Daphne, it was found to have already considerably passed opposition (when it is nearest the earth), and that only a few observations of it could be obtained before it was lost in the sun's rays. From the positions thus determined, the elements of the planet's orbit were calculated by M. Pape of Altona, but in consequence of the small extent of the arc on which they rested, it was feared that considerable difficulty would be experienced in rediscovering the planet upon coming to opposition in the following year. However, guided by an ephemeris founded on M. Pape's elements, M. Goldschmidt discovered an asteroid on the 9th of September, 1857, which he supposed to be Daphne, and under the impression that his surmise was correct, a considerable number of positions of the asteroid were obtained at various observatories throughout Europe. It is only within the last few months that M. Ernest Schubert, a German astronomer, has found that the orbit calculated from the observations of 1857 is totally irreconcilable with the positions of Daphne obtained in the previous year. It was plain, therefore, that the observations of 1856 and 1857, referred to two distinct bodies, and that the object discovered by M. Goldschmidt on the 9th of September, 1857, was in reality not Daphne, but a new asteroid. Since the date just mentioned, however, nine additional asteroids had been discovered, to each of which the proper characteristic number had been prefixed. In order, therefore, not to disturb this arrangement, astronomers have agreed to distinguish the asteroid originally observed on the 9th of September, 1857, by the numerical symbol o). In the mean time, the asteroid Daphne has not been seen since 1856, the year of its discovery. The ten asteroids attributed to Mr. Hind in the foregoing list, and also the asteroid discovered by M. Marth, were discovered at the observatory of Mr. Bishop, Regent's Park, a gentleman who, by the munificent patronage which he has extended towards astronomy during a long series of years, has earned for himself an honourable name in the annals of science. It is worthy of remark, that M. Goldschmidt, the discoverer of twelve asteroids, is by profession an historical painter. His discoveries have been all made during his leisure hours in the evening, with a telescope of comparatively moderate dimensions fitted up in an apartment of his private residence. Allusion has been made to the useful services rendered by the Berlin charts in the discovery of asteroids. Recently they have been superseded in so far as this object is concerned by charts of a more limited range, but which include stars of a smaller magnitude. The construction of such charts has been rendered necessary by the faint aspect of the asteroids discovered in recent years (being generally inferior in brightness to stars of the tenth magnitude). It is of course reasonable to suppose that the brighter asteroids would have been detected in the first instance. Charts for the discovery of further asteroids are in the course of being published at the expense of Mr. Bishop. Similar charts, constructed by M. Chacornac are also emanating from the Imperial Observatory, Paris. The theory of the movements of the asteroids is almost entirely due to the German astronomers, more especially Gauss, Encke, and Hansen. Upon the discovery of one of those bodies, the first step is to determine the elements of the orbit from a limited number of observations. For this purpose the method given by Gauss in his immortal work, the Theoria Motus,' or a modification of it by Encke (Berlin Jahrbuch,' 1854), is usually employed. The results thus obtained are subsequently corrected by a discussion of all the observations made during the opposition of the asteroid (on which occasion alone it is generally visible), taking into account the effects of perturbation, more especially those due to Jupiter the principal disturbing body. In consequence of the great magnitude of the eccentricities and inclinations of their orbits, the methods usually employed in computing the perturbations of the older planets, are not generally applicable to the asteroids. It has accordingly been found necessary to have recourse to what has been called the method of quadratures. In other words, instead of computing the perturbations from general formulæ, involving mean values of the elements as determined by the discussion of a long series of observations, the astronomer calculates their numerical effects for a succession of special epochs, by a process not adapted to mean elements (and therefore not admitting of direct application to any time whether past, present, or future), but to osculating elements corresponding to some given epoch. These elements, being used as the starting point of calculation, are subsequently corrected at the close of each of the intervals into which the period of perturbation is divided. In more recent times MM. Hansen and Encke have devised methods for obtaining general formulæ of perturbation which are applicable to all the asteroids. Tables of Flora founded on formulæ derived from the development of Professor Encke's method, have been calculated by Dr. Brünnow. Professor Hansen has applied his theory to the asteroid Egeria. The following synopsis [see cols. 644, 645, and 646] of the elements of all the asteroids hitherto discovered has been derived from the Berlin 'Jahrbuch' for 1861, with the exception of the elements of the In the case of Vesta, tables have been calculated by Santini and Daussy from general formulæ representing