minary. Very frequently, he goes so near the sun as to be lost altogether in his rays. When he begins to make his appearance in the evening after sunset, he can scarcely at first be distinguished in the rays of the twi light. But the planet disengages itself more and more, and is seen at a greater distance from the sun every successive evening; and having got to the distance of about 22° 5, it begins to return again. During this interval, the motion of Mercury, referred to the stars, is direct; but when it approaches within 18° of the sun, it appears for some time stationary; and then its motion begins to be retrograde. The planet continues to approach the sun, and at last plunges into his rays in the evening, and disappears. Soon after, it may be perceived in the morning, before sunrise, separating further and further from the sun, his motion being retrograde as before he disappeared. At the distance of 18° it becomes stationary, and assumes a direct motion, continuing, however, to separate, till it comes to 22°.5 of distance; then it returns again to the sun, plunges into his rays, and appears soon after in the evening, after sunset, to repeat the same career. The angular distance from the sun, which the planet reaches on both sides of that luminary, varies from 16° to nearly 28°. The duration of a complete oscillation, or the interval of time that elapses before the planet returns again to the point from which it set out, varies also from 100 to 130 days. The mean arc of his retrogradation is about 134°; its mean duration twenty-three days; but the quantity differs greatly in different retrogradations. In general, the laws of the movements of Mercury are very complicated; he does not move exactly in the plane of the ecliptic; sometimes he deviates from it more than 5o. Some considerable time must have elapsed before astronomers suspected that the stars which were seen approaching the sun in the evening and in the morning were one and the same. The circumstance, however, of the one never being seen at the same time with the other, would gradually lead them to the right conclusion. The apparent diameter of Mercury varies as well as that of the sun and moon, and this variation is obviously connected with his position relatively to the sun, and with the direction of his movement. The diameter is at its minimum when the planet plunges into the solar rays in the morning, or when it disengages itself from them: it is at its maximum when the planet plunges into the solar rays in the evening, or when it disengages itself from them in the evening; that is to say, when the planet passes the sun in its retrograde motion, its diameter is the greatest possible; when it passes the sun in its direct motion, it is the smallest possible; and the mean length of the apparent diameter of Mercury is 11". Sometimes, when the planet disappears during its retrograde motion, that is to say, when it plunges into the sun's rays in the evening, it may be seen crossing the sun under the form of a black spot, which describes a chord along the disk of the sun. This black spot is recognized to be the planet, by its position, its apparent diameter, and its retrograde motion. These transits of Mercury, as they are termed, are real annular eclipses of the sun; they demonstrate that the planet is an opaque body, and that it borrows its light from the sun. When examined by means of telescopes, magnifying about 200 or 300 times, he appears equally luminous throughout his whole surface, without the least dark spot. But he exhibits the same difference of phases with the moon, being sometimes horned, sometimes gibbous, and sometimes shining almost with a round face, though not entirely full, because his enlightened side is never turned directly towards us; but at all times perfectly well defined without any ragged edge, and perfectly bright. Like the moon, the crescent is always turned towards the sun. These different phases throw considerable light on the orbit of Mercury. See VENUS. MERCURY, in heraldry, a term used, in blazoning by planets, for the purple colour in the arms of sovereign princes. See BLAZONING. MERGER, in law, is where a less estate in lands, &c. is drowned in the greater; as if the fee come to the tenant for years or life, the particular estates are merged in the fee; but an estate tail cannot be merged in an estate in fee; for no estate in tail can be extinct by the accession of a greater estate to it. MERGUS, in natural history, the Merganser, a genus of birds of the order Anseres. Generic character: bill serrated, slender, and hooked at the point; nostrils small, oval, and near the middle of the bill; feet four-toed, the outer one before longer than the middle one. There are ten species, of which we shall notice the following. M. merganser, the goosander, weighs