Barometer, an instrument of modern invention, for measuring and ascertaining the weight of the atmosphere, as well as the height of maintains, and likewise foretelling, with tolerable accuracy, the probable changes of the weather. That such an instrument must be of extensive utility to every person engaged in the active pursuits of life, whether those of gardening and agriculture, or in the various departments of the domestic and useful arts, will be universally admitted. Hence we propose to bestow a considerable share of attention on this interesting subject.

History and analysis:—When Galileo, in the beginning of the seventeenth century, discovered that water could not ascend in a pump, unless the sucker reached within 33i feet of its surface in the well, he justly concluded that the ascent of water in pumps, was effected by the pressure of the atmosphere, and not by the power of suction ; that a column of water 33 feet high was a counterpoise to one of air of an equal diameter and base, the height or which extended to the top of the atmosphere;i and that consequently the water could not be at traded any farther by the sucker. This important discovery induced his great pupil Torri-CKixt, to substitute, a column of mercury for that of water; because the former fluid being about 14 times heavier than the latter, he wanted, according to that proportion, only about 291/2 inches of quicksilver to determine the accuracy of his experiment. He accordingly found that, after Having filled a glass tube with mercurv, and inverted it in a bason of the same semi-metal, it descended in the tube till it became stationary at about 291/2 inches above the surface of that contained in the lower jel. Many years, however, elapsed after this experiment, before any notice was taken of the circumstance, that this pressure of the air considerably varied at different times, though the tube was uniformly kept in the same situation. Indeed, these variations in the mercurial column, were too obvious to remain long unnoticed; and philosophers began minutely to mark their degrees. As soon, therefore, as this point was properly attended to, they observed that the changes in the rise and fail of the mercury were in general very speedily succeeded by variations in the weather. Hence the instrument obtained the name of weather-glass, for which purpose it has, since that period, been generally employed.

It is surprizing that the ancients were unacquainted with the laws by which the ambient air presses on our bodies as well as on all inanimate matter; and that Otto Gukricke, a German, to whom the world is indebted for the discovery of the air-pump, was the first who excited the attention of philosophers to this important subject. Although TORRICELLi had previously ascertained, by his experiment made in the year 1646, thai the mercury in a tube of four feet in length did not remain stationary at 291/2 inches, but varied according to the greater or less de-of density of the atmosphere; not appear that he applied great discovery to the purpose of predicting the future, or impending changes of the wea ther. Even prior to that date, namely, in 1643, experiments were institute.1, both at Florence and Rome, with the Torricellian tube; and in l648 Perifer, a Frenchman, made use of two similar in-struments; one of which he left in a valley, while with the other he ascended one of the highest mountains of Auvergne: and thus he observed, that the atmosphere on the top of the hill did not press upon the mercury with a force equal to that observed in a lower situation, where he found the quicksilver much lower than in the former region; and hence he judiciously concluded, that the air must be lighter, or more rarefied, in proportion to the altitude of places. But that these remarkable changes were in any degree connected with the present, or future, constitution of the weather, was doubtless discovered by Gue-ricke, though Mr. BoYle, whose talents and genius deserve the highest admiration, improved and applied it to philosophical purposes. This assertion is amply supported by a passage in a printed work still extant, and entitled "Casp. Schotti Technica curiosa" (ii. 12. p. 52); by which it appears, from a letter addressed to the author by Gue-riCKE, that the latter had constructed a tube filled with mercury, on the top of which fluid he placed a small wooden figure, that rose and fell with the quicksilver, pointing out with its finger, or index, the variations in the gravity of the air, and at the same time the concomitant changes of the weather.

The compound barometer, the tube of which is filled not only with mercury, but likewise with another coloured fluid, was in-vented by Cartesius and HuvGENS, much about the same time; but the latter considerably improved it, by using a double tube, and mixing the water with one sixth part of aqua-fortis, to prevent its congelation in winter.