the effects of perturbation. In the 'Connaissance de Temps' for 1846, Damoiseau has given the general formula for the perturbations of Juno and Ceres. of the neighbouring planets Mars and the earth. M. le Verrier has considered the subject from this point of view. His researches are founded on the twenty-six asteroids which had been hitherto discovered, but the conclusion at which he arrived is equally applicable in the present day. He found that if the aggregate mass of the asteroids was equal to the earth's mass, its attraction would disturb the place of the perihelion of Mars to the extent of 11" in a century. Now, if there really existed from such a cause a variation in the perihelion of the planet, amounting to even the fourth part of this quantity, it could not escape observation in the present advanced state of practical astronomy. The conclusion therefore is that the aggregate mass of the asteroids does not amount to one-fourth of the earth's mass. ASTHMA. [BRONCHITIS.] ASTRAGAL, a moulding used in architecture, and applied principally to the upper ends of the shafts of columns and to their bases. It is also used in the entablatures of the Roman Doric, the Ionic, Corinthian, and Composite orders. The term is derived from the Greek ȧorpayaλos, which signifies the bone on which the tibia rests, and sometimes a vertebra. The form of this moulding is semicircular, projecting from a vertical diameter. The surface is usually worked plain, although there are Roman examples of its being carved to represent leaves, as in the arch of the goldsmiths at Rome, or reeds bound together, as in the pedestal of Trajan's column. The astragal cut into beads is common to Greek and Roman architecture. 9.4 15 9.7 10:0 10.1 11.1 Calypso 17 12.5 " Melpomene Atalanta 3 11.8 The asteroids, Iris, Hygeia, Egeria, Thetis, Phocea, Urania, Pomona, Leucothea, Lætitia, Ariadne, Nysa, Aglaja, Doris, and Pandora do not come to opposition in 1859. The asteroid (se) which is not included in the list from which the foregoing numbers are extracted, may be stated to be about the 10-11th magnitude when it comes to opposition. Since the asteroids exhibit no appreciable discs by the measurement of which their absolute diameters might be determined, as in the case of the other planets, astronomers have endeavoured to accomplish the same object by means of photometric results, founded on their apparent brightness, combined with an assumption relative to their power of reflecting the solar light. Professor Stampfer, of Vienna, has endeavoured in this way to determine the absolute magnitudes of several of the asteroids. By photometric experiments he found that the planets Mercury, Venus, and Jupiter, possess the same capacity for reflecting the solar light. By extending this principle to the asteroids, the following results have been obtained (Bruhns, 'De Planetis Minoribus,' Berlin, 1856) relative to the absolute magnitude of those bodies. No trustworthy conclusion can be arrived at respecting the aggregate number of the asteroids. If, indeed, we suppose them to be all of the average size of those already discovered, we might then be enabled to form some opinion on the subject, for the zone in which they perform their revolutions contains only a definite number of stars of the average apparent magnitude of those bodies. But the more the search for new asteroids is prosecuted and the greater their aggregate number becomes, the smaller the more recently-discovered ones turn out to be, and the obvious inference therefore is, that there probably exist many more of such bodies, which from their minute magnitude have hitherto escaped detection. But if it is impossible to assign any limit to the number of the asteroids, the result is different when we consider their aggregate mass. In this case it may be ascertained by calculations founded on the theory of gravitation, that if the quantity of matter contained in the totality of those bodies exceeded a certain amount, their attractive force could not fail to produce sensible perturbations in the movements d Sections of astragal mouldings, and elevations of astragal mouldings carved: -a, section of an astragal form the three columns of the temple of Jupiter [Stator], in the Campo Vaccino, at Rome; b, astragal used in the base of the Ionic order of the temple of Minerva Polias at Priene; c, enriched astragal used in the arch of the goldsmiths at Rome; d, enriched astragal of the pedestal of Trajan's column at Rome; e, astragal cut into beads. The apparent use of the astragal is, to bind the parts of columns and entablatures together, for which purpose it is employed both at the top of the shaft where the capital commences, and at the bottom where the base terminates. Many of the parts also of the entablature are bound together with the astragal moulding. In Egyptian architecture, bands curved after the manner of astragals seem to bind the reeds of which the shaft of the column often appears to be formed. In the choragic monument of Lysicrates at Athens, supposed to be one of the oldest examples of the Corinthian order, it has been conjectured that the hollow between the top of the shaft and the lower part of the capital of the column formerly received a bronze ring of the form of an astragal, by which means, if the conjecture be well founded, the parts, from the contrast of colour, would appear to be more distinctly bound together. The most remarkable example of the use of the astragal in Grecian architecture is in the base employed in the Ionic temple of Minerva