and fair weather, the mercury is higher than either a little before or after, or in the rain; and that it commonly descends lower after rain than it was before it. And he ascribes these effects to the vapours with which the air is charged in the former case, and which are dispersed by the falling rain in the latter. If it chance to rise higher after rain, it is usually followed by a settled serenity. And that there are often great changes in the air, with out any perceptible alteration in the barome

ter.

Mr. Patrick's Rules for judging of the Weather.-These are esteemed the best of any general rules hitherto made:

1. The rising of the mercury presages, in general, fair weather; and its falling, foul weather, as rain, snow, high winds, and

storms.

2. In very hot weather, the falling of the mercury indicates thunder.

3. In winter, the rising presages frost: and in frosty weather, if the mercury falls 3 or 4 divisions, there will certainly follow a thaw. But in a continued frost, if the mercury rises, it will certainly snow.

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

5. In foul weather, when the mercury rises much and high, and so continues for 2 or 3 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 2 or 3 days before the rain comes; then expect a great deal of wet, and probably high winds.

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

8. You are not so strictly to observe the words engraven on the plates, as the mercury's rising and falling; though in general it will agree with them. For if it stands at much rain, and then rises up to changeable, it presages fair weather; though not to continue so long as if the mercury had risen higher. And so, on the contrary, if the mercury stood at fair, and falls to changeable, it presages foul weather; though not so much of it as if it had sunk lower.

Upon these rules of Mr. Patrick the following remarks are made by Mr. Rowning. That it is not so much the absolute height of the mercury in the tube that indicates the weather, as its motion up and down: wherefore, to pass a right judgment of what weather is to be expected, we ought to know whether the mercury is actually rising or falling; to which end the following rules are of use.

1. If the surface of the mercury is convex, standing higher in the middle of the tube than at the sides, it is a sign that the mercury is then rising.

2. But if the surface be concave, or hollow in the middle, it is then sinking. And,

3. If it be plain, or rather, a very little convex, the mercury is stationary: for mercury being put into a glass tube, especially a small one, naturally has its surface a little convex, because the particles of mercury attract one another more forcibly than they are attracted by glass. Farther,

4. If the glass be small, shake the tube; then if the air be grown heavier, the mercury will rise about half a 10th of an inch higher than it stood before; but if it be grown lighter, it will sink as much. And, it may be added, in the wheel or circular barometer, tap the instrument gently with the finger, and the index will visibly start forwards or backwards according to the tendency to rise or fall at that time. This proceeds from the mercury's sticking to the sides of the tube, which prevents the free motion of it till it be disengaged by the shock: and therefore, when an observation is to be made with such a tube, it ought to be first shaken; for sometimes the mercury will not vary of its own accord, till the weather is present which it ought to have indicated.

And to the foregoing may be added the forlowing rules, more accurately drawn from later and more close observation of the motions of the barometer, and the consequent changes in the air in this country.

1. In winter, spring, and autumn, the sudden falling of the mercury, and that for a large space, denotes high winds and storms; but in summer it denotes heavy showers, and often thunder: and it always sinks lowest 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. Also, if, after rain, the wind change into any part of the north, with a clear and dry sky, and the mercury rise, it is a certain sign of fair weather.

2. After very great storms of wind, when the mercury has been low, it commonly rises again very fast. In settled fair and dry weather, except the barometer sink much, expect but little rain; for its small sinking then is only for a little wind, or a few drops of rain and the mercury soon rises again to its former station. In a wet season, suppose in hay-time and harvest, the smallest sinking of the mercury must be minded; for when the constitution of the air is much inclined to showers, a little sinking in the barometer then denotes more rain, as it never then stands very high. And if, in such a season, it rise suddenly, very fast, and high, expect not fair weather more than a day or two, but rather that the mercury will fall again very soon, and rain immediately to follow: the slow gradual rising, and keeping on for 2 or 3 days, being most to be depended on for a week's fair weather. And the unsettled state of the quicksilver always denoting uncertain and changeable weather, especially when the mercury stands any where about the word changeable on the scale.

3. The greatest heights of the mercury, in this country, are found upon easterly and north-easterly winds; and it may often rain or snow, the wind being in these points, and the

barometer sink little or none, or it may even be in a rising state, the effect of those winds counteracting. But the mercury sinks for wind, as well as rain, in all the other points of the compass; but rises as the wind shifts about to the north or east, or between those points: but if the barometer should sink with the wind in that quarter, expect it soon to change from thence; or else, should the fall of the mercury be much, a heavy rain is then likely to ensue, as it sometimes happens.