The conical or pendent barometer was contrived by M. Amontons ; the well-known diagonal barometer, by Sir Samuel Moke-land ; the wheel barometer, by Dr. Robert Hooke, who likewise improved Huvgens's double barometer above-mentioned ; - the rectangular horizontal barometer was invented by Bernouilli, or rather Cassini ; and the credit of another invention, by which four tubes (containing mercury, and a lighter fluid alternately) are con-necked together, is likewise due to Amontons.

The marine barometer was also invented in the beginning of the 18th century, by Dr. R. Hooke, and afterwards rendered more perfect by M. Passement, who, by the simple, but effectual expedient, of twisting the middle part of the common, or Torricellian tube, into a spiral of two revolutions, prevented the oscillations of the mercury on ship-board. By this ingenious contrivance, the shocks which the mercury sustains from the motions of a ship, are effectually broken; as, from the turns of the instrument, the impulses are transmitted in contrary directions.

But of all the instruments contrived for the purpose of ascertaining the specific gravity of the atmosphere, that of Magellan, in the year 1/65, deserves a distinguished place; as it points out not only the changes occurring in the atmosphere, but likewise, and in a very sensible manner, the different degrees of temperature of the air.

Among the travelling barometers, that of M. de Luc, improved by J. F. Luz, is the most useful and compendious.—A wind-baro-meter has also been contrived by Mr. Wilkes, of which we pro-to give an account under the article WlND'.

A great variety of other useful instruments have been invented since the days of Torricelli, upon similar principles, and with mechanical additions more or less complicated ; but it must be acknowledged, that the simple barometer of his invention is the most exact, though not the most sensible balance for weighing the atmosphere, and has therefore been most gene-rally adopted.

Rationale.—M. de Luc, not satisfied with the different hypotheses brought forward by Wallis, Halley, Leibnitz, MairAn, etc. to explain the variations of the barometer, has satisfactorily refuted the conjectures of all his predecessors, and endeavoured to establish his own, which is founded on a supposition, that a column of air, loaded with vapours, is lighter than a column of pure air of equal bulk. He consequently asserts, 1. Tha the density of air is the immediate and only cause that supports the mercury; and 2. That the more elastic the air is, the less does it is and weigh upon its base : but though there is considerable ingenuity in this explanation, it is no mean conclusive.

Soon after the publication of this theory, M. Beguelin, in 17/3, endeavoured to prove, that the variation of the whole mass of the atmosphere, as well as that of the spring or elasticity in a part of this mass, are the (wo general causes of the variations of the barometer ; and that the primitive causes of this remarkable effect are, heat, cold', dryness, and moisture, with their different combinations.

We cannot enter into the particulars relative to the operation of these various causes, but shall briefly observe, that heat dilates the air; cold contracts it, and draws together those parts which it is the property of heat to separate : hence, the natural effect of the former is expansion 3 the consequences of the latter, condensation, compression of its spring, and an increase of pressure, on account of which, the mercury rises in the barometer.

Respecting the effects of dryness and moisture, it deserves to be remarked, that, if the latter diminishes the pressure of the air, by relaxing its spring, it on the other hand loads it with watery particles, which very considerably augment its mass, so that it may be difficult to determine how far the moisture of the air exerts its influence on the rise and fall of the barometer. The same observation may be applied to dryness, which dispels the watery particles that increased the weight of the air; so that there appears to be a perpetual conflict between the effect of spring and mass, between. elasticity and weight; and experi-ence alone can inform us whether, in this contest of the elements, the effect of elasticity is much superior to that of weight. Such is the plausible explanation of Beguelin, and we have only to regret, that the result of his inquiries, still leaves the subject involved in much doubt and difficulty because even this theory does not account for the changes of the atmosphere:

