BAROMETER

, an instrument for measuring the weight or pressure of the at mosphere; and by that means the variations in the state of the air, foretelling the changes in the weather, and measuring heights or depths, &c.

This instrument is founded on what is called the Torricellian experiment, related below, and commonly consists of a glass tube, open at one end; which being first filled with quicksilver, and then inverted with the open end downwards into a bason of the same, the mercury descends in the tube till it remains at about the height of 29 or 30 inches, according to the weight or pressure of the atmosphere at the time, which is just equal to the weight of that column of the quicksilver. Hence it follows that, if by any means the pressure of the air be altered, it will be indicated by the rising or falling of the mercury in the tube; or if the barometer be carried to a higher station, the quicksilver will descend lower in the tube, but when carried to a lower place, it will rise higher in the tube, according to the difference in elevation between the two places.

History of the Barometer.—About the beginning of the last century, when the doctrine of a plenum was in vogue, it was a common opinion among philosophers, that the ascent of water in pumps was owing to what they called nature's abhorrence of a vacuum; and that thus fluids might be raised by suction to any height whatever. But an accident having just discovered, that water could not be raised in a pump unless the sucker reached to within 33 feet of the water in the well, it was conjectured by Galileo, who flourished about that time, that there might be some other cause of the ascent of water in pumps, or at least that this abhorrence was limited to the finite height of 33 feet. Being unable to satisfy himself on this head, he recommended the consideration of the difficulty to Torricelli, who had been his disciple. After some time Torricelli fell upon the suspicion that the pressure of the atmosphere was the cause of the ascent of water in pumps; that a column | of water 33 feet high was a just counterpoise to a column of air, of the same base, and which extended up to the top of the atmosphere; and that this was the true reason why the water did not follow the sucker any farther. And this suspicion was soon after confirmed by various experiments. Torricelli considered, that if a column of water 33 feet high were a counterpoise to a whole column of the atmosphere, then a column of mereury of about 2 feet and a half high would also be a counterpoise to it, since quicksilver is near 14 times heavier than water, and so the 14th part of the height, or near 2 feet and a half, would be as heavy as the column of water. This reasoning was soon verisied; for having filled a glass tube with quicksilver, and inverted it into a bason of the same, the mercury presently descended till its height, above that in the bason, was about two feet and a half, just as he expected. And this is what has, from him, been called the Torricellian experiment.

The new opinion, with this confirmation of it, was readily acquiesced in by most of the philosophers, who repeated the experiment in various ways. Others however still adhered to the old doctrine, and raised several pretended objections against the new one; such as that there was a film or imperceptible rope of mercury, extended through the upper part of the tube, which suspended the column of mercury, and kept it from falling into that in the bason. This and other objections were however soon overcome by additional confirmations of the true doctrine, particularly by varying the elevation of the place. It was hinted by Descartes and Pascal, that if the mercury be sustained in the tube by the pressure of the atmosphere, by carrying it to a higher situation, it would descend lower in the tube, having a shorter column of the atmosphere to sustain it, and vice versa. And Pascal engaged his brother-in-law, M. Perier, to try that experiment for him, being more conveniently situated for that purpose than he was at Paris. This he accordingly executed, by observing the height of the quicksilver in the tube, first at the bottom of a mountain in Auvergne, and then at several stations, or different altitudes, in ascending, by which it was found that the mercury fell lower and lower all the way to the top of the mountain; and so confirming the truth of the doctrine relating to the universal pressure of the atmosphere, and the consequent suspension of the mercury in the tube of the barometer. Thus, by the united endeavours of Torricelli, Descartes, Pascal, Mersenne, Huygens, and others, the cause of the suspension of the quicksilver in the tube of the barometer, became pretty generally established.

It was some time however after this general consent, before it was known that the pressure of the air was various at different times, in the same place. This could not however remain long unknown, as the frequent measuring of the column of mercury, must soon shew its variations in altitude; and experience and observation would presently shew that those variations in the mercurial column, were always succeeded by certain changes in the weather, as to rain, wind, frosts, &c. Hence this instrument soon came into use as the means of foretelling the changes of the weather; and on this account it obtained the name of the weather-glass, as it did that of barometer from its being the measure of the weight or pressure of the air. We may now proceed to take a view of its various forms and uses.