about four pounds, and is twenty-eight inches long. It is common in the northern regions of Europe and Asia, and is found in the Orknies during the whole year. It builds sometimes on trees, but generally in the holes and fissures of rocks, and feeds on fish. Its flesh is strong, and seldom applied for food. See Aves, Plate IX. fig. 5. The M. serrater, or red-breasted goosander, is considerably less than the former, is found also in the same latitudes, and breeds in the north of Scotland, particularly in Loch Mari, in the county of Ross. It dives excellently, and is extremely alert on the water. About the season of its moulting, however, the natives of Greenland often kill it by darts, as the birds are less active than usual in that state of weakness, and suffer the enemy to approach more nearly than at other times. These birds, like the former, and indeed the other species of the genus, subsist in a great degree on fish. They fly near the surface of the water, with great apparent vigour, though seldom to any great distance. Their sharp, serrated, and hooked bills are admirably adapted to secure their prey, which is scarcely ever observed, notwithstanding all its lubricity, to elude their grasp. See Aves, Plate IX. fig. 4. For the Smew, see Aves, Plate IX. fig. 6. MERIDIAN, in astronomy, a great circle passing through the poles of the world, and both the zenith and nadir, crosses the equinoctial at right angles, and divides the sphere into two hemispheres, the eastern and western: it has its poles in the east and west points of the horizon. It is called meridian, because when the sun cometh to the south part of this circle, it is then mid-day; and then the sun has his greatest altitude for that day. These meridians are various, and change according to the longitudes of places; so that they may be said to be infinite in number, for all places from east to west have their several meridians: but there is (or should be) one fixed, which is called the first meridian. Ptolemy chose to make that the first meridian which passes near the Fortunate Islands, at about the distance of one degree from them; and reckons from thence to the east through Africa and Asia; choosing to begin at a place inhabited, and which was then the bounds and limits of the known part of the earth to the west, and to end at the eastern shore of Scain in Asia; but America being discovered not many ages ago, and long after Ptolemy's time, the first meridian was removed more to the west. Some made that the first meridian which passes through the isle of St. Nicholas, which is one of those near Cape Verd; and Hondius chose the isle of St. James to be the first in his map. Others chose that which passes through the isle del Corvo, one of the Azores, be canse the needle was found not to decline from the north there and in the adjacent seas, but to lie in the meridian line; and this beginning Mercator chooses. But seeing there are other places where the needle points to the north, and it doth not so in every part of that meridian, geographers thought this not a sufficient reason; some fixing it at the shore of Brasil, that runs out into the sea. Later geographers choose to begin at the mountain Teneriffe, in the Fortunate or Canary islands, which is counted one of the highest on the earth; and the rather, because they thought some remarkable place should be chosen that might be most known to future ages; and so Ptole my's first meridian, though long observed, was not laid aside without good reason. The French, since the year 1634, have taken that which goes through the west part of the isle of Faro, one of the Canaries. Astronomers also have taken divers places for the first meridian; the followers of Tycho fix it at Uraniburg, an island in the Danish streights, and calculate the celestial motions to that place, and from thence accommodate them to the rest. Others choose other places, according to the authors of the ephemeris they use, who calculate the ephemeris, and the planets places for the meridian of their own place ; as Riccioli, who fixed his first meridian at Bologna; Mr. Flamsteed, at the Royal Obrervatory at Greenwich; and the French, at the Observatory at Paris. See OBSERVATORY. But without regard to any of these rules, our geographers and map-makers frequently assume the meridian of the place, or the capital of the country, for the first meridian; and thence reckon the longitudes of their places. is a suggestion that the meridians vary in In the Philosophical Transactions, there time. This seems very probable from the old meridian line in the church of St. Petronio at Bologna, which is found to vary no less than eight degrees from the true meridian of that place at this time; and from that of Tycho Brahe at Uraniburg, which M. Picart observes varies eighteen minutes