Polias at Priene; which has been imitated by Mr. Cockerell in the portico in the front of Hanover Chapel, Regent-street. In the temple of Jupiter Olympius, at Athens, the astragal at the top of the column appears to have a channel cut underneath it. (Stuart, vol. iii.) This, however, is very unusual. [COLUMN.] ASTREA, one of the group of small planets revolving between Mars and Jupiter. [ASTEROIDS.] ASTRINGENTS (from astringo, to constringe, or bring closer together) are agents which contract the fibres of the muscles and blood-vessels, and lessen the flow of fluids, whether it be the secretions of the glands proceeding from their natural orifices in excessive quantity, or the contents of the blood-vessels escaping by their exhalent extremities, or by an unnatural opening (or rupture). They produce this effect, generally by a vital, but sometimes by a chemical action. Their power is manifested first, and often solely, on the part to which they are applied; yet in many instances it is extended by sympathy very rapidly over the whole body, as is observed when the acerb juice of the sloe is brought in contact with the tongue. The sensation then experienced may be considered the best general test of the presence of astringency, which cannot be ascribed to any one prin while watery grasses, among which no astringent plants grow, favour the generation of worms. ciple, but is owing to tannin, gallic acid, and hæmatine, in vegetable astringents, and is possessed by acids, and many metallic salts among mineral agents; and is also one of the effects of the application of cold The primary sympathetic effect of several of the astringents which to the body. In vegetables, the astringent principles are found chiefly ultimately enter into the circulation, is the most valuable in some of in the bark (as oak), the root (as rhatany and tormentil), and the wood the cases in which they are employed, such as dilute sulphuric acid, (as logwood). As wood and bark form parts of exogenous trees only, it which often checks hæmorrhage by closing a bleeding vessel, before any is only from this section of the vegetable kingdom that any astringent of it can be conceived to have been conveyed directly to the bleeding principles can be obtained. [See explanation of the term exogenous, orifice; it checks the flow of blood in the same way as cold suddenly under the article AGE OF TREES.] Sir Humphry Davy found that the applied to the surface or skin does. The tonic effect of many inner layer of the bark possessed the greatest quantity of the astrin- astringents, after their use for some time, first on the digestive organs, gent principle: this is the natural consequence of the mode in which and afterwards upon the whole system, and more especially upon any the sap descends from the leaves, namely, through this inner layer of weak organ, must be admitted, and borne in mind in forming our estibark, whence it occasionally passes into the wood, which will then be mate of their utility in a curative point of view. Without attempting found to possess principles similar to those of the bark. Most astrin- to account for the ultimate cause of the action of astringents, to dc gent vegetables are red, owing to the presence of an acid in excess, which successfully seems impracticable in the present imperfect state which is often manifest to the taste, as in rumex or sorrel. In of our knowledge, it may be stated, that under their influence a tension metallic astringents, when super-salts, the excess of acid is also very of the parts is produced, during which the muscular and vascular strucperceptible to the taste, as in alum, which is a supersulphate of alumina tures acquire an increase of power, and secreting surfaces and glands and potassa. produce less fluid, but more natural secretions. Some indeed lessen the action of the heart, and so stop the flow of blood from dilated or ruptured vessels, such as the preparations of lead, which, though in some degree astringent, ought to be considered at sedatives; while others which combine with, and neutralise the unhealthy or excessive secretions, as lime and its carbonate with the secreted fluids of the intestinal canal, are more properly termed absorbents than astringents. When astringents are applied directly to the bleeding vessels, such as to external wounds, or to the nostrils or gums, they are termed styptics, and in such cases they often act chemically as well as vitally. The particular principle to which any substance is indebted for its astringent power, may be ascertained by appropriate tests. When tannin exists in plants, its presence may be proved by an insoluble precipitate taking place on the addition of a concentrated solution of gelatin. The precipitate is a compound in definite proportions of tannin and gelatin, being forty-six of tannin and fifty-four of gelatin. Gelatin has therefore been proposed by Sir Humphry Davy as a test of the quantity of tannin in different astringent vegetables. (See Philosophical Transactions,' 1803.) But in the practical application of this test there are some sources of fallacy difficult to guard against. (See Papers by Dr. Bostock in Nicholson's 'Journal,' vol. xxiv. 1809, and by Mr. E. B. Stephens, in 'Annals of Philosophy,' New Series, vol. x. p. 401.) Tannin rarely exists alone, though it probably does so in catechu, but mostly along with gallic acid. Extractive is also a frequent accompaniment of tannin, and is of considerable service, assisting its action in the process of tanning. Gallic acid strikes a bluish-black