The Common Barometer. This is represented at fig. 1, plate iv, such as it was invented by Torricelli. AB is a glass tube, of 1/4, or 1/3, or 1/2 inch wide, the more the better, and about 34 inches long, being close at the top A, and the open end B immersed in a bason of quicksilver CD, which is the better the wider it is. To fill this, or any other barometer; take a clean new glass tube, of the dimensions as above, and pour into it well purified quicksilver, with a small funnel either of glass or paper, in a fine continued stream, till it wants about half an inch or an inch of being full; then stopping it close with the finger, invert it slowly, and the air in the empty part will ascend gradually to the other end, collecting into itself such other small air bubbles as unavoidably get into the tube among the mercury, in filling it with the funnel: and thus continue to invert it several times, turning the two ends alternately upwards, till all the air bubbles are collected, and brought up to the open end of the tube, and when the part filled shall appear, without speck, like a fine polished steel rod. This done, pour in a little more quicksilver, to sill the empty part quite full, and so exclude all air from the tube: then, stopping the orifice again with the finger, invert the tube, and immerse the finger and end, thus stopped, into a bason of like purified quicksilver; in this position withdraw the finger, so shall the mercury descend in the tube to some place as E, between 28 and 31 inches above that in the bason at F, as these are the limits between which it always stands in this country on the common surface of the earth. Then measure, from the surface of the quicksilver in the bason at F, 28 inches to G, and 31 inches to H, dividing the space between them into inches and tenths, which are marked on a scale placed against the side of the tube; and the tenths are subdivided into hundredth parts of an inch by a sliding index carrying a vernier or nonius. These 3 inches, between 28 and 31, so divided, will answer for all the ordinary purposes of a stationary or chamber barometer; but for experiments on altitudes and depths, it is proper to have the divisions carried on a little higher up, and a great deal lower down. In the proper filling and otherwise fitting up of the barometer, several circumstances are to be carefully noted; as, that the bore of the tube be pretty wide, to allow the freer motion of the quicksilver, without being impeded by an adhesion to the sides; that the bason below it be also pretty large, in order that the surface of the mercury at F may not sensibly rise or fall with that in the tube; that the bottom of the tube be cut off rather obliquely, that when it rests on the bottom of the bason there may be a free passage for the quicksilver; and that, to have the quicksilver very pure, it is best to boil it in the tube, which will expel all the air from it. This barometer is commonly sitted up in a neat mahogany case, together with a thermometer and hygrometer, as represented in plate 4, fig. 13.

As the scale of variation is but small, being included within 3 inches in the common barometer, several contrivances have been devised to enlarge the scale, or to render the motion of the quicksilver more sensible.

Descartes first suggested a method of increasing the sensibility, which was executed by Huygens. This | was effected by making the barometrical tube end in a pretty large cylindrical vessel at top, into which was inserted also the lower or open end of a much finer tube than the former, which was partly filled with water, to give little obstruction by its weight to the motion of the mercury, while it moved through a pretty long space of the very sine tube by a small variation of the mercury below it, and so rendered the small changes in the state of the air very sensible. But the inconvenience was this, that the air contained in the water gradually disengaged itself, and escaped through into the vacuum in the top of the small tube, till it was collected in a body there, and by its elasticity preventing the sree rise of the fluids in the tubes, spoiled the instrument as a barometer. And this, it may be observed by-the-bye, is the reason why a water barometer cannot succeed. This barometer is here represented in fig. 2, where CD is the vessel, in which are united the upper or small water tube AC, with the lower or mercurial one CB.

To remedy this inconvenience, Huygens thought of placing the mercury at top, and the water at bottom, which he thus contrived. ADG (fig. 3) is a bent tube hermetically sealed at A, but open at G, of about one line in diameter, and passing through the two equal cylindrical vessels BC, EF, which are about 20 inches apart, and of 15 lines diameter, their length being 10. The mercury being put into the tube, will stand between the middle of the vessels EF and BC, the remaining space to A being void both of air and mercury. Lastly, common water, tinged with a 6th part of aqua regis, to prevent its freezing, is poured into the tube FG, till it rises a foot above the mercury in DF. To prevent the water from evaporating, a drop of oil of sweet almonds floats on the top of it. But the column of water will be sensibly affected by heat and cold, which spoils the accuracy of the instrument. For which reason other contrivances have been made, as below.