from the modern meridian. If Wallis says, the change must arise from a there be any thing of truth in this hint, Dr. change of the terrestrial poles (here on earth, of the earth's diurnal motion) not of their pointing to this or that of the fixed stars; for if the poles of the diurnal motion remain fixed to the same place on the earth, the meridians which pass through these poles must be the same. But this notion of the changes of the meridian, seems overthrown by an observation of M. Chazelles, of the French academy of sciences, who, when in Egypt, found that the four sides of a pyramid, built 3000 years ago, still looked very exactly to the four cardinal points; a position, which could never be looked on as fortuitous. The meridian on the globe or sphere, is represented by the brazen circle, in which the globe hangs and turns. It is divided into four times 90, or 360°, beginning at the equinoctial. See GLOBE. On it, each way from the equinoctial, on the celestial globes, is counted the south and north declination of the sun or stars; and on the terrestrial globe, the latitude of places north or south. There are two points of this circle, which are called the poles of the world; and a diameter continued from thence through the centre of either globe, is called the axis of the earth or heavens, on which they are sup. posed to turn round. On the terrestrial globes there are usually thirty-six meridians drawn, one through every tenth degree of the equator, or through every tenth degree of longitude. The uses of this circle are, 1. To set the globes to any particular latitude. 2. To shew the sun's or a star's declination, right ascension, or greatest altitude, &c. "To find the sun's meridian altitude or depression at night, by the globes." Bring the sun's place to the meridian above the horizon for his altitude at noon; which will shew the degrees of it, counted from the horizon. For his midnight depression below the north point of the horizon, you must bring the opposite point to the sun's present place, as before to the meridian; and the degrees there intercepted between that point and the horizon, are his midnight depression. Meridian line is an arch, or part of the meridian of a place, terminated each way by the horizon. Or it is the intersection of the plane of the meridian of the place with the plane of the horizon, vulgarly called a north and south line, because its direction is from one pole towards the other. It is of great use in astronomy, geography, dialling, &c. and on its exactness all depends; whence divers astronomers have taken infinite pains to have it to the last precision. MERIDIAN line, on a dial, is a right line arising from the intersection of the meridian of the place, with the plane of the dial; this is the line of twelve o'clock, and from hence the division of the hour-lines begin. See DIAL. MERIDIAN, magnetical, is a great circle passing through the magnetical poles, to which the magnetic needle, or needle of the mariner's compass, conforms itself. MERIDIAN altitude of the sun and stars, is their altitude when in the meridian of the place where they are observed, Or it may be defined, an arch of a great circle perpendicular to the horizon, and comprehended between the horizon and the sun or star then in the meridian of the place. "To take the meridian altitude with a quadrant." If the position of the meridian be known, and the plane of an astronomical quadrant be placed in the meridian line, by means of the plumb-line suspended at the centre, the meridian altitudes of the stars, which are the principal observations whereon the whole art of astronomy is founded, may easily be determined. The meridian altitude of a star may likewise be had by means of a pendulum-clock, if the exact time of the star's passage over the meridian be known. Now it must be observed, that stars have the same altitude for a minute before and after their passage by their meridian, if they be not in or near the zenith; but if they be, their altitudes must be taken every minute when they are near the meridian, and their greatest altitudes will be the meridian altitudes sought. MERIDIONAL DISTANCE, in naviga. tion, is the same with the departure, easting or westing, or the difference of longitude between the meridian under which the ship now is, and any other meridian she was before under. MERIDIONAL PARTS, MILES, OF MINUTES, in navigation, are the parts by which the meridians in Mr. Wright's chart (com. monly though falsely called Mercator's) do increase as the parallels of latitude decrease and as the cosine of the latitude of any place, is equal to the radius or semidiameter of that parallel; therefore, in the true sea-chart, or nautical planisphere, this radius being the radius of the equinoctial, or whole sine of 90°, the meridional parts at