Nevertheless, experience has furnished us with a sufficient number of facts, from which we may, with tolerable precision, ascertain the present, and predict the future state of the surrounding element, by the practical use of the barometer. Thus, 1. The rising of the quick-silver generally presages fair weather, as its falling does the contrary, or rain, snow, high winds and storms; 2. In very hot weather, the sudden falling of the mercury portends thunder; 3. In winter, the rising indicates frost; and in frosty Weather, if the mercury falls three or four divisions, there will Certainly follow a thaw ; but if it rise in a continued frost, it will always be accompanied with snow; 4. When foul weather quickly succeeds after the falling of the mercury, it will not be of long duration ; nor are we to expect a continuance of fair weather, when it succeeds the rise of the quick-silver ; 5.If, in foul weather, the mercury happens to ascend considerably, and continues in an advancing state for two or three days successively, then we may expect also a continuance of fair weather ; 6. If, in clear weather, the mercury falls remarkably for faro or three days together, before the rain sets in, it is then highly probable that it will be succeeded by much rain, and perhaps high winds ; 7. The unsettled motion of the mercury indicates changeable weather ; 8. Respecting the words engraved on the register-plate, they cannot be strictly relied upon to correspond-exactly- with the state of the weather; though it will in general agree with them as to the mercury rising and falling. These words deserve to be particularly noticed when the mercury removes from Changeable" upwards ; as those on the lower part should be adverted to, when the quicksilver falls from "Changeable" downwards. In other cases, they are of no use; for as its rising in any part forebodes a tendency to fair, and its filling to foul weather, it follows, that though it descend in the tube from Settled to Fair, it may never-ss be attended with a little rain; and when it rises from the words "Much Rain" to "Rain, " it shews only an inclination to become fair, though the wet weather may still continue in a less considerable degree than it was when the mercury in to rise. But if the mercury, after having fallen to "Much Rain, " should ascend to "Chang able, " it predicts fair weather, though of a shorter continuance than if the quicksilver had risen still higher; and so, on the contrary, if the mercury stood at "Fair, " and descends to "Changeable, " it presages foul weather, though not of such duration, as if it had fallen .lower.

These observations are founded on experience, and we are indebted for them chiefly to Mr. Patrick, who has investigated this subject with considerable precision. It appears from the result of these facts, that the height of the mercury is not the principal criterion for ascertaining the probable changes of the weather, but rather the relative motion of that fluid in the tube. Hence, to enable us to judge right-ly of the impending variations, we ought to possess a correct knowledge, whether the mercury is actually rising or falling. For this purpose, the following rules, stated by Mr. Rowing, may be of advantage : I. If the surface of the mer-cury be convex, standing higher in the middle of the tube than at the sides, it generally indicates the rising of this fluid metal; 2. If its surface be concave, is is then sinking; 3. If it appear level, it is stationary: 4. If, after shaking the tube of a small glass, the mercury - rises about half a tenth of an inch higher than it stood before, it is a proof that the air has become heavier; but if it sinks as much, it follows that the atmosphere is lighter. Hence, in making observations on the weather, such a glass should always be previously shaken; because the metal which adheres to the sides of the tube, prevents its free motion, till disengaged by a slight agitation of the instrument. These phenomena are peculiar to places situated at a distance from the equator, and therefore deserve to be attended to in our climate : on the contrary, at St. Helena, they would be of little or no service ; for, according to the accurate observations of Dr. Halley, made in that island, the mercury remained stationary in all weathers. This judicious naturalist has furnished us with the following curious account of these phenomena, and their causes; of which we shall give a concise abstract : 1. In calm weather, when there is a prospect of rain, the mercury is commonly low ; 2. In serene and settled weather it is generally high ; J. On the approach of tempestuous winds, though unaccompanied with rain, the mercury sinks lower than on any other occasion; 4. The greatest height of the mercury is observable during the prevalence of easterly or north-easterly winds; 5. In calm, frosty weather, the quicksilver generally stands high; 6. After very great tempests, when the mercury has been very low, it generally again quickly ascends; 7. The barometer in the northern regions exhibits greater variations than in those lying more towards the South; 8. Within and near the tropics, there is little or no alteration in the mercury, as before observed. Hence Dr. Halley is of opinion, that the principal cause of the rise and the fall of the quicksilver must be attributed partly to the variable winds in the temperate zone, the great inconstancy of which, in Britain, is well-known; and partly to the uncertain exhalation and precipitation of the vapours floating in the at-mosphere, which is at one time more saturated than at another, and consequently heavier; though the precipitation of aqueous vapours chiefly depends on the previous degree of evaporation.