The Horizontal or Rectangular Barometer, fig. 4, was invented by J. Bernoulli and Cassini; where AB is a pretty wide cylindrical part at the top of the tube, which tube is bent at right angles at C, the lower part of it CD being turned into the horizontal direction, and close above at A, but open at the lower end D, where however the mercury cannot run out, being there opposed by the pressure of the atmosphere. This and the foregoing contrivance of Huygens are founded on the theorem in hydrostatics, that fluids of the same base press according to their perpendicular altitude, not according to the quantity of their matter; so that the same pressure of the atmosphere sustains the quicksilver that fills the tube ACD, and the cistern B, as would support the mercury in the tube alone. Hence, having fixed upon the size of the scale, as suppose the extent of 12 inches, instead of the 3, in the common barometer from 28 to 31, that is 4 times as long; then the area of a section of the cylinder AB must be 4 times that of the tube, and consequently its diameter double, since the areas of circles are as the squares of their diameters: then for every natural variation of an inch in the cylinder AB, there will be a variation of four inches in the tube CD.—But on account of the attrition of the mercury against the sides of the glass, and the great momentum from the quick motion in CD, the quicksilver is apt to break, and the rife and fall is no longer equa- ble; and besides, the mercury is apt to be thrown out of the orifice at D by sudden motions of the machine.

The Diagonal Barometer of Sir Samuel Moreland, fig. 5, is another method of enlarging the natural scale of three inches perpendicular, or CD, by extending it to any leught BC in an oblique direction. This is liable in some degree to the same inconvenience, from friction and breaking, as the horizontal one; and hence it is found that the diagonal part BC cannot properly be bent from the perpendicular more than in an angle of 45°, which only increases the scale nearly in the proportion of 7 to 5.

Doctor Hook's Wheel Barometer, fig. 6. This was invented about 1668, and is meant to render the alterations in the air more sensible. Here the barometer tube has a large ball AB at top, and is bent up at the lower or open end, where an iron ball G floats on the top of the mercury in the tube, to which is connected another ball H by a cord, hanging freely over a pulley, turning an index KL about its centre. When the mercury rises in the part FG, it raises the ball, and the other ball descends and turns the pulley with the index round a graduated circle from N towards M and P; and the contrary way when the quicksilver and the ball sink in the bent part of the tube. Hence the scale is easily enlarged 10 or 12 fold; being increased in proportion of the axis of the pulley to the length of the index KL. But then the friction of the pulley and axis is some obstruction to the free motion of the quicksilver. Contrivances to lessen the friction &c, may also be seen in the Philos. Trans. vol. 52, art. 29, and vol. 60, art. 10.

The Steelyard Barometer, for so that may be called which is represented by fig. 7, which enlarges the scale in the proportion of the shorter to the longer arm of a steelyard. AB is the barometer tube, close at A and open at B, immersed in a cylindrical glass cistern CD, which is but very little wider than the tube AB is. The barometer tube is suspended to the shorter arm of an index like a steelyard, moving on the fulcrum E, and the extremity of its longer arm pointing to the divisions of a graduated arch, with which index the tube is nearly in equilibrio. When the pressure of the atmosphere is lessened, the mercury descends out of the tube into the cistern which raises the tube and the shorter arm of the index, and consequently the extremity of the longer moves downwards, and passes over a part of the graduated arch. And on the contrary this moves upwards when the pressure of the atmosphere increases.

The Pendant Barometer, fig. 8, was invented by M. Amontons, in 1695. It consists of a single conical tube AB, hung up by a thread, the larger or open end downwards, and having no vessel or cistern, because the conical sigure supplies that, and the column of mercury suftained is always equal to that in the common barometer tube; which is effected thus; when the pressure of the air is less, the mercury sinks down to a lower and wider part of the tube, and consequently the altitude of its column will be less; and on the contrary, by a greater pressure of the atmosphere the mercury is forced up to a higher and narrower part, till the length of the column CD be equal to that in the tube of the common barometer.—The inconvenience of this barometer is, that as the bore must be made very small, to prevent the mercury from falling out by an accidental shake, the | friction and adhesion to the sides of the tube prevent the free motion of the mercury.

Mr. Rowning's Compound Barometers. This gentleman has several contrivances for enlarging the scale, and that in any proportion whatever. One of these is described in the Philos. Trans. N° 427, and also in his Nat. Philos. part 2; and another in the same part, which is here represented at fig. 9. ABC is a compound tube, hermetically sealed at A, and open at C; empty from A to D, filled with mercury from thence to B, and from hence to E with water. Hence by varying the proportions of the two tubes AF and FC, the scale of variation may be changed in any degree.

