:

down to G through a wooden square pipe, where the two separate harves, so that any substance that will plumb may be seen immersed in the small vessel, G, pass between the prongs of its forked end may be full of water, above a small stage, in order to keep brought into the field of view, and be seen magnified the line from oscillating. This vessel G may be by the eye-glass, used on the principle of a simple raised or lowered by the screw that supports it. The microscope: let the thread of the plumb-line be this secondary microscopes have each the same adjust- interposed body, which indeed will cover only a small ments as the above-mentioned microscopes; and the portion of the field of view; but as the plumb-line is plumb-line has also its point of suspension so ad- not to be moved, except by the screw at the point of justable, that it can be brought into the foci of the suspension, nor even touched by any external object, upper and lower eye-pieces so as to bisect the fields the microscope must necessarily be brought to it, of view, when the microscopes are both properly and placed in such a manner that the thread will biadjusted. sect the field of view; this is done by fitting the forked The plumb-line serves two separate, and very im- end of the measuring-bar into the upper frame E, portant purposes; for its peculiar application to both first, in such a way that it may be made to slide in of which we are indebted to the ingenious Ramsden and out any number of times to the same situation; first it not only serves to set the vertical axis perpen- then the adjustments of the frame, or of the cock of dicular in one position, but by being carried round suspension, will bring the thread into the field of in azimuth with the axis and all the other append-view; let now the object be a round dot on a slip of ages, serves to show if the perpendicular direction of ivory, or mother-of-pearl would be better, perhaps, the said axis is preserved with respect to all the points and its image may be so adjusted that the plumbof east, west, north, and south; and if any deviation line will bisect it in its magnified state. This ingeis detected by the thread being at one side of the nious contrivance of producing an image in the open original situation, then one of the adjustments of the air has been denominated Ramsden's ghost by suciron circles under the inferior pivot of the axis, as ceeding instrument makers, from the name of its ineffected by the handle of the compound-joint under ventor. Let now the measuring-bar, which we will the mahogany ring, M, must be made to verify the suppose to be too short, be laid and supported horiposition; and, secondly, the horizontal axis of the zontally in a direction just perpendicular to the plane vertical circle is made perfectly level by the same of the circle, and let there be a thick pin screwed into plumb-line; this is effected by an additional appara- its end next the circle, which by being unscrewed tus, in a very ingenious, as well as very accurate will approach the plane of the circle till it just manner, which may be thus explained without a touches it as the circle revolves, then the distance figure suppose a bar of metal to be made of such from the extreme end of this pin to the plumb-line length as, when used as a horizontal measure, would is exactly gauged; it is of no importance what may just reach from the divided face of the vertical circle be the total length of this gauge, provided it be kept to a point directly opposite it in one of the pillars at unaltered; remove, in the next place, the measuringthe upper end; and suppose again, this measuring rod and its apparatus at each end in statu quo, to a bar to be applied below to a point at the lower part similar fitting made for it in the inferior frame I F; of the said pillar, to try if in this situation it will also and if, when the plumb-line bisects the image of the touch the graduated face of the same circle; then, if dot here as it did before, which the adjustments of the distance is found to be precisely the same in both the frame only must now effect, the pin at the oppocases, the conclusion would be from such a rough site end turns out to touch the plane of the circle that the pillar and the plane of the vertical below at the same right angle that it did above, then circle are parallel, or very nearly parallel, to each the plane is perpendicular, and the axis necessarily other; now, as the circle was originally made by horizontal; but if there is any deviation, the adjustbeing turned on its own pivots in a large frame, its ing-screw, p, on the bearing frame, DD, must rectify axis is necessarily at right angles to its plane, and one-half of this deviation, and the pin which screws consequently also to the surface of the pillar. Hence, into the measuring-bar the other half; after a few if the pillar were perfectly perpendicular, the axis, trials above and below, the horizontal position may on a supposition that the measures were accurately be given to the axis in question to the exactitude of taken, would be perfectly horizontal. But we know a single second; for we have said, that a microscope that a plumb-line is perpendicular whenever it is at may be depended on to that degree of accuracy in rest, therefore any contrivance that will measure reading off the micrometer's scale. When the ververy minutely the distance from the plumb-line to tical circle is truly fixed, a second measuring-bar the plane of the circle, both above and below, will may be added at the lower frame, while the first redetermine whether or not the axis is horizontal; mains at the upper one, and the turning the circle this contrivance is what we have to describe: round on its axis would show both above and below