Our plan does not permit us to accompany these propositions with the explanations given by Dr. Hal-ley ; and, as the theoretical part has been strongly contested by other philosophers, though not refuted, till the late Dr. Black, of Edinburgh, endeavoured to prove the fallacy of all preceding theo-ries, we shall conclude this subject: with a summary outline of his doctrine.

According to Dr. Black, 1. Vapour is formed by an intimate union between fire and water, by which the fire, or heat, is so totally enveloped, and its action so entirely suspended by watery particles, that it loses its properties of giving light and heat, and consequently is in a latent state; 2. If the atmosphere is affected by any unusual degree of heat, it becomes incapable of supporting a column of mercury so long as before ; for which reason that in the barometer sinks. - From these preliminary axioms it follows, that as vapour is formed by the union of fire and water, or the solution of the latter in the former, it is impossible that the vapour can be condensed, until this union or solution be effected. The beginning of the condensation of the vapour, then, or the first symptoms of approaching rain, must be the separation of the fire which lies hidden or involved in the vapour. This may at first be slow and partial, or it may be sudden and violent: in the former case, the rain may come on slowly, and after considerable intervals ; in the latter, it will be quick, and in a large quantity. With regard to the effect of this separation between the fire and water, we shall only observe, that as it is gradual and slow, the barometer may in-dicate rain for a considerable time before it appears: or, if the sensible heat communicated from the vapour to the atmosphere, should be absorbed by the colder parts, or be carried off by any unknown means, or prevented from affecting the specific gravity of the air, the barometer will undergo no change ; and yet the vapour, being deprived of the heat necessary to sustain it, must descend in rain ; and thus it happens, that the indications of the barometer do not always hold true, respecting the changes of the weather. Hence also it appears, that, though the specific gravity of the air is diminished, unless that diminution proceed from a discharge of the latent heat contained in the vapours, no rain will follow; and thus the sinking of the barometer may prognosticate wind as well as rain, or sometimes no change whatever.

Such is the ingenious theory of Dr. Black, who certainly has produced the most philosophic explanation yet offered on this apparently intricate subject; and though his reasoning may not enable us to solve all the difficulties occurring in meteorological investigations, we do not hesitate to say, that it has placed the nature and causes of these phenomena in the clearest point of view, as far as it is possible to account for them, in the present state of physical science. We shall, therefore, conclude with observing, that though the wind is certainly not the sole cause of raising and depressing the mercury, yet, in our climate, it has a remarkable influence on the state of the barometer. For, if the mercury falls, when the wind blows from those quarters which generally are productive of much rain, such as the south and south-east winds are in the environs of London, there is no doubt but wet weather will speedily follow : on the contrary, if the quicksilver rises, while the wind blows from dry quarters, such as the northerly and easterly regions are to Britain, then it is highly probable that the weather will become fair. But if the mercury rises during a southerly wind, or falls while it blows from the north; in both cases the prognostics are extremely doubtful 3 as it frequently happens, that the weather does not correspond to the temporary rising or falling of the quicksilver.

A very curious new phenomenon deserves to be mentioned, concerning the state of this instrument, en particular occasions. M. Schmidt, Professor of Mathematics at Pforte, in Germany, lately observed, that his barometer became luminous in the day time, while the horizon was covered with thunder-clouds 5 and that on the approach of a tempest, there ap-d on the surface of the mercury, a small and distinct luminous globe, which could be perceived in day-light, at the distance of several yards. We relate this singular fact; and, not being in possession of farther particulars, we can only conjecture, that such a phenomenon may perhaps be ascribed to the greater rarefaction and electricity of the air in the serene climate of Germany, previous to the explosion of thunder ; and Likewise to a more phosphorescent nature of the mercury ; a property which it doubtless acquires when submitted to the repeated processes of distillation, and other means of purifying that peculiar metallic flid. - See also Spider.