The Marine Barometer. This was first invented by Dr. Hook, to be used on board of ship, being contrived so as not to be affected or injured by the motion of the ship. His contrivance consisted of a double thermometer, or a couple of tubes half filled with spirit of wine; the one sealed at both ends, with a quantity of air included; the other sealed at one end only. The former of these is affected only by the warmth of the air; but the other is affected both by the external warmth and by the variable pressure of the atmosphere. Hence, considering the spirit thermometer as a standard, the excess of the rise or fall of the other above it will shew the increase or decrease of the pressure of the atmosphere. This instrument is deseribed by Dr. Halley, in the Philos. Trans. N° 269, where he says of it, “I had one of these barometers with me in my late southern voyage, and it never failed to prognosticate and give early notice of all the bad weather we had, so that I depended thereon, and made provision accordingly; and from my own experience I conclude, that a more useful contrivance hath not for this long time been offered for the benefit of navigation.”

Mr. Nairne, an ingenious artist in London, has lately invented a new kind of Marine Barometer; which differs from the common barometer by having the lower part of the tube, for about 2 feet long, made very small, to check the vibrations of the mercury, which would otherwise arise from the motions of the ship. This is also assisted by being hung in gimbals, by a part which subjects it to be the least affected by such motions.

Another sort of Marine Barometer has also been invented by M. Passemente, an ingenious artist at Paris. This contrivance consists only in twisting the middle of the tube into a spiral of two revolutions; by which contrivance the impulses which the mercury receives from the motions of the ship are destroyed, by being transmitted in contrary directions.

The Statical Baroscope, or Barometer, of Mr. Boyle, &c. This consists of a large glass bubble, blown very thin, and then balanced by a small brass weight. Hence these two bodies being of unequal bulk, the larger will be very much affected by a change of the density of the medium, but the less not at all as to sense: So that, when the atmosphere becomes denser, the ball loses more of its weight, and the brass weight preponderates; and contrariwise when the air grows lighter.

Mr. Caswell's Baroscope, or Barometer. This is described in the Philos. Trans. vol. 24, and seems to be the most sensible and exact of any. It is thus described: Suppose ABCD, (fig. 10) is a bucket of water, in which is the baroscope xrezyosm, which consists of a body xrsm, and a tube ezyo, which are both concave cylinders, made of tin, or rather glass, and communicating with each other. The bottom of the tube zy has a leaden weight to sink it, so that the top of the body may just swim even with the suiface of the water by the addition of some grain weights on the top. When the instrument is forced with its mouth downwards, the water ascends into the tube to the height yu. To the top is added a small concave cylinder, or pipe, to keep the instrument from sinking down to the bottom: md is a wire: and mS, de are two threads oblique to the surface of the water, which perform the office of diagonals: for while the instrument sinks more or less by an alteration in the gravity of the air, where the surface of the water cuts the thread is formed a small bubble, which ascends up the thread while the mercury of the common baroscope ascends, and vice versa.

It appears from a calculation which the author makes, that this instrument shews the alterations in the air 1200 times more accurately than the common barometer. He observes, that the bubble is seldom known to stand still even for a minute; that a small blast of wind, which cannot be heard in a chamber, will sensibly make it sink; and that a cloud passing over it always makes it descend, &c.

While some have been increasing the sensibility of the barometer by enlarging the variations, others have endeavoured to make it more convenient by reducing the length of the tube. M. Amontons, in 1688, first proposed this alteration in the structure of barometers, by joining several tubes to one another, alternately filled with mercury and with air, or some other fluid; and the number of these tubes may be increased at pleasure: but the contrivance is perhaps more ingenious than useful.

M. Mairan's reduced Barometer, which is only 3 inches long, serves the purpose of a manometer, in shewing the dilatations of the air in the receiver of an airpump; and instruments of this kind are now commonly applied to this use.

The Portable Barometer, is so contrived that it may be carried from one place to another without being disordered. The end of the tube is tied up in a leathern bag not quite full of mercury; which being pressed by the air, forces the mercury into the tube, and keeps it suspended at its proper height. This bag is usually inclosed in a box, through the bottom of which passes a screw, by means of which the mercury may be forced up to the top of the tube, and prevented from breaking it by dashing against the top when the instrument is removed from one station to another. It seems Mr. Patrick first made a contrivance of this kind: but the portable barometer has received various improvements since; and the most complete of this kind has been described by M. De Luc, in his Recherches, vol. 2, pa. 5 &c, together with the apparatus belonging to it, the method of construction and use, and the advantages attending it. Improvements have also been suggested by Sir George Shuckburgh, and Col. Roy, which have been carried into execution, with farther improvements also, by Mr. Ramsden, and other ingenious artists in London. |

