METAL, in heraldry. There are two metals used in heraldry, by way of colours, viz. gold and silver, in blazon called or and argent. In the common painting of arms these metals are represented by white and yellow, which are the natural colours of those metals. In engraving, gold is expressed by dotting the coat, &c. all over; and silver, by leaving it quite blank. It is a general rule in heraldry, never to place metal upon metal, nor colour upon colour; so that if the field be of one of the metals, the bearing must be of some colour; and if the field be of any colour, the bearing must be of one of the metals. METALLURGY, comprehends the whole art of working metals from the state of ore to the utensil; hence assaying, gilding, refining, smelting, &c. are only branches of metallurgy. In a more limited sense, it includes only the operations which are followed in separating metals from their ores. See ASSAYING, &c. METAPHOR, in rhetoric, a trope, by which we put a strange word for a proper word, by reason of its resemblance to it; or it may be defined, a simile or comparison intended to enforce and illustrate the thing we speak of, without the signs or forms of comparison. { Uranium Tantaliuma Tin granular Iridium and brittle Rhodium Palladium Nickel fusible and Platina Gold METEOR, in physiology, a moveable igneous body, congregated in the air by means not thoroughly understood, and varying greatly in size and rapidity of motion. Many attempts have been and are still made to account for the formation and ignition of these grand objects. Dr. Woodward, of the old school, seems to have ap proached nearer to modern opinions, founded on recent observations, than any other writer on the subject. That gentleman supposed them to originate from mineral particles raised from the earth by subterraneous heat, accompanied by vapours from the same strata, which furnished the minerals, and being condensed by the pressure of the atmosphere, partake of the immediate action of the bodies they intersect in their passage. Derham thought the ignis fatuus a vapour on fire; Beccaria, on the contrary, suppposed them to be vapour forced out of the earth by the descent of rain or snow, and not decidedly burning, but rather of the nature of cold phosphori. Franklin conjectures, in the Memoirs of the Manchester Society, that the dense fog of 1783 may have been produced by smoke arising from the combustion of some of those vast globes, "which we happen to meet with in our rapid course round the METAPHYSICS. See PHILOSOPHY, sun." The generality of the meteors ob mental. served resemble each other, except in size, which cannot be ascertained with certainty, on account of the apparent diminution of bodies through distance. The most remarkable of late times were those of 1783 and 1805: the former was very luminons, and its supposed diameter 1000 yards: the latter passed with such astonishing rapidity, that amazement had not subsided ere it vanished, consequently very little dependence can be placed on what has been said concerning its bulk and shape; the light which it emitted was a pale blue, and almost as instantaneous as a flash of lightning, and the rushing of the enormous body produced a sound like very distant thunder. Some of the smaller meteors explode after a certain interval of burning, and it has been uniformly asserted that they deposited stones; the apparent improbability of this assertion long prevented persons of enlightened minds from crediting it, and till Dr. Chladni published a dissertation on the subject in 1794, which induced Mr. King to collect every instance, ancient and modern, calculated to establish the fact with the public, and this was no easy task. Mr. Howard followed the example of those two gentlemen; but went further, and actually procured specimens of the substances alluded to, which having compared, he proceeded to analyze by chemical means. He found them entirely different from all known stones, and exactly resembling each other, even in their component parts. It has been said that the stones, thus incontestibly proved by different authorities, and from various places, to have fallen after the explosion of meteors, are heated and luminous when they reach the earth: the force of their descent buries them some depth into it, and they have been seen under these circumstances in Italy, Germany, France, England, and India. The meteors either really do, or appear to, move horizontally, and are said to descend ere they explode. The stones are of different sizes, and from a few ounces in weight to several tons: they are generally circular, and inva riably covered with a rough black crust, which, according to Howard, is principally composed of oxide of iron. The process adopted by that gentleman produced a result which has since been confirmed by Klaproth and Vauquelin. We shall give the analysis of two of these substances, by Howard, of a stone which fell in Yorkshire; and by Vanqnelin and Fourcroy, of another that fell at Laigle in France, 1805. The conjectures which these extraordinary productions have occasioned are visionary in the extreme: indeed