Several members of a German meteorological society have registered many observations upon the barometer. The most noted of the observers are M. M. Steilehner, Planer, Chiminello, and Hemmer. The first says, that he found, by several comparative observations, that the greatest fall of the barometer does not happen in very remote places at the same time, but that it is earlier towards the west, and later towards the east, and that the difference of the time is nearly equal to the difference of the meridians of the places; an assertion which indeed deserves to be more accurately examined.

M. Planer observed the barometer for a whole year, six times every day, or every fourth hour of the natural day, at two, six, and ten in the morning, and at the same hours after noon; and found, in general, that the barometer, between ten in the morning and two in the afternoon, and between ten at night and two in the morning, was less in its rising, and greater in its fall; and that the contrary was the case between the hours of six and ten in the evening and morning. M. Chiminello observed the barometer twenty-two times a day for three years, but he left a chasm in the night which he supplied by calculation. The principal positions which he thence deduced are, that the barometer falls towards noon, as well as towards midnight.

M. Hemmer from a great number of accurate observations, deduced the three following general rules.

1st, When the sun passes the meridian, the barometer, if in the act of falling, continues to fall, and the falling is accelerated.

2d, When the sun passes the meridian, the barometer, if in the act of rising, falls, or becomes stationary, or rises more slowly.

3d, When the sun passes the meridian, the barometer, which is stationary, falls, if it has not risen before or after being stationary; in which case it usually becomes stationary during the sun's passage.

From a register kept by a Mr. Dunbar near the banks of the Missisippi, in N. lat. 31. 28, we find, that, for the space of about four days before, and six days after the summer solstice, the barometer regularly rises from about 9 P.M. to about 6 A.M. then falls till the return of the former hour in the evening, then rises again as before, &c. in the alternate periods. In the first four days the direction is ascending, and the elevation of a line drawn through the mean is about of an inch. In the latter six days the mean line is perfectly horizontal,

the elevation each night amounting to 1 and the depression each day to the same, but occupying double time.

Humboldt made some interesting observations at Carraccas, in South America, near the equator. "I have read," says he, "in the Transactions of the Bengal Society, that the barometer rises and falls there regularly every 24 hours. Here, in South America, its motion is more astonishing. There are four atmospherical tides every 24 hours, which depend only on the attraction of the sun. The mercury falls from 9 in the morning to 4 in the evening: it rises from 4 to 11 o'clock: it falls from 1 o'clock till past 4 in the morning: and it reascends from that time till 9 o'clock. Neither winds, storms, nor earthquakes, have any influence on this motion." Being fearful of extending this article beyond its due length, we refer to No. 4 of the Retrospect of Philosophical, &c. Discoveries, for Mr. Horsburgh's observations on a diurnal variation of the Barometer between the tropics; and for a Comparison of some Observations on the diurnal variations of the Barometer, made in Peyrouse's Voyage round the World, with those made at Calcutta by Dr. Balfour.

Cause of the Phenomena of the Barometer. -To account for the foregoing phenomena of the barometer, many hypotheses have been framed, which may be reduced to two general heads, viz, mechanical and chemical. The chief writers upon these causes are, Pascal, Beal, Wallis, Garcin, Garden, Lister, Halley, Garsten, De la Hire, Mariotte, Le Cat, Woodward, Leibnitz, De Mairan, Hamberger, D. Bernoulli, Muschenbroek, Chambers, De Luc, Black, &c; and an account of most of their hypotheses may be seen at large in M. De Luc's Recherches sur les Modifications de l'Atmosphere, vol. 1. chap. 3; see also the Philos. Trans, and various other works on this subject. It may suffice to notice here slightly a few of the principal of them.

Dr. Lister accounts for the changes of the barometer from the alterations by heat and cold in the mercury itself; contracting by cold, and expanding by heat. But this, it is now well known, is quite insufficient to account for the whole of the effect.