Fig. 11 represents this instrument, as inclosed in its mahogany case by means of three metallic rings aaa. This case is a hollow cone, so shaped within as to contain steadily the body of the barometer, and is divided into three branches from b to c, forming three legs or supports for the instrument when observations are making, and sustaining it at the part d of the case, as it appears in Fig. 12, by an improved kind of gimbals, in which its own weight renders it sufficiently steady at any time. In the part of the frame fg where the barometer tube appears, is made a long slit or opening, that the column of mercury may be seen against the light, and the vernier piece f brought down to coincide very nicely with the edge of the mercury. When the instrument is fixed in its stand, the screw t is to be turned to let the mercury down to its proper station, and a peg at i must be loosened, to admit the external air to act upon the mercury contained in the box k. The proper adjustment, or mode of observing what is called the zero or o division of the column of mercury, is by observing it in the transparent part of the box k, which has a glass tube or reservoir for the quicksilver, and an edged piece of metal attached to the external part of it; with the edge of which the mercury is to be brought into contact by turning the screw l to the right or left as occasion requires. The vernier piece at f, which determines the altitude of the mercurial column, is first brought down by the hand to a near contact, and then accurately adjusted by turning the screw e at the top. The divisions annexed to the tube of this instrument may be of any sort, or of any degree of smallness, according to the purposes it is intended to serve. To accommodate it to the use of foreigners as well as natives, there are commonly added scales of both French and English inches, with their subdivisions to any extent required. It is usual to place the French scale of inches on the right side at fg, from 19 to 31 inches, measured from the zero or surface of the mercury in the box k below; each inch being divided into lines or 12th parts, and each line subdivided by the vernier into 10th parts, or 120th parts of inches; by means of which therefore the length of the mercurial column may be determined to the 120th part of a French inch. The other scale, which is placed on the left side of the instrument, is divided into English inches, and each inch into 20th parts, which by a vernier are subdivided into 25th parts, or 500th parts of inches; by this means shewing the height of the mercury to the 500th part of an English inch. But this vernier is figured double or each division is accounted 2, which reduces the measures to 1000ths of an inch for the conveniency of calculation, in measuring altitudes of hills &c.

A thermometer is always attached to the instrument, as a necessary appendage to it, being fastened to the body at h, and sunk into the surface of the frame, to preserve it from injury: the degrees of this thermometer are marked on two scales, one on each side of it, viz, the scale of Fahrenheit, and that of Reaumur; the freezing point of the former being at 32, and of the latter at o. Also on the right hand side of these two scales there is a third, called a scale of correction, placed oppositely to that of Fahrenheit, with the word add and subtract marked; which shews the necessary correction of the observed altitude of the mercury at any given temperature of the air, indicated by the thermometer.

There are several other pieces of mechanism about the instrument, which will be evident by inspection; and the manner of making the observations, with the necessary calculations, are fully explained in M. de Luc's Recherches fur les Modifications de l'Atmosphere, and the Philos. Trans. vol. 67 and 68, before cited.

The Common Chamber Weatherglass, is also usually fitted up in a neat mahogany frame, and other embellishments, to make it an ornamental piece of furniture. It confists of the common tube barometer, with a thermometer by the side of it, and an hygrometer at the top, as exhibited in fig. 13.

To the foregoing may be added a new sort of Barometer, or Weather Instrument by the Sound of a Wire. This is mentioned by M. Lazowski in his Tour through Switzerland: it is as yet but in an imperfect state, and was lately diseovered there by accident. It seems that a clergyman, though near-sighted, often amused himself with firing at a mark, and contrived to stretch a wire so as to draw the mark to him to see how he had aimed. He observed that the wire sometimes sounded as if it vibrated like a musical cord; and that after such soundings, a change always ensued in the state of the atmosphere; from whence he came to predict rain or sine weather. On making farther experiments, it was found that the sounds were most distinct when extended in the plane of the meridian. And according to the weather which was to follow, it was found that the sounds were more or less soft, or more or less continued; also fine weather, it is said, was announced by the tones of counter-tenor, and rain by those of bass. It has been said that M. Volta mounted 15 chords in this way at Pavia, to bring this method to some precision, but no accounts have yet appeared of the success of his observations.

The Phænomena and Observations of the BarometerAn ingenious author observes that, by means of barometers we may regain the knowledge that still resides, in brutes, and which we forfeited by not continuing in the open air, as they mostly do; and, by our intemperance, corrupting the crasis of our organs of sense. The phænomena of the barometer are various; but authors are not yet agreed upon the causes of them; nor is the use of it, as a weather-glass, yet perfectly ascertained, though daily observations and experience lead us still nearer to precision. Mr. Boyle observes that the phænomena of the barometer are so precarious, that it is exceedingly difficult to form any certain general rules concerning the rise and fall of the mercury. Even that rule fails which seems to hold the most generally, viz, that the mercury is low in high winds. The best rules however that have been deduced by several authors are as follow.

