have greater alte. rations of the barometer than the more foutherly.

"8. Within the tropics, and near them, thofe accounts we have had from others, and my own obfervations at St Helena, make very little or no variation of the height of the mercury in all weathers.

"Hence I conceive, that the principal caufe of the rife and fall of the mercury is from the variable winds which are found in the temperate zone, and whofe great inconftancy here in England is notorious.

"A fecond cause is, the uncertain exhalation and precipitation 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 depends in a great meafure upon the former. Now from these principles I fhall endeavour to explicate the: feveral phenomena of the barometer, taking them in the fame order I have laid them down. Thus,

"1. The mercury's being low inclines it to rain, becaufs

Barometer. because the air being light, the vapours are no longer fupported thereby, being become fpecifically heavier than the medium wherein they floated; fo that they defcend towards the earth, and, in their fall, meeting with other aqueous particles, they incorporate together, and form little drops of rain: but the mercury's being at one time lower than another, is the effect of two contrary winds blowing from the place where the barometer ftands; whereby the air of that place is carried both ways from it, and confequently the incumbent cylinder of air is diminished, and accordingly the mercury finks: A6, for inftance, if in the German Ocean it fhould blow a gale of wefterly wind, and, at the fame time, an easterly wind in the Irish Sea; or, if in France it fhould blow a northerly wind, and in Scotland a foutherly; it must be granted, that that part of the atmosphere impendant over England would thereby be exhausted and attenuated, and the mercury would subfide, and the vapours which before floated in these parts of the air of equal gravity with themselves would fink to the earth.

66 2. The greater height of the barometer is occafioned 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 cylinder of air being increased both in height and weight, the mercury preffed thereby muft needs ftand 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 have no inclination to precipitate and fall down in drops, which is the reafon of the ferene good weather which attends the greater heights of the mercury.

"3. The mercury finks the loweft of all by the very rapid motion of the air in ftorms of wind. For the tract or region of the earth's furface, wherein the winds rage, not extending all round the globe, that ftagnant air which is left behind, as likewife that on the fides, cannot come in fo fast 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 less 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 agitation of its particles is the reason why the vapours are diffipated, and do not condenfe into drops fo as to form rain, otherwife the natural confequence of the air's rarefaction.

"4. The mercury ftands higheft upon the easterly and north-easterly wind; becaufe in the great Atlantic ocean, on this fide the 35th degree of north latitude, the winds are almoft always wefterly or fouth-wefterly; so that whenever here the wind comes 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 our second remark, the air muft needs be heaped over this ifland, and confequently the mercury muft ftand high as often as these winds blow. This holds true in this country; but is not a general rule for others, where the winds are under different circumftances: and I have fometimes feen the mercury here as low as 29 inches upon an easterly wind; but then it blew exceedingly hard, and fo comes to be accounted for by what was observed in the third remark.

5. In calm frofty weather the mercury generally Barometer, ftands high; because (as I conceive) it feldom freezes but when the winds come out of the northern and northeaftern quarters, or at leaft unless thofe winds blow at no great diftance off. For the north parts of Germany, Denmark, Sweden, Norway, and all that tract from whence north-eaftern winds come, are fubject to almoft continual froft all the winter: and thereby the lower air is very much condensed, and in that state is brought hitherward by thofe winds, and, being accumulated by the oppofition of the westerly wind blowing in the ocean, the mercury muft needs be preffed to a more than ordinary height; and as a concurring caufe, the fhrinking of the lower parts of the air into leffer room by cold, muft needs caufe a defcent of the upper parts of the atmosphere, to reduce the cavity made by this contraction to an equilibrium.

"6. After great ftorms, when the mercury has been very low, it generally rifes again very faft: I once obferved it to rife one inch and an half in lefs than fix hours after a long-continued ftorm of fouth-weft wind. The reafon is, because the air being very much rarefied by the great evacuations which fuch continued ftorms make thereof, the neighbouring air runs in the more swiftly to bring it to an equilibrium; as we fee water runs the faller for having a greater declivity.

66

7. The variations are greater in the more northerly places, as at Stockholm greater than at Paris (compared by M. Pafchal); because the more northerly parts have ufually greater forms of wind than the more foutherly, whereby the mercury fhould fink lower in that extreme; and then the northerly winds bringing in the more denfe and ponderous air from the neighbourhood of the pole, and that again being checked by a foutherly wind at no great diftance, and fo heaped, muft of neceflity make the mercury in such case fland higher in the other extreme.

"8. Laftly, this remark, that there is little or no variation near the equinoctial, does above all others confirm the hypothefis of the variable winds being the caufe of thefe variations of the height of the mercury; for in the places above named there is always an easy gale of wind blowing nearly upon the fame point, viz. E. N. E. at Barbadoes, and E. S. E. at St Helena; fo that there being no contrary currents of air to exhauft or accumulate it, the atmosphere continues much in the fame ftate: however, upon hurricanes, the moft violent of ftorms, the mercury has been obferved very low; but this is but once in two or three years, and it foon recovers its settled ftate, about 291 inches."

19

This theory we find controverted in Chambers's Objections Cyclopædia, under the word BAROMETER. The by Mr principal objections are, "That if the wind was the fole Chambers. agent in raifing or depreffing the mercury, the alterations of its height in the barometer would be only relative or topical; there would ftill be the fame quantitity fupported at feveral places taken collectively: thus what a tube at London loft, another at Paris, Pifa, or Zurich, &c. would gain. But the contrary is found to be the cafe; for, from all the obfervations hitherto made, the barometers in feveral diftant parts of the globe rife and fall together. This is a very furprifing fact; and deferves to be well examined. Again, fetting

this, fee the articles EVAPORATION, COLD, CONGE- Barometer. LATION, &c. 2. If the atmofphere is affected by any unufual degree of heat, it thence becomes incapable of fupporting fo long a coluinn of mercury as before, for which reafon that in the barometer finks. This appears from the obfervations of Sir William Beefton already mentioned; and likewife from thofe of De Luc, which fhall be afterwards taken notice of.