The changes in the weight or pressure of the atmosphere inust therefore be regarded as the principal cause of those in the barometer. But then, the difficulty will be to assign the cause of that cause, or whence arise those alterations that take place in the atmosphere, which are sometimes so great as to alter its pressure by the 10th part of the whole quantity. It is probable that the winds, as driven about in different directions, have a great share in them; vapours and exhalations, rising from the earth, may also have some share; and some perhaps the flux and reflux occasioned in the air by the sun and moon; as well as some chemical causes operating between the different particles of matter.

M. De Luc supposes that the changes observed in the pressure of the atmosphere, are

chiefy produced by the greater or less quantity of vapours floating in it: as others have attribated them to the same cause, but have given a different explanation of it. His opinion is, that vapours diminish the specific gravity, and seruently the absolute weight, of those columns of the atmosphere into which they are received, and which, notwithstanding this adEstare, still remain of the same height with the adjoining columns that consist of pure or dry a. He afterwards vindicates and more fully ains this theory, and applies it to the solution of the principal phenomena of the barometer, epending on the varying density and weight of the atmosphere.

Dr. James Hutton, in his Theory of Rain, printed in the Transactions of the Royal Somety of Edinburgh, vol. 1, gives ingenious and pausible reasons for thinking that the lessenig the weight of the atmosphere by the fall of is not the cause of the fall of the barome: but that the principal, if not the only use, arises from the commotions in the athere, which are chiefly produced by sudvenetanges of heat and cold in the air. The *ter," says he, "is an instrument necesconnected with motions in the atmosphere, Li is not equally affected with every motion in that fluid body. The barometer is chiefly arcted by those motions by which there are paired accumulations and abstractions of this fei, in places or regions of sufficient extent to affe: the pressure of the atmosphere upon the rice of the earth. But as every commotion the atmosphere may, under proper condiwe, be a cause for rain, and as the want of action in the atmosphere is naturally a use of fair weather, this instrument may be Fate of great importance for the purpose of orological observations, although not in de certain and more simple manner in which ars been, with the increase of science, successfully applied to the measuring of bts. See RAIN.

In the Encyclopædia Britannica there is another theory of the changes in the barometer, a depending on the heat in the atmosphere, botas producing commotions there, but as Atering the specific gravity of the air by the the zes of heat and cold. The principles of theory are, 1st, that vapour is formed by timate union between the elements of fire and water, by which the fire or heat is so tobuy envelosed, and its action so perfectly susd by the aqueous particles, that it not loses its properties of burning and of givFeht, but becomes incapable of affecting most sensible thermometer, in which case ai said to be in a latent state: and 2d, that #thesimsphere be affected by any unusual dep-of heat, it thence becomes incapable of sporting so long a column of mercury as bef for which reason it is that the barometer

But M. Hemmer, the result of whose obserwims we have already stated, rejects the foreEz theories and makes the subsequent remarks. VOL. II.

"We e may ask those who ascribe the falling of the barometer to heat, why should this cause be so closely confined to the period between 11 in the morning and the middle of the first hour after midnight? Why does it not take place earlier or later, when the mercury rises continually the whole day, or several days either fall a little towards noon, or at least become stationary or rise slowly? and why does not this rather take place several hours after noon, the time at which the heat generally is the greatest? But that heat is not the cause of this phenomenon is proved by the falling, of the mercury at midnight; which is no less common and certain than the falling at noon: on account of the cold, it ought then rather to rise or to fall. We may assert the same thing of vapour as of heat, as these have the closest connection with each other. As a certain accumulation of vapours dissolved in the atmosphere makes the air lighter, and obliges the barometer to fall, the power with which it acts on the instrument must be in proportion to its quantity. When it obliges the barometer, therefore, to fall towards noon, why does the latter so often rise afterwards? What then lessens the quantity of these vapours when the heat, which by its nature promotes their solution, still increases, or at any rate does not become less? How does it happen that the barometer falls in the night? as about that time the vapours are, for the most part, precipitated from the atmosphere. Why do they exercise the greatest force exactly at inid night? and, by what law of nature are they more connected with that than with any other period? It may here evidently be seen, that the constant falling of the barometer about noon and midnight can be occasioned neither by heat nor by the effect of vapours.

"Besides these two causes, and the winds, we are acquainted with no others in our atmosphere which can make the barometer to fall. But as the wind often becomes calm about noon and midnight, or, when it blows, is of such a nature that it does not make the column of air which presses on the barometer lighter, it is evident that the falling of the mercury about these times cannot arise from the wind.