Dr Halley's Rules for judging of the Weather.

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

2. In serene, good, and settled weather, the mercury is generally high.

3. Upon very great winds, though they be not accompanied with rain, the mercury sinks lowest of all, according to the point of the compass the wind blows from. |

4. The greatest heights of the mercury are found upon easterly or north-easterly winds, other circumstances alike.

5. In calm frosty weather, the mercury commonly stands high.

6. After very great storms of wind, when the mercury has been very low, it generally rises again very fast.

7. The more northerly places have greater alterations of the barometer than the more southerly, near the equator.

8. Within the tropics, and near them, there is little or no variation of the barometer, in all weathers. For instance, at St. Helena it is little or nothing, at Jamaica 3-10ths of an inch, and at Naples the variation hardly ever exceeds an inch; whereas in England it amounts to 2 inches and a half, and at Petersburgh to 3 1/3 nearly.

Dr Beal, who followed the opinion of M. Pascal, observes that, cæteris paribus, the mercury is higher in cold weather than in warm: and in the morning and evening usually higher than at mid-day.—That in settled 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, without any perceptible alteration in the barometer.

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 fo 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 engraved 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 salling; 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 added, in the wheel or circular barometer, tap the instrument gently with the singer, 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 following additional rules, more accurately drawn from later and more clofe 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 bein low, it commonly rises again very fast. In settled sair and dry weather, except the barometer fink 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.

Cause of the Phænomena of the Barometer.

To account for the foregoing phænomena 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, Muschenbrock, 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 must theresore 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 moon; as well as some chemical causes operating between the different particles of matter.

Dr. Halley thinks the winds and exhalations sufficient; and on this principle gives a theory, the substance of which may be comprised in what follows:

1st, That the winds must alter the weight of the air in any particular country; and this, either by bringing together a greater quantity of air, and so load- ing the atmosphere of any place; which will be the case as often as two winds blow from opposite parts, at the same time, towards the same point: or by sweeping away some part of the air, and giving room for the atmosphere to expand itself; which will happen when two winds blow opposite ways from the same point at the same time: or lastly by cutting off the perpendicular pressure of the air; which is the case when a single wind blows briskly any way; it being found by experience, that a strong blast of wind, even made by art, will render the atmosphere lighter; and hence the mercury in a tube below it, as well as in others more distant, will considerably subside. See Philos. Trans. N° 292.

2dly, That the cold nitrous particles, and even the air itself condensed in the northern regions, and driven elsewhere, must load the atmosphere, and increase its pressure.

3dly, That heavy dry exhalations from the earth must increase the weight of the atmosphere, as well as its elastic force; as we find the specific gravity of menstruums increased by dissolved salts and metals.

4thly, That the air being rendered heavier by these and the like causes, is thence better able to support the vapours; which being likewise intimately mixed with it, make the weather serene and fair. Again, the air being made lighter from the contrary causes, it becomes unable to support the vapours with which it is replete; these therefore precipitating, are collected into clouds, the particles of which in their progress unite into drops of rain.

Hence he infers, it is evident that the same causes which increase the weight of the air, and render it more able to support the mercury in the barometer, do likewise produce a serene sky, and a dry season; and that the same causes which render the air lighter, and less able to support the mercury, do likewise generate clouds and rain.

But these principles, though well adapted to many of the particular cases of the barometer, seem however to fall short of some of the principal and most obvious ones, besides being liable to several objections.

Leibnitz accounted for the fall of the mercury before rain by another principle, viz, That as a body specifically lighter than a fluid, while it is sustained by it, adds more weight to that fluid than when, by being reduced in bulk, it becomes specifically heavier, and descends; so the vapour, after it is reduced into the form of clouds, and descends, adds less weight to the air than it did before; and hence the mercury sinks in the tube.—But here, granting that the drops of rain formed from the vapours always increasing in size as they fall lower, were continually accelerated also in their motion, and so the air suffer a continued loss of their weight as they descend; it may however be objected, that by the descent of the mercury the rain is foretold a much longer time before it comes, than the vapour can be supposed to take up in falling: that many times, and in different places, there falls a great deal of rain, without any sinking of the mercury at all; as also that there often happens a fall of the mercury without any rain ensuing: and that sometimes the mercury will suddenly sink, in a short space | of time, half an inch or more, which answers to 7 inches of rain, or about one third of the whole quantity falling in the whole year.