measure,

Conceive the said bar of measurement to terminate when any alteration takes place in the true position, at one end after the manner of a two-pronged fork, from whatever cause. But instead of using the plane and suppose one-half of a compound microscope, viz. of the circle itself, Ramsden judged it better to fix a the object, object-lens, and body of the instrument, little bridge, ay, over the object-glass of the teleto be carried by one prong of the fork, and the eye- scope, with a prominence, h, which he made to come glass in a separate tube, borne by the other prong; in contact with the pins of the measuring-rod above and it is easy to apprehend, that the image of any and below successively, by which means the contact small object, whatever it may be, may by the adjust- is more nicely observed, and the method equally acment of the object-lens be made to fall into the open curate. Whenever the line of collimation of the tespace between the prongs, which image may again lescope is thus adjusted, it will be certain to describe be rendered distinct to the eye by the focal adjust- a semi-circle in the heavens, when turned half round, ment of the eye-glass; we have now got a measuring-which will be truly perpendicular to the horizon, bar with a compound microscope carried by it, in whether that semi-circle be in the meridian or in

any given azimuth. Whenever Piazzi rectified the mouth, when the finder has pointed the tube properly superior and inferior micrometers and plumb-line, he to its object.

took care to use the zeros of the vertical circle as the Most observatories possess an instrument called points that bisected the circle best into two equal an armillary sphere. In the present day, it is more semi-circles; and he gave as a reason, that he found used for elucidating the general principles of astrothese did not deviate more than a quarter of a se-nomy, than for practical research; it may, however, cond from their true places. When the vertical circle be advisable to give a description and graphic deliis used in taking altitudes, it may be clamped by a neation of the instrument.

piece, k, on the pillar P, which when loose will allow

a quick motion, but when fast will only permit a very slow one by means of the handle, V, of the compoundjoint, which like that at Q is connected with the tangent-screw out of sight. These, we believe, are all the essential parts of this highly-important in

strument.

It consists, as is seen in the figure, of an artificial sphere, composed of a number of circles of metal, put together so as to represent the ecliptic zodiac, tropics, and other imaginary circles in the heavens, in their natural order.

The theory of the telescope would not be understood by the reader without a preparatory acquaintance with the science of optics, and we purpose under the article TELESCOPE giving a very full account of that form of the instrument which is best fitted for astronomical purposes; but there is a very important part of the furniture of the observatory which must be noticed here, we mean the frame-work and supports for the telescope. One of these, contrived by Sir William Herschel, is represented in the centre of the observatory (Plate, ASTRONOMY, fig. 2), in which A is the elevated mouth of a seven-feet tube, and B the place of the large speculum that reflects the rays of light back to the small diagonal plane metal near C, which, by a second reflection, brings them to a focus at the eye-piece below C, as seen in the engraving. Above Ċ is the finder, the upper end of which has a small achromatic object-glass, and the lower end the eye-glass. The upper end of the tube rests on a support, D, which is capable of being raised or depressed by a pinion on the axis of the handle under D, while the lower end rests on the horizontal bar of the frame E F, which is suspended by a pulley over F; the four pivots a, b, c, and d, of the said frame, sliding in the open grooves, seen near those letters in the main frame, keep the small frame This meridian is likewise moveable within the silin any given situation, and allow a free motion, first vered horizon, so as to admit the elevation or depresdown the vertical, and then down the inclined pieces sion of the poles, f. The use of this sphere is to that compose the main frame, as low as to G and H; assist the imagination in conceiving the apparent and when the lower end of the tube has been de-motion of the celestial bodies. The student must pressed into this situation, the tube may have an suppose himself to be placed upon the earth, with elevation approaching towards the zenith: for not the various circles in the heavens revolving round only is the upper end elevated by the handles at Jhim from east to west.