M. Laplace supposes them to be fragments ejected by volcanos in the moon: Sir William Hamil ton and Mr. King, on the contrary, imagine that they are concretions formed in the atmosphere. METEOROLOGY, is the science of studying the phenomena of the atmosphere, and the term by which is expressed all the ob servations that tend to make them a system. There are many most important meteorological phenomena, and those may be classed under five distinct heads; for instance, the alterations that occur in the weight of the atmosphere, those that take place in its temperature, the changes produced in its quantity by evaporation and rain, the excessive agitation to which it is frequently subject, and the phenomena arising from electric and other causes, that at particular times occasion or attend the precipitations and agitations alluded to. All the above phenomena prove to demonstration that constant changes take place, the consequences of new combina. tions and decompositions rapidly follow. ing each other. The majority of meteorolo. gical alterations depend on these chemical changes, and were we accurately acquainted with the peculiarities of all the substances which form the component parts of the atmosphere, nothing would be more easy than to explain the result of their mutual action; but as that is unfortunately Hot the case, we must be contented to build upon strong probabilities supported in many instances by positive experiment. It is singular that this science should have remained for so long a period in a state of comparative neglect, when it is recollected that almost all the operations necessary for the support of human life, and almost all the comforts of corporeal feeling, depend upon the state of the atmosphere, and yet nothing was attempted to any purpose towards investigating the laws of meteorology till the seventeenth century, when the most important discoveries of the barometer and thermometer occurred, which was followed in the eighteenth by the invention of excellent hygrometers and electrometers; by these the philosopher finds himself competent to make accurate and satisfactory observations. Scientific persons, who have particularly turned their attention to this pursuit, have undertaken the laborious task of collecting and methodically arranging numbers of the observations just mentioned, and after attentively comparing and examining them have formed theories of the weather of more or less probable accuracy; but the science is of such difficulty that though those theories deserve every praise, weare compelled to acknowledge the phenomena of the weather is still very imperfectly understood. This acknowledgment, how ever, reflects no discredit on those ingenious men, as it is impossible that any thing like certainty should be attained, till observations that can be depended upon are procured from all parts of the globe, the atmosphere has been more accurately explored, and the chemical changes occurring in it are correctly ascertained. To render our explanation of this subject as satisfactory as circumstances will permit we shall proceed in the succession before pointed out; with respect to the changes in the weight of the atmosphere, it is generally known that the instrument called the barometer shews the weight of a body of air immediately above it extending to the extreme boundary of the atmosphere, and the base of which is equal to that of the mercury contained within it. As the level of the sea is the lowest point of observation, the column of air over a barometer placed at that level is the longest to be obtained; in this case the mean height of the barometer is thirty inches. According to the experiments of Sir George Shuckburgh in the Channel and the Mediterranean Sea in the temperature of 55° and 60°, this was found to be the case, and the result is confirmed by those of M. Bouguer on the coast of Peru in the temperature of 84o, and Lord Mulgrave in latitude 80°. From these data, it is evident that the mean height of the barometer decreases in proportion with its elevation above the level of the sea, and in proportion to the consequent shortening of the columns of air; hence it is used for measuring heights. The keeping of a barometer in one particular place does not make the mercury stationary, as it will vary by rising or falling to the extent of several inches, of necessity the weight of the air which balances the mercury must be subject to the same changes; this circumstance proves that the gravity of the air in any given situation varies greatly, being at one time light and another heavy, an effect which must be caused by changes in its quantity, and a fact that demonstrates the air of every place liable to perpetual alterations, which must arise from the accumulation of air in particular places, and a reduction in others, "or," as Dr. Thomson observes, "part of the atmosphere must be alternately abstracted altogether, and restored again by some constant, though` apparently irregular process." The variations of the barometer between the tropics are very trifling, and it is worthy of observation, it does not descend more