Mr. De Luc supposes that the changes observed in the pressure of the atmosphere, are chiefly produced by the greater or less quantity of vapours floating in it: as others have attributed them to the same cause, but have given a different explanation of it. His opinion is, that vapours diminish the specific gravity, and consequently the absolute weight, of those columns of the atmosphere into which they are received, and which, notwithstanding this admixture, still remain of the same height with adjoining columns that consist of pure or dry air. He afterwards vindicates and more fully explains this theory, and applies it to the solution of the principal phenomena of the barometer, as depending on the varying denfity and weight of the atmosphere.

Dr. James Hutton, in his Theory of Rain, printed in the Transactions of the Royal Society of Edinburgh, vol. 1, gives ingenious and plausible reasons for thinking that the lessening the weight of the atmosphere by the fall of rain, is not the cause of the fall of the barometer; but that the principal, if not the only cause, arises from the commotions in the atmosphere, which are chiefly produced by sudden changes of heat and cold in the air. “The barometer, says he, is an instrument necessarily connected with motions in the atmosphere; but it is not equally affected with every motion in that fluid body. The barometer is chiefly affected by those motions by which there are produced accumulations and abstractions of this fluid, in places or regions of sufficient extent to affect the pressure of the atmosphere upon the surface of the earth. But as every commotion in the atmosphere may, under proper conditions, be a cause for rain, and as the want of commotion in the atmosphere is naturally a cause of fair weather, this instrument may be made of great importance for the purpose of meteorological observations, although not in the certain and more simple manner in which it has been, with the increase of science, so successfully applied to the measuring of heights.” See Rain.

In the Encyclopædia Britannica there is another theory of the changes in the barometer, as depending on the heat in the atmosphere, not as producing commotions there, but as altering the specific gravity of the air by the changes of heat and cold. The principles of this theory are, 1st, That vapour is formed by an intimate union between the elements of fire and water, by which the fire or heat is so totally enveloped, and its action so perfectly suspended by the aqueous particles, that it not only loses its properties of burning and of giving light, but becomes incapable of affecting the most sensible thermometer, in which case it is said to be in a latent state: and 2d, That if the atmosphere be affected by any unusual degree of heat, it thence becomes incapable of supporting so long a column of mercury as before; for which reason it is that the barometer sinks.

From these axioms it would follow, that as vapour is formed by an union of fire with water, whether by attraction or a solution of the water in the fire, the vapour cannot be condensed till this union, attraction, or solution, is at an end. Hence the beginning of the condensation of the vapour, or the first signs of approaching rain, must be the separation of the fire which is latent in the vapour. In the beginning, this may be either slow and partial, or it may be sudden and violent: in the first case, the rain will come on slowly, and after a considerable time; but in the other, it will come very quickly, and in a great quantity. But Dr. Black has proved, that when fire quits its latent state, however long it may have lain dormant and insensible, it always reassumes its proper qualities, and affects the thermometer just the same as if it had never been absorbed. The consequence of this is, that in proportion as the latent heat is discharged from the vapour, those parts of the atmosphere into which it is discharged must be sensibly affected by it; and in proportion to the heat communicated to those parts, they will become specifically lighter, and the mercury will sink of course.

In the Memoirs of the Literary Society of Manchester, vol. 4, is also a curious paper on this subject, viz, Metevrological Observations made on different Parts of the Western Coast of Great Britain: arranged by T. Garnett, M. D. This paper is composed of materials furnished by several observers; those of Mr. Copland, surgeon at Dumfries, are of special importance. This gentleman is of opinion that the changes of the barometer indicate approaching hot and cold weather, with much more certainty than dry and wet. “Every remarkable elevation of the barometer, says he, where it is of any duration, is followed by very warm or dry weather, and moderate as to wind, or by all of them; but heat seems to have most influence and connexion; and when it is deficient, the continuance of the other two will be longer and more remarkable; therefore the calculation must be in a compound ratio of the excess and deficiency of the heat, and of the dryness of the weather in comparison of the medium of the sea son; and with regard to the want of strong wind, it appears to be intimately connected with the last, as they shew that no precipitation is going on in any of the neighbouring regions.”

In his 14th and 15th remarks, he had said,

‘14th, That the barometer being lower, and continuing so longer than what can be accounted for by immediate falls, or stormy weather, indicates the approach of very cold weather for the season; and also, cold weather, though dry, is always accompanied by a low barometer, till near its termination.’

‘15th, That warm weather is always preceded and mostly accompanied by a high barometer; and the rising of the barometer in the time of broken or cold weather, is a sign of the approach of warmer weather: and also if the wind is in any of the cold points, a sudden rise of the barometer indicates the approach of a southerly wind, which in winter generally brings rain with it.’