each degree of latitude must increase, as the secants of the arch, coutained be. tween that latitude and the equinoctial, decrease. The tables, therefore, of meridio nal parts, which we have in books of navigation, are made by a continual addition of secants; they are calculated in some books for every degree and minute of latitude; and they will serve either to make or graduate a Mercator's chart, or to work the Mercator's sailing. To use them, you must enter the table with the degree of latitude at the head, and the minute on the first column towards the left hand, and in the angle of meeting you will have the meridional parts. Having the latitudes of two places, to find the meridional miles or minutes between them, consider whether one of the places lies on the equator, or both on the same side of it, or, lastly, on different sides. 1. If one of the proposed places lies on the equator, then the meridional difference of latitude is the same with the latitude of the other place, taken from the table of meridional parts. 2. If the two proposed places be on the same side of the equator, then the meridional difference of latitude is found by subtracting the meridional parts answering to the least latitude, from those answering to the greatest, and the difference is that required. 3. If the places lie on different sides of the equator, then the meridional difference of latitude is found by adding together the meridional parts answering to each latitude, and the sum is that required. MERLON, in fortification, is that part of a parapet which is terminated by two embrasures of a battery. Its height and thickness is the same with that of the parapet; but its breadth is generally nine feet on the inside, and six on the outside. It serves to cover those on the battery from the enemy; and is better when made of earth well beat and close, than when built with stone; because these fly about and wound those they should defend. MEROPS, in natural history, the beeeater, a genus of birds of the order Picæ. Generic character: bill quadrangular, somewhat curved, compressed, and pointed; nostrils small, at the base of the bill; tongue slender, and in some species ciliated; the outer toe somewhat connected with the middle one. Gmelin notices twenty-six species, and Latham twenty. We shall mention only M. apiaster, or the common beeeater: this is about ten inches long, and found in many countries of Europe, though never observed in Great Britain. It is particularly fond of bees, but will eat various other insects; many of which it seizes; and like the swallow, on the wing. When in sects are with difficulty to be found, it feeds on many species of seeds. In the markets of Italy it is frequently to be seen among the poulterers collections. It builds in the deep holes to be found on the banks of rivers. In the island of Candia these birds are often taken by boys, in the same manner as swallows, by a line, with an insect attached to a hook at the end of it. The cockchafer is chiefly employed for this purpose, notwithstanding its being thus fastened, it continues its flight, and is thus the most effectual of the decoys used on those occasions. MESEMBRYANTHEMUM, in botany, fig-marigold, a genus of the Icosandria Pentagynia class and order. Natural order of Succulenta Ficoidea, Jussien. Essential character: calyx five cleft; petals numerous, linear; capsule fleshy, inferior, many seeded. There are seventy-five species, of which M. nodiflorum Egyptian fig-marigold is a native of Egypt, where they cut up the plants, and burn them for pot ash: it is esteemed the best sort for making hard soap and the finer glass: it is an annual plant, with diffused, decumbent stems; calyxes five-toothed, two of the teeth larger, leaf shaped; petals flat, narrow, connate at the base; stigmas usually five. M. crystallinum, diamond fig marigold, or ice plant: this is also an annual, distinguished by its leaves and stalks, being closely covered with pellucid pimples full of moisture, which, when the sun shines on them, reflect the light, appearing like small bubbles of ice, whence its name; many call it the dia. mond ficoides. MESENTERY, a thick fat membrane, placed in the midst of the intestines, particularly of the smaller ones, whence it has the name. MESPILUS, in botany, a genus of the Icosandria Pentagynia class and order. Natural order of Pomaceæ. Rosacea, Jussieu. Essential character: calyx five cleft; petals five; berry inferior, five seeded. There are nine species, of which M. germanica, Dutch medlar; this tree never rises with an upright trunk, but sends out crooked deformed branches, not far from the ground; the leaves are large, entire, and downy on their under side; flowers very large, as is also the fruit, which is rounder and approaches nearer to the shape of an apple. This