The earth is situated in the centre of this sphere, in the same plane with the rational horizon, which horizon, d, is generally represented by a broad silvered circle, divided into degrees, &c. The whole machinery is usually supported upon a brass stand, and is moveable about an axis, within a brass meridian like the common globes.

sented in Plate, ASTRONOMY, fig. 2. It consists of a circular building with a moveable roof of similar form. The latter is made to revolve on friction rollers, and a contrivance of a winch and pulley enables the observer to elevate or depress the moveable ceiling in the lantern at pleasure. Various instru

for the quick, and at D for the slow motions; but Having in the preceding pages entered pretty fully the lower one is depressed by the handle at I, round into the construction of the instruments employed in which the cord is coiled, that goes round a fixed practical astronomy, it may now be necessary to furroller at K, and two others at L and M, before it nish a notion of the sort of edifice best fitted for conembraces the pulley N, and is hooked to a pin at Otaining them. A very useful observatory is repreabove the frame. The rest of the main frame is so clearly exhibited in the engraving, that no farther description of it is necessary. In some of the instruments of this construction, when the handle J is omitted, and a quicker motion in altitude is required, and also a greater elevation than can be given simply by the handle at D, the second square stem that car-ments are seen round the room, and amongst the ries the pinion of the handle, is raised by hand, and kept to its elevation by means of a second rack, when this squared stem is lowered again. The quick motion in azimuth is given by sliding the lower end of the tube gently along the bar on which it rests, or by moving the whole frame, which moves on castors; but the slow motion is produced by a screw. It is scarcely necessary to add that the eye of the observer is applied to the side of the tube near its

rest an armillary sphere, to which allusion has already been made. Shutters for presenting the tube of the telescope in various aspects are also employed, towards one of which the large instrument is directed.

We do not purpose in the present place entering into the history of observatories, as that branch of the subject will be discussed under the article itself; but it may be advisable to give a description of a very excellent modern establishment in the sister

kingdom, erected without reference to expense for | nent, pursued by academicians freed from the embarthe express purpose of practical astronomy. rassments of professional labour, and when we look The observatory belonging to Trinity College, at their numerous and well-appointed observatories, Dublin, commonly called the Dublin Observatory, we shrink from the comparison which is thus forced was begun in the year 1783. It was founded by Dr. upon our attention. We feel as if it were a species Francis Andrews, provost of that college, who be- of treason to record the fact, that within the wide queathed a large income for this purpose, which was range of the British islands there is only one obto commence upon a particular contingency happen- servatory, and scarcely one supported by the governing in his family. When this event had taken place, ment! We say scarcely one, because we believe that the College, with their wonted zeal for the promotion some of the instruments in the observatory at Greenof science, determined not to lose time by waiting for wich were purchased out of the private funds of the the accumulation; but advanced from their own Royal Society of London. The observatories of Oxfunds a sum considerably exceeding the amount of ford, Cambridge, Dublin, Edinburgh, Armagh, and the original bequest. Glasgow, are all private establishments, to the support of which government contributes nothing." The consequence of this is, that many of them are in a state of comparative inactivity, and of but little service to the scientific world.

They chose for their professor of astronomy and observer, the Rev. Dr. Usher, a man of extensive learning and indefatigable research, who was directed to proceed to England, to order from Mr. Ramsden the best instruments he could make, without any limitation of expense.

The apparatus first ordered was a transit instrument of six-feet focal length, with a four-feet axis, bearing four inches and a quarter aperture, with three different magnifying powers up to 600. An entire circle of ten-feet diameter, on a horizontal axis for measuring meridian altitudes; an equatorial instrument, with circles of five feet in diameter: an achromotic telescope, mounted on a polar axis, and carried by an heliostatic movement. Regulators were also ordered from Mr. Arnold, without any limitation of price. The situation chosen for the observatory is on elevated ground, about four miles from Dublin. The foundation is a solid rock of limestone, of several miles extent; and the soil is very favourable, being a calcareous substance, called limestone gravel, which is remarkable, absorbs rain, and thus contributes to dry the atmosphere.

Works of reference on astronomy have not multiplied to the same extent as the printed treatises on many other sciences. The labour of a long life is necessary to the formation of a good book; and till the formation of Mr. Babbage's modes of calculation, a series of tables would alone occupy years of mathematical labour.