"What then remains but to seek out of our atmosphere for the cause of this phenome non? It is evident that it is to be found in the sun; for, as by its attraction it moves the water of the ocean, it must exercise a still stronger action on the column of air; and from what is known respecting the flux and reflux of the sea, the whole phenomenon may be easily explained. In the first place, as the atmospheric flux and reflux themselves must depend on the sun's passage over the meridian, it must also take place both at the diurnal and the nocturnal passage of the sun; because the flux and reflux of the atmosphere, like those of the sea, must happen at the same time on opposite sides of the terrestrial hemisphere. In the last place, the atmospheric flux which produces

the falling of our barometer is only an effect of the flux which takes place between the tropics, where the air dur ng the passage of the sun ascends, and therefore proceeds thence from us, as happens in all the tides of the sea which lie beyond these circles. That these tides, which arise by communication, are transmitted more speedily and by a longer duration in the atmosphere than in the ocean, may be concluded from this circumstance, that the particles of the air have less gravity and less mutual adhesion." Phal. Mag. No. 42.

The Barometer applied to the measuring of Andes.-The secondary character of the baroacter, namely as an instrument for measuring accesible heights or depths, was first proposed by Pascal and Descartes, as has been before observed; and succeeding philosophers have been at great pains to ascertain the proportion between the fall of the barometer and the height to which it is carried; as Halley, Mariotte, Maraldi, Scheuchzer, J. Cassini, D. Bernoulli, Horrebow, Bouguer, Schuckburg, Roy, and more especially De Luc, who has given a critical and historical detail of most of the attempts that have at different times been made for applying the motion of the mercury in the baromicter to the measurement of accessible heights. To this purpose is applied the portable barometer, before described, which should be made with all the accuracy possible. Various rules have been given by the writers on this subject, for computing the height ascended from the given fall of the mercury in the tube of the barometer, the most accurate of which was that of Dr. Halley, till it was rendered much inore accurate by the indefatigable researches of De Luc, who introduced into it the corrections of the columns of mercury and air on account of heat. And other corrections and modifications of the same have been given by various philosophers, as Robison, Trembley, Englefield, Prony, &c. A very commodious rule is that investigated by Dr. Hutton, which is

this:

M

m

is the altitude in fa

Viz. 10000 × log. of thoms, in the mean temperature of 31o; and for every degree of the thermometer above that, the restilt must be iacrewed by so many times its 435th part, and diminished when below it: in which theorem M denotes the length of the column of mercury in the barometer tube at the bottom, and m that at the top of the hill, or other eminence; which lengths may be expressed in any one and the saine sort of micaSures, whether feet, or inches, or tenths, &c., and either English or French, or of any other nation; but the result is always in fathoms, of 6 Engish feet each.

The precepts, in words, for the practice of measurements by the barometer, are these following:

11. Observe the height of the barometer at the bottom of any height or depth proposed to be misured; together with the temperature of the mercury by means of the thermometer at

tached to the barometer, and also the temperature of the air in the shade by another thermomcter which is detached from the barometer.

2dly. Let the same thing be done also at the top of the said height or depth, and as near to the same time with the former as may be. And let those altitudes of mercury be reduced to the same temperature, if it be thought necessary, by correcting either the one or the other, viz. augmenting the height of the mercury in the colder temperature, or diminishing that in the warmer, by its 9/00th part for every deeee of difference between the two; and the altitudes of mercury, so corrected, are what are denoted by M and m in the algebraic formula above stated.

3dly. Take out the common logarithms of Che two heights of mercury, so corrected, and subtract the less from the greater, cutting off. from the right hand side of the remainder three places for decimals, so shall those on the left be fathoms in whole numbers, the tables of logarithms being understood to be such as have seven places of decimals.

4thly. Correct the number last found, for the difference of the temperature of the air, as follows: viz. take half the sum of the two temperatures of the air, shewn by the detached thermometers, for the mean one; and for every degree which this differs from the standard temperature of 31°, take so many times the 435th part of the fathoms above found, and add them if the mean temperature be more than 319, but subtract them if it be below 310; so shall the sum or difference be the true altitude in fathoms, or being multiplied by 6, it will give the true altitude in English feet.

Example. Let the state of the barometers and thermometers be as follows, to find the altitude: viz.