precipitate with all the salts of iron, but a solution of the persulphate is the ordinary test. Hæmatine exists in logwood, along with tannin and extractive. It may be known by combining with oxide of lead without undergoing any change. Before proceeding to consider the cases in which astringents may be advantageously used, an enumeration of the most common and valuable substances may be given. Of vegetable astringents the chief are barks, as of oak and willow, the best kind of the former of which is obtained from the Quercus robur of Linnæus (the true British oak) which is synonymous with the Quercus pedunculata of Willdenow, while the inferior sort is obtained from the Quercus sessiflora of Salisb., which is synonymous with the Quercus robur of Willdenow. The best willow-bark is procured from the Salix pentandra, or sweet bay-leaved willow, though very excellent bark is yielded by the Salix Russelliana, or Bedford willow. Roots, as of tormentil (Potentilla tormentilla); bistort (Polygonum bistorta); common avens (Geum urbanum), which are British plants; and rhatany (Krameria triandra), rhubarb (Rheum leaves of arctostaphylos (Uva ursi), petals of the Rosa gallica, fruits of Prunus spinosa, or sloe-thorn (Punica granatum), and secreted juices of many plants, as kino, from Petrocapus marsupium, and several others; catechu, from Acacia catechu, and galls, from Quercus infectoria; in all of which the astringent principle is tannin, with more or less of gallic acid; log-wood (Hamatoxylon Campechianum), in which hæmatine as well as tannin possesses an astringent property; and Egle Marmelos, or Indian Bael. Acetic acid must also be classed among the vegetable astringents. The effect of astringents which is due to their chemical action is nearly the same in dead as in living animal matter; their long-con-palmatum); pomegranate (Punica granatum), which are exotic plants; tinued application to the skin will produce a condition similar to that of a tanned hide. They are therefore sometimes employed to effect this, when internal parts are exposed, to change them from a secreting to a non-secreting surface-such as in irreducible prolapsed uterus. Their use in this way however is very limited, whilst their vital action is extensive and important. The chief effects of astringents are to contract the muscular and vascular tissues, to diminish secretion, and lessen irritability, and in many instances to impart strength or increased tone to an organ or part. Their action is always greatest on the part to which they are applied. When a drop of diluted acetic or sulphuric acid is applied to the skin, whiteness of the part is observed, which soon disappears, and the natural colour, or even a more intensely red one, follows. If this is frequently repeated, the structure of the part is changed; it ceases to secrete, is no longer pliant, but becomes stiff and inflexible. The loss of colour is owing to the diminished calibre of the blood-vessels, which no longer admit the red globules. During the absence of these, the sensibility of the part is less than natural; just as cold and torpid fingers lose their fineness of touch. Nearly similar effects may be supposed to follow the internal administration of astringents, the action of which is greatest on the intestinal canal, and less on parts remote from this; yet it deserves to be remarked, that as the intestinal canal is a mucous membrane, and possesses a muscular structure, parts of a similar structure are more influenced by astringents introduced into the stomach than other parts are hence, increased secretion from the mucous membrane of the lungs, or from the lining membrane of the bladder, or flow of blood from arteries, is more effectually checked by astringents than increased exhalation from serous surfaces. There is reason to believe that the astringent principle of many plants does not enter into the circulation, but passes along the whole course of the intestinal canal without being absorbed; for Sir Humphry Davy found, that when tannin is present in grasses, as it is in that of aftermath crops, it is voided in the dung of the animals which feed upon it. (See Davy, Elements of Agricultural Chemistry, Appendix,' p. lxi.) But that of other plants enters the system so rapidly, that the astringency of the Uva Ursi, or bear's whortleberry, can be detected in the urine forty-five minutes after it has been swallowed. In the case of those which do not enter into the circulation, any beneficial effect which they exert upon remote organs must be attributed to that sympathy which exists in so great and unquestionable a degree between the stomach and almost every organ of the body. That such vegetable substances, while passing along the intestinal canal, promote the fulfilment of its functions, is obvious, from the effects following the use of food in which astringent principles are absent. Plants possessing astringent powers and bitter principles, such as tormentil and the bog-bean, are very efficacious in preventing the rot in sheep (as has been already stated under ANTHELMINTICS), The mineral astringents are- -diluted sulphuric acid, and salts of iron, zinc, copper, silver, and the salts of lead. Cold, in whatever way applied, is also a valuable astringent, particularly in the form of ice. In treating of the employment of astringents as curative agents, it is necessary to distinguish between their action as local, direct, and often chemical, and their action as general, influencing remote organs, their effects upon which are vital rather than