Of the numerous works on astronomy, we shall only mention here the latest and most important manuals and elementary works: Astronomie par de Lalande, 3rd edit., Paris, 1792, 3 vols., 4to. (there is an abridgment of it-Abrége d'Astronomie par de Lalande, Paris, 1795); Astronomie Theorique et Pratique, par Delambre, Paris, 1814, 3 vols., 4to.-a work important for professional astronomers; Schubert's Theoretical Astronomy, Petersburgh, 1798, 3 vols., 4to., and the new French edition of the same work, 1822; Biot's Traité Elémentaire d'Astronomie Physique, 2nd edit, Paris., 1810, 3 vols.; Laplace's Expo

The plan of the building unites at once both ele-sition du Système du Monde, 5th edit., Paris, 1824 (a gance and convenience: it fronts the east, and the lower range of windows and doors are twenty-three in number. In the centre there is a magnificent dome of three stories high, with a moveable roof for the equatorial instrument, which is placed upon a pillar of sixteen feet square, of the most substantial masonry, and surrounded by a circular wall of a foot distance, that supports the moveable dome, and also the floors, which in no part touch the pillar; thus, no motion of the floor or wall can be communicated to the instrument. The aperture for observation in the dome is two feet and a half wide.

general exposition of the results developed in the large work, Mécanique Céleste); Bode's Illustrations of Astronomy (which is confined to the less difficult propositions of geometry and astronomy), 3rd edit., Berlin, 1808, 2 vols.: together with this work, we may mention Bürjas' Manual of Astronomy, Berlin, 1794, 5 vols., which requires, however, more extensive knowledge. Excellent, though very condensed, is Bohnenberger's Astronomy, Tübingen, 1811. Piazzi's Italian Manual of Astronomy is a good work. Among the English treatises are Woodhouse's Elementary Treatise on Astronomy, 1823, and Ferguson's Lectures on But the most important erection belonging to this Astronomy, a popular work; also Vince's Complete establishment is behind the main building, and at System of Astronomy, 3 vols., 4to., with additions, right angles to it, in order to obtain an uninterrupted | 1814. To astronomers, practical and theoretical, view both north and south. This is the meridian | Bessel's Observations at the Observatory of Königsberg, or transit room, which contains both the transit in- which have appeared in folio since 1813, are indisstrument and the circle. It is thirty-seven feet long, twenty broad, and twenty-one high. Fine pillars of Portland stone are erected for both instruments, and the floor is so framed as to let all the pillars rise totally detached from it and such was Dr. Usher's attention to extreme accuracy, that he first ascertained the pillars to be perfectly homogeneous, lest any variety in their substance might admit of a difference in their expansion or contraction by heat, cold, or other changes of atmosphere.

We cannot conclude this view of practical astronomy without quoting a remark made by a very enlightened critic in the Quarterly Review, who says, "when we look at the state of science on the Conti

pensable. Notices of astronomical tables may be found in the larger astronomical treatises mentioned. With respect to astronomical periodicals, Zach's Monatl. Correspondenz zur Beförderung der Erd- und Himmels- Kunde, with which is connected Lindenau's and Bohnenberger's Astronom. Zeitschr., is continued under the title Correspondence Astronomique, Géographique, &c. du baron de Zach. Schumacher has also published, in Copenhagen, since 1822, Astronomische Nachrichten. The latest observations may be also found in the Paris Connaissance des Tems, and in the Berlin Astronomisches Jahrbuch, which has been published for more than fifty years. The history of astronomy may be found at large in Montucla's

Histoire des Mathématiques, 4 vols., 4to.; in Delam- | thrust at the same time with the antagonist's blow or bre's Histoire de l'Astronomie Ancienne, celle du thrust. It is, of course necessary that a tempo blows Moyen Age et Moderne, Paris, 1817, 5 vols., 4to.; should be made in such a way as to afford at the and in Bailly's Histoire de l'Astronomie, of which the same time a guard against the other's thrust, or to first volume appeared in 1771, containing the history prevent its full effect. This kind of blow takes of ancient astronomy, and the three other volumes, place particularly in fighting with the broad-sword, 1779 and 1782, containing the history of modern when the antagonist leaves himself much exposed. astronomy, followed, in-1787, by his Traité de l'As- ATEMPO GIUSTO, in Music, signifies, to sing or tronomie Indienne, which latter work, however, must play in proper time. be used with precaution, on account of the inclination of the author to adopt theories on insufficient grounds, which indeed is one of the faults of his school. Mrs. Somerville's excellent and learned treatise on the Mechanism of the Heavens, lately published, deserves the highest praise, both for general research and high mathematical attainments. This work will alone give a character to the literature of the age in which we live, and tend to place our fair country-woman foremost in the ranks of astronomical science.

ASTROSCOPE, an astronomical instrument, composed of two cones, on whose surface the constellations are delineated, by means of which the situation of the stars may easily be known.