In the two following remarks, Mr. Copland had explained certain phenomena from a principle similar to that on which Dr. Darwin has so much insisted: (Betanic Garden, I. notes p. 79, &c.)

‘That the falling of the barometer may proceed from a decomposition of the atmosphere occurring a- | round or near that part of the globe where we are placed, which will occasion the electricity of the atmosphere to be repelled upwards in fine lambent portions; or driven downwards or upwards in more compacted balls of fire; or lastly, to be carried along with the rain, &c, in an imperceptible manner to the surface of the earth: the precipitation of the watery parts generally very soon takes place, which diminishes the real gravity of the atmosphere, and also by the decomposition of some of the more active parts, the air loses part of that elastic and repulsive power which it so eminently possessed, and will therefore press with less force on the mercury of the barometer than before, by which means a fall ensues.

‘That the cause of the currents of air, or winds, may also be this way accounted for: and in very severe storms, where great decompositions of the atmosphere take place, this is particularly evident, such as generally occur in one or more of the West India islands at one time, a great loss of real gravity, together with a considerable diminution of the spring of the air immediately ensues; hence a current commences, first in that direction whence the air has most gravity, or is most disposed to undergo such a change; but it being soon relieved of its superior weight or spring on that side, by the decomposition going on as fast as the wind arrives on the island, it immediately veers to another point, which then rushes in mostly with an increase of force; thus it goes on till it has blown more than half way round the points of the compass during the continuation of the hurricane. For in this manner the West India phenomena, as well as the alteration of the wind during heavy rains in this country, can only be properly accounted for.’ See remark No. 4.

Mr. C.'s 4th aphorism is, ‘That the heaviest rains, when of long continuance, generally beign with the wind blowing easterly, when it gradually veers round to the south; and that the rain does not then begin to cease till the wind has got to the west, or rather a little to the northward of it, when, it may be added, it commonly blows with some violence.’

Many other observations on the barometer, the weather, &c. may be seen in various parts of the Philos. Trans. And for other curious papers on the same, and other subjects connected with the barometer, see the Gentleman's Magazine for 1789, p. 317; also Greu's Journal of Nat. Philos. printed at Leipzig 1792, for the influence of the sun and moon upon the barometer.

The Barometer applied to the measuring of Altitudes.

The secondary character of the barometer, namely as an instrument for measuring accessible 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, Shuckburgh, Roy, and more especially by 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 barometer to the measurement of accessible heights. And for this purpose serves the portable barometer, before described, (fig. 11 and 12, plate 4,) 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 more accurate by the indefatigable researches of De Luc, by introducing into it the corrections of the columns of mercury and air, on account of heat. And other corrections and modifications of the same may be seen inserted under the article ATMOSPHERE, where the most correct rule is deduced from one single experiment only. This rule is as follows:

The Rule for Computing Altitudes, is this,

Viz, 10000 X log. of M/m is the altitude in fathoms, in the mean temperature of 31°; and for every degree of the thermometer above that, the result must be increased 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 same sort of measures, whether feet, or inches, or tenths, &c, and either English, or French, or of any other nation; but the refult is always in fathoms, of 6 English feet each.

And the Precepts, in words, for the practice of measurements by the barometer, are these following:

1st, Observe the height of the barometer at the bottom of any height or depth, proposed to be measured; together with the temperature of the mercury by means of the thermometer attached to the barometer, and also the temperature of the air in the shade by another thermometer 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 9600th part for every degree 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.

3dly, Take out the common logarithms of the 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 7 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 sor 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 31°, but subtract them if it be below 31°; 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 1. Let the state of the barometers and thermometers be as follows, to find the altitude: viz.

Example 2. To sind the altitude of a hill, when the state of the barometer and thermometer, as observed at the bottom and top of it, is as follows; viz,

See this rule investigated under the article PNEUMATICS, at the end.

N. B. The mean height of the barometer in London, upon an average of two observations in every day of the year, kept at the house of the Royal Society, for many years past, is 29.88; the medium temperature, or height of the thermometer, according to the same, being 58°. But the medium height at the surface of the sea, according to Sir Geo. Shuckburgh (Philos. Trans. 1777, p. 586) is 30.04 inches, the heat of the barometer being 55°, and of the air 62°.

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ABCDEFGHKLMNOPQRSTWXYZABCEGLMN

Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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BALLUSTER
BALLUSTRADE
BAND
BANQUET
BARLOWE (William)
* BAROMETER
BAROSCOPE
BARREL
BARRICADE
BARRIER
BARROW (Isaac)