chemical; also between their mere astringent power and their tonic power. The beneficial effects of many of the above-named astringents in checking increased secretion, is doubtless often due to their tonic power; for as in a weak state of the system or any particular gland, the secretions are generally profuse in quantity, a return to the healthy proportion and quality can only be insured by increasing the power or tone of the body or gland, which astringents do by bringing the living tissues into a closer or more compact state, and which tonics do by heightening the vitality of the debilitated structures. Hence astringents are beneficially employed in diseases where a laxity of the muscular and vascular tissues exists, accompanied with imperfect discharge of the functions of the secreting organs. The stomach and intestinal canal being the channel by which is conveyed the material necessary for the nourishment and vigour of the system, and for maintaining a capacity to discharge their functions in the other organs of the body, an impaired state of the structure and functions of this canal extends to every other part. The re-establishment of its healthy condition is a primary object in endeavouring to cure many diseases. Of these intermittent and remittent fevers may be taken as an example, since in these there is always great debility of the digestive organs, and of all the parts which have the most intimate sympathy with them, such as the skin. Astringents possessed of a tonic power have, therefore, mostly been resorted to, in order to remove this debility; cinchona-bark, willow-bark, and many others, have been used with this intention. These, however, are to be avoided whenever any acute inflammation exists, which must first be subdued by appropriate means before tonic astringents can be safely or advantageously used. In diseased states of the intestinal canal, in which greatly increased or unhealthy secretions take place, as diarrhoea, dysentery, and cholera, the most careful inquiry should be made into the cause of the disease, that if it has its origin in an inflammatory 651 ASTRINGENTS. ASTROLABE. condition of the mucous membrane of the intestine, or is owing to the substances, and they seem to have dipped the coarse cloths in which presence of any acrid substance, the former may be overcome by anti- the mummies were enveloped in some astringent liquid, which tanned phlogistic measures, and the latter be removed by purgatives. When the skin, and rendered it less subject to change, as well as excluded with this view is supposed to have been some sort of kino. The same the increased flow from the intestines is connected with increased the air from the interior of the body. The article employed by them determination towards these parts, owing to the application of cold to the outer surface suppressing the secretion of the skin, which has the substance is said to be used by the Chinese to dye cotton for their This property of astringents may be usefully applied for the pregreatest sympathy with the internal surface, and which consequently nankeens, but perhaps a peculiar cotton. is excited to double action, a preternatural quantity of secreted fluid is produced. The most effectual, as well as only safe means of diminish-servation of all kinds of cordage, fishing-lines, and nets, which last ing this, in the early stages of its occurrence, is the employment of much longer if steeped in an infusion of oak-bark. Though inferior in diaphoretics, or such medicines as restore the action of the skin; after preserving power to the plan of Mr. Kyan, it may be applicable in some For further information on astringents, see Dr. A. T. Thomson's which, should the discharge continue, mild astringents may be used, cases where his is inadmissible. [ANTISEPTICS.] of which logwood or tormentil is the best. A preliminary treatment is much recent valuable matter is brought together. likewise required in dysentery: in the common cholera a purgative Elements of Materia Medica and Therapeutics,' vol. ii., in which should generally be given before any astringent is administered. heat. In the bilious cholera of autumn, after the employment of suitable purgatives, nothing seems to act more effectually as an astringent than the infusion of cusparia, or angustura bark, with dilute nitric acid; to which, in some cases, a small portion of laudanum may be added at first, but afterwards omitted. Nor in the slighter cases of epidemic cholera, has more marked benefit followed the use of any means than has resulted from the employment of this combination, which speedily checks the liquid discharges, and restores the circulation and animal Diarrhoea, or looseness of bowels, proceeding from acid secretions, is best removed by the astringents which combine chemically with these -such as lime, or its carbonate, which are rendered more suitable by uniting them with aromatics, an excellent form of which is supplied by the mistura crete, or chalk mixture of the pharmacopoeia. The next most important class of diseases in which astringents may be employed are termed hæmorrhages, or a discharge of blood, either from the exhalant extremities of the arteries, when they are gorged or when they are too much relaxed, or from the wounded or ruptured The above distinction refers to the differcoats of any blood-vessel. ences between active and passive hæmorrhage, or that which takes place when the system is too full of blood and the vessels propel it with great force; the other, which takes place when the power of the vessel