than half as much in that part of the globe for every two-hundred feet of elevation as it does beyond the tropics, which we learn from the Journal de Physique; besides, the barometer rises about two-thirds of a line twice during each day in the torrid zone. We are informed by M. Horsburgh that from latitude 26° north to latitude 27° south, which includes the space termed the tropical seas, the mercury attained its greatest elevation at eight in the morning, from which hour till noon it continued stationary, it then began to fall and descended till about four o'clock, when it reached the lowest point of depression. In the interval between four and five the mercury rose, and continued to rise till about nine or ten P. M. when it had once more arrived at its most elevated point, where it remained stationary till near midnight, when it fell and continued to fall, till at four A. M. it had descended as low as it had been at four in the afternoon; from that period till seven or eight it continued rising, and at the latter hour it had attained the highest point of elevation. The gentle man who made these observations termed the elevations and depressions now described equatropical motions, and asserts, that they were regularly performed while the barometers were on the sea, but they were seldom observed on a river, or when the instruments were on shore. This circumstance leads us to concur with Dr. Thomson, in supposing that the singular fact is to be ascribed to the motion of the ship, "which by regularly agitating the mercury, might make its elevations and depressions more sensible and correct than when the baro meter continues stationary." The range of the barometer increases gradually as the latitude advances towards the poles, till in the end it amounts to two or three inches. The following table, composed by the writer just cited, will explain the gradual increase alluded to, which he compiled from the best authorities. The range of the barometer is considerably less in North America than in the corresponding latitudes of Europe, particularly in Virginia, where it never exceeds 1.1. The range is more considerable at the level of the sea than on mountains, and in the same degree of latitude it is in the inverse ratio of the height of the place above the level of the sea. M. Cotte composed a table which has been published in the Journal de Physique, from which it appears extremely probable that the barometer has an invariable tendency to rise between the morning and the evening, and that this impulse is most considerable from two in the afternoon till nine at night, when the greatest elevation is accomplished; but the elevation at nine differs from that at two by four-twelfths, while that of two varies from the elevation of the morning only by one-twelfth, and that in particular climates the greatest elevation is at two o'clock The observations of M. Cotte confirm those of Mr. Luke Howard, and from them it is concluded that the barometer is influenced by some depressing cause at new and full moon, and that some other makes it rise at the quarters. This coincidence is most considerable in fair and calm weather; the depression in the interval between the quarters and conjunctions amounts to onetenth of an inch, and the rise from the conjunctions to the quarters is to the same amount. The range of this instrument is found to be greater in winter than in summer; for instance, the mean at York during the months from October to March inclusive, in the year 1774 was 1.42, and in the six summer months 1.016. The more serene and settled the weather is the higher the barometer ranges, calm weather with a tendency to rain depresses it, high winds have a similar effect on it, and the greatest elevation occurs with easterly and northerly winds, but the south produces a directly contrary effect. According to the Asiatic Researches it is always observed to be highest with north and north-west winds, and the reverse when the south-east prevails; it falls rapidly previous to violent tempests, and is greatly agitated while they continue. It has been remarked by Mr. Copland in the Transactions of the Society of Manchester, that "a high barometer is attended with a temperature above, and a low barometer with one below, the monthly mean." Various but almost altogether unsuccessful attempts have been made to explain the phenomena we have enumerated; that of Mr. Kirwan carries considerable plausibility, though it is not considered quite satisfactory. In order that his ideas on the subject may be clearly understood, we shall give what may be considered an abstract of his theory, improved by Dr. Thomson. The density of the atmosphere is evidently greatest at the poles, and least at the equator, as the centrifugal force at the latter, the distance from the centre of the earth, and the heat, all contributing to lessen the density of the air, are at their maximum, when at the pole it is exactly the reverse. In every part of the world the mean height of the barometer placed at the level of the sea will be found to be 30 inches, consequently, the weight of the atmosphere is