ASTROSCOPIA; the art of examining the stars by telescopes, or, in plainer terms, practical astronomy. ASTROTHEMATA, in Astrology, the positions of the stars in a plan of the heavens.

ASTROTHESIA; the same as ASTRUM, which see. ASTRUM ; a constellation or assemblage of stars. ASTRUM, in Alchemy, a term employed to denote the power imparted by chemical mixture.

ASYMPTOTE; commonly, a straight line, which approaches a curve line, so that the distance between them is constantly diminishing, although they can never meet, even if indefinitely continued. Hence Leibnitz called infinite spirits the asymptotes of the Deity. An asymptote may also be a curve.

ATABAL; a kind of tabor used among the Moors, which is probably a word of Moorish extraction.

ATHANASIA, in ancient Medicine; an epithet given to a kind of antidotes, supposed to have the power of prolonging life, even to immortality. In the Augustan Dispensatory we still find a medicine under the appellation of "athanasia magna," recommended against dysenteries and hæmorrhages.

ATHANATI; a peculiar body of cavalry, among the ancient Persians, consisting of 10,000 men, always complete, because, when any one of them died another was immediately put into his place.

ATHANOR, in Chemistry; a digesting furnace, which retained the heat for a long period of time, and was so contrived that it might be increased or diminished at pleasure.

ATHENA, in Medicine; a plaster or liniment recommended against wounds of the head and nerves, of which we find descriptions given by Oribasius, Ælius, and Æginata.

ATHENATORIUM, in Chemistry; a thick glass cover, to be luted to a cucurbit when the alembic is taken off.

ATHENORIUS CATAPOTIUM, in Medicine; a cough pill mentioned by Celsus, consisting of myrrh, pepper, castor, and opium.

ATHENIPPUM, in Medicine; an affusion for the eyes. ATHEROMA, in Surgery, is a soft, pultaceous, uninflamed tumor; generally contained within a cyst, or bag. The cure of this swelling consists in its removal with a scalpel. It is very common on the finger joints.

ATHWART, when used in navigation, implies across the line of the course.

ATHWART-HAWSE; the situation of a ship when she is driven by the wind, tide, or other accident, across the fore-part of another. This phrase is equal

ATABULUS; a provincial wind much felt in Apulia, of a dry and noxious quality. The ancient naturalists speak of the atabulus in terms of horror, on account of the ravages it made among the fruits. ATAXY signifies irregularity of crisis, and parox-ly applied when the ships bear against each other, or ysms of fevers.

ATARAXIA; exemption from disease or vexation. "The sceptics," says Glanville," affected an indifferent equiponderous neutrality, as the only means to their ataraxia and freedom from passionate disturbances." So much for the clearness and simplicity of a much-praised author.

ATCHE, in Commerce; the smallest silver coin current in Turkey, worth about one-third of a sterling.

penny ATCHIEVEMENT, in Heraldry, denotes the arms of a person or family, together with all the exterior ornaments of the shield, as helmet, mantle, crest, scrolls, and motto, together with such quarterings as may have been acquired by alliances, all marshalled in order. Hatchment, as a funeral memento, is a vulgarism of the above.

ATEBRAS, in Chemistry; a subliming vessel. ATEGAR; a weapon among the Saxons, which seems to have been used as a hand dart.

A TEMPO, in music; of similar signification with a battuta, and, like that expression, seldom used but when the time has been interrupted. A tempc, in any kind of fencing or fighting, means a blow or

when they are at a small distance: the transverse position of the former to the latter being principally understood. Athwart the fore-foot denotes the flight of a cannon-ball from one ship across the course of another, to intercept the latter, and oblige her to shorten sail, that the former may come near enough to examine her.

ATLANTIDES, in Astronomy; another name for the

Pleiades.

ATLANTIDES, in Architecture; pillars used like the Caryatides, to support an edifice.

ATLAS, in Anatomy; the first vertebræ of the neck, which supports the head.

ATLAS, in Commerce; a very beautiful species of Chinese manufacture, now but little used. It consisted of a mixture of metal and silk, and was bought with avidity at very high prices.

ATMOSPHERE. This is the ordinary title of the elastic fluid in which we live and breathe. The weight and pressure of the atmosphere has already been examined under the article AIR, and will be further discussed when treating of the BAROMETER; but there are some interesting facts connected with the atmosphere which may properly have a place here.