Barometer. afide all other objections, it is impoffible, on Dr Halley's hypothefis, to explain the mercury's fall before, and rife after, rain. For fuppofe two contrary winds fweeping the air from over London: We 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 vapours indeed may be let lower; but it will only be till they come into an air of the fame specific gravity with themfelves, and there they will ftick as before. Laftly, it is impoffible, according to the laws of fluids, that the air above any place could be exhaufted by the blowing of two contrary winds from it: for, fuppofe a north-caft and fouth-west wind both blow from London at the fame time, there will be two others at the fame time blowing towards it from oppofite points, viz. a N. W. and S. E. one, which will every moment reftore the equilibrium, fo that it can never be loft in any confiderable degree at least."

20

Hy; othefis of Ar

21

Mr Leibnitz accounted for the finking of the mercury before rain upon another principle, viz. That as Leibnitz. a body specifically lighter than a fluid, while it is fufpended by it, adds more weight to that fluid than when, by being reduced in its bulk, it becomes fpecifically heavier, and defcends; fo the vapour, after it is reduced into the form of clouds, and defcends, adds lefs weight to the air than before; and therefore the mercury falls. To which it is anfwered, 1. That Refuted. when a body defcends in a fluid, its motion in a very little time becomes uniform, or nearly fo, a farther acceleration of it being prevented by the refiftance of the fluid; and then, by the third law of nature, it forces the fluid downwards with a force equal to that whereby it tends to be farther accelerated, that is, with a force equal to its whole weight. 2. The mercury by its defcent foretells rain a much longer time before it comes, than the vapour after it is condenfed into clouds can be fuppofed to take up in falling. 3. Suppofing that as many vapours as fall in rain during a whole year were at once to be condensed into clouds, and even quite ceafe to gravitate upon the air, its gravity would fcarce be diminished thereby fo much as is equivalent to the defcent of two inches of mercury in the barometer. Befides, in many places between the tropics, the rains fall at certain feasons in very great quantities, and yet the barometer fhows there very little or no alteration in the weight of the atmosphere.

22 Infufficient

of Mr Chambers.

Mr Chambers gives an hypothefis fomewhat fimilar hypothefis to that of Leibnitz: but as it is liable to the objections jult now mentioned, efpecially the laft, we forbear to give any particular account of it; and fhall attempt, upon other principles, to give a fatisfactory folution of this phenomenon.

23 Another theory.

The neceffary preliminaries to our hypothefis are, 1. That vapour is formed by an intimate union between the element of fire and that of water, by which the fire or heat is fo totally enveloped, and its action fo entirely fufpended by the watery particles, that it not only lofes its properties of giving light and of burning, but becomes incapable of affecting the moft fenfible thermometer; in which cafe, it is faid by Dr Black, the author of this theory, to be in a latent ftate. For the proofs of

This

These axioms being established, it thence follows, that as vapour is formed by an union of fire with water, or if we pleafe to call it an elective attraction between them, or folution of the water in the fire, it is impoffible that the vapour can be condensed until this union, attraction, or folution, be at an end. The beginning of the condenfation of the vapour then, or the firit fymptoms of an approaching rain, muft be the feparation of the fire which lies hid in the vapour. may be at firft flow and partial, or it may be fudden and violent: in the firft cafe, the rain will come on flowly, and after a confiderable interval; and in the other, it will be very quick, and in great quantity. But Dr Black hath proved, that when fire quits its latent ftate, however long it may have lain dormant and infenfible, it always affumes its proper qualities again, and affects the thermometer as though it had never been abforbed. The confequence of this must be, that in proportion as the latent heat is discharged from the vapour, it muft fenfibly affect thofe parts of the atmosphere into which it is ditcharged; and in proportion to the heat communicated to thefe, they will become fpecifically lighter, and the mercury fink of courfe. Neither are we to imagine that the quantity of heat difcharged by the vapour is inconfiderable; for Dr Black hath fhown, that when any quantity of water, a pound for inftance, is condenfed from the vapour of a common ftill, as much heat is communicated to the head and refrigeratory as would have been fufficient to heat the pound of water red hot, could it have borne that degree of fenfible heat.

The caufes by which this feparation between the. fire and water is, or may be, effected, come to be confidered under the articles RAIN, CONDENSATION, VAPOUR, &c. Here we have only to obferve, that as the feparation may be gradual and flow, the barometer may indicate rain for a confiderable time before it happens: or if the fenfible heat communicated from the vapour to the atmosphere fhall be abforbed by the colder parts, or by any unknown means carried off, or prevented from affecting the fpecific gravity of the air, the barometer will not be affected; and yet the water being deprived of the heat neceffary to fuftain it, must defcend in rain; and thus it is found that the indications of the barometer do not always hold true. Hence alfo it appears, that tho' the fpecific gravity of the air is diminished, unless that diminution proceeds from a discharge of the latent heat contained in the vapours, no rain will follow; and thus the finking of the barometer may prognofticate wind as well as rain, or fometimes nothing at all.

The difficulty, however, on this hypothefis, is to account for the barometer being stationary in all weathers between the tropica; whereas it ought to move up and down there as well as here, only more fuddenly, as the changes of weather there are more fudden than here. But it must be confidered, that in these climates, during

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