tree bearing the largest fruit is now generally cultivated: the Nottingham medlar has a more poignant taste, but the fruit is considerably less. a MESSERSCHIMIDIA, in botany, so, named from Daniel Gottlieb Messerschmid genus of the Pentandria Monogynia class and order. Natural order of Asperifoliæ. Borragineæ, Jussieu. Essential character: corolla funnel form, with a naked throat; berry suberous, bipartile, each two seeded. There are two species, viz. M. fructicosa, and M. arguzia. MESUA, in botany, so called from John Mesue, a physician, a genus of the Monadelphia Polyandria class and order. Natural order of Guttiferæ, Jussieu. Essential character: calyx simple, four leaved; corolla four petalled; pistil one; nut four cornered, one seeded. There is but one species, viz. M. ferrea, a native of the East Indies it is much cultivated in Malabar, for the beauty of the flowers; it bears fruit from the nut at six years old, and continues frequently bearing during three centuries. It is a very large tree, spreading like the lime; the flowers resemble our sweetbriar roses, having only four white petals; the fruit when ripe has a rind like that of the chesnut, with three or four kernels within of the substance, shape, and taste of ches nuts. METACARPUS, in anatomy, that part of the hand between the wrist and the fingers. METALS, according to strict definition, are inflammable bodies, being all capable of combining with oxygen, and many of them, during this combination, exhibit the phenomena of combustion. Formerly only seven metals were known, but modern discoveries have added to the number about twenty others, which are distinguished by their great specific gravity, considerable tenacity and hardness, opacity, and property of reflecting the greater part of the light which falls on their surface, giving rise to what is denominated the metallic lustre or brilliancy. See LUSTRE. To these have been added two others by Mr. Davy, who has discovered the method of decomposing potash and soda, and producing therefrom the new metals called by that professor Potassium and Sodaium. See POTASSIUM and and SODAIUM, under which terms a more particular account of these metals will be given. Of the others the principal characteristic is their superior specific gravity. In this they exceed all other bodies, the lightest being about six times heavier than water, the common standard, while the specific gravity of the heaviest substance with which we are ac. quainted, that is not metal, is less than five times heavier than water. Opacity is another leading property of metals; even when beat to the greatest possible thinness, they transmit scarcely any light: from the union of the two qualities, density and opacity, arises that of lustre. By their opacity and the denseness of their texture, they reflect the greater part of the light that falls on their surface. From their density they are susceptible of a fine polish, by which their lustre is increased. Colour is not a characteristic property of metals, but it serves to distinguish them from each other. Their colours are generally shades of white, grey, or yellow. Tenacity distinguishes a number of the metals, and is not possessed in any great degree by other bodies: hence arises their MALLEABILITY and DUCTILITY, which see. Some of the metals are neither malleable nor ductile. Metals are less hard than the diamond and many fossils, and their elasticity follows the same order as their hardness. Both these qualities are greater in combinations of the metals than in the individual metals, and both may be increased by raising the metal to a bigh temperature, and then suddenly cooling it. Metals are the best conductors of caloric; their expansibilities are various, and are probably nearly in the order of their fusibilities. Mercury melts at so low a temperature, that it can be obtained in the solid state only at a very low temperature; others, as platina, can scarcely be melted by the most intense heat, which we can excite. In congealing, some of the metals expand considerably, especially iron, bismuth, and antimony; the others contract, some of them to a great extent, the contraction of mercury being equal to the rd of the whole volume. Metals may be volatilized; at the degree of 600 quicksilver may be volatilized; and zinc and arsenic at a temperature not very remote from this; many others may be dissipated in the focus of a large burning mirror, or by a powerful galvanic battery. Metals are the best conductors of electricity. Metals are susceptible of combination; they have an affinity to oxygen, hydrogen, carbon, sulphur, phosphorus, and to each other, and when combined with oxygen, to all the acids, to the alkalies, and to the earths. The metals, independently of potassium and sodaium, may be thus enumerated and arranged. |