is greatly below the natural standard. In the former, astringents cannot safely be employed at the commencement of the flow of blood, but time should be allowed for the vessels to unload themselves; or a vein should be opened, cooling saline medicines administered, cold air admitted freely to the surface of the body, and, under competent medical attendance, opium or laudanum may be given; after which, astringents will either not be required, or if so, may be safely used. In passive hæmorrhage they may be employed from the commencement; and perhaps, in most cases, a saturated solution of alum in the infusion of roses is to be preferred, though the tincture of the muriate of iron is very eligible when the kidney is the source of the bloody discharge, as acetate of lead is when the lungs are the organs whence the blood flows. So long as lead is kept in the state of an acetate, its administration is perfectly safe: it should therefore always be accompanied with dilute acetic acid. Bleeding from the nostrils or gums may be checked by the direct application of styptics; such as preparations of zinc or copper. Nitrate of silver will frequently stop the flow of blood from a leech bite; collodion more effectually still. Cold should, in most cases, be employed along with the other means; even alone it is often successful, especially in the form of water poured from a height in uterine hæmorrhage. Ruspini's styptic, which is said to be a solution of gallic acid in alcohol, is sometimes useful, where other means have failed. Matico is very useful. The application of astringents to more limited examples of loss of tone or increased flow of secreted fluids, need not be extensively noticed here. After acute inflammation of the eye, proper antiphlogistic means having been used, astringent applications are very serviceable, especially those of zinc and nitrate of silver, either in solution or made into an ointment. Scrofulous inflammation of the eye is often Salivation, or benefited by them, if internal means be also used. excessive flow of saliva, occurring either spontaneously or from the use of mercury or other means, is often effectually checked by nitrate of silver, or decoction of the rhus glabrum, or by iodine, or infusion of cloves. Nitrate of silver, by lessening the inflammation which gives rise to them, also frequently removes morbid discharges from other mucous surfaces besides those we have specially noticed; an effect which also often follows the use of diluted chloride of soda. The colliquative sweats of hectic fever are best checked by giving internally dilute sulphuric acid, and sponging the skin with vinegar and water, or by oxide of zinc. Astringent substances are decomposed by, or decompose, many others, which therefore should not be given at the same time with them; such, for example, as ipecacuanha with most of the vegetable astringents which contain tannin, by which an insoluble tannate of emetina is formed: when kino is united with calumba, a purgative action follows. All astringent vegetables containing tannin, except oak-bark, decompose tartrite of antimony, and are therefore the best antidotes to it, especially tea. The ancient Egyptians would appear to have been acquainted with the power of astringents in preserving vegetable as well as animal (For the employment of astringents in the arts, see DYEING AND TANNING; and also Library of Entertaining Knowledge'-Vegetable ASTROLABE, from two Greek words, signifying to take the stars. Substances; Materials of Manufactures, p. 178, 1st edition). It has an earlier and a later meaning. As used by Ptolemy, it may stand for any circular instrument used for observations of the stars; but in the 16th and 17th centuries it signified a projection of the Planisphere. To this small projection, which had a graduated rim, sphere upon a plane, being used in the same sense as the word sights were added, for the purpose of taking altitudes; and in this state it was the constant companion and badge of office of the astrologer. circular rim, with sights attached, called an astrolabe, was used for In later times, before the invention of Hadley's quadrant, a graduated taking altitudes at sea, as further described in Bion, Traité des Instrumens de Mathématique,' Hague, 1723. In the older sense of the word, every one of our modern astronomical instruments is a part of the If a solid circle be fixed in any one position, and a tube be fixed upon astrolabe, the principle of which we proceed to describe. its centre, round which it may be allowed to move, as in the adjoining diagram; and if the line c D be drawn upon the circle, pointing towards obvious that, by turning the tube A B towards any other object p in the any object Q in the heavens which lies in the plane of the circle, it is plane of the circle, the angle BOD will be the angle subtended by the two objects P and Q at the eye, or their angular distance upon a common globe. This angle may be measured, if the circumference of the circle be graduated. Thus, suppose the plane of the circle to pass then when the tube points towards the star, NOB its north polar dis through the poles N and s, and CD to point towards the equator; tance, or BOD its declination, may be measured; or if the circle be fixed in the plane of the equator, and CD be made to point towards towards the star, then the angle DOB will be the right ascension of the vernal equinox at the same moment at which the tube points the star. A collection of circles, such as the Armillary Sphere, might therefore, by furnishing