the same in all places; its weight depending on its density and height; where the former is greatest the height must be the least, and where its density is least the height is the greatest. Arguing from these facts it will, therefore, appear that the height of the atmosphere must be least at the poles, and greatest at the equator, decreasing gradually in the interval, and thus forming the resemblance of two inclined planes, meeting at the high est part above the equator. The difference of the mean heat between the pole and the equator, when the sun is in our hemisphere in the summer, does not vary so much as in the winter, as the heat at that period in northern countries equals that of the torrid zone; hence the thermometer rises to 85° in Russia during the months of July and August, of necessity the rarity of the atmosphere and its height increases; in consequence, the upper part in the northern hemisphere inclines less, but that of the southern, from different causes, must be much more inclined; during our winter the exact réverse takes place. The pressure of the superincumbent column in a great measure canses the density of the atmosphere, and therefore decreases in proportion to the height as the pressure of the column constantly decreases, yet the density in the torrid zone does not decrease so rapidly as in the temperate and frigid, as the column is longer, and because there is a larger proportion of air in the upper part of it. This fact agrees with the assertion of M. Cassan, "that the barometer only sinks half as much for every two hundred feet of elevation in the torrid, as in the temperate zones." The density at the equator, though less at the surface of the earth, must equal at a certain height, and still higher exceed the density in the températe zones, and at the poles. It is ascertained that a current of air constantly ascends at the équator, part at least of which reaches to and remains in the highest parts of the atmosphere; the fludity of that body prevents it from accumulating above the equator, and hence it must descend the declined plane before mentioned. The surface of the atmosphere being more inclined in the northern hemis phere during our winter than that of the southern, more of the current must flow on the northern than on the southern, from which cause the quantity of our atmosphere is greater in winter than that of the southern hemisphere'; in the summer it is just the contrary; consequently the range of the barometer is less in summer than in winter, and the greatest mercurial heights occur during winter. The heat of any given place in a great measure influences the density of its atmos. phere; that density will be most consider able where it is coldest, and its column shortest. Chains of mountains, the summits of which are covered with snow great part of the year, and highlands, must be colder than places less elevated in the same latitude, and the column of air over them much shorter. The current of air above must be impeded and accumulaté while on its passage over these places towards the poles, which causing an agitation, it will be communicated to, and indicated by, the barometer in a singular manner. These accumulations occur over the north-west parts of Asia and North America, and this raises the barometer, and canses less variation in it there than in Europe. It is precisely so on the Pyrenees, the Alps, and the mountains in Africa, Turkey in Europe, Tartary, and Tibet. After the accumulations have existed some time are surrounding atmosphere becomes incapable of balancing the density of the air, when it descends with violence, and occasions cold winds, which raise the barometer; it is to this that we are to attribute the rise of the barometer, almost always attending northeast winds in Europe, which is the effect of accumulations near the pole, or in the north-west parts of Asia; it is thus besides that the north-west wind from the mountains of Tibet raises the barometer at Calcutta. It may be supposed that in the polar regions large quantities of air are casually compressed; when this is the case the southern atmosphere must rush in to replace it, which occasions south-west gales and the fall of the barometer. The mean heat of our hemisphere varying in successive years, the density of the atmosphere, and necessarily, the quantity of equatorical air passing towards the poles, cannot be otherwise than variable, hence occurs the different ranges of the barometer in successive years; at some particular periods, more considerablé accumulations take place in the highest parts of Asia, and the south of Europe, than at others, which may be produced by early falls of snow, or the interruption of the sun's rays by long continued fogs; at such times the atmosphere in the polar regions becomes proportionably lighter, and this causes the prevalence of southerly winds in some winters more than in others. The heat of the torrid zone never greatly varying, the height and density of the atmosphere undergoes but few changes, thence arises the comparatively small range of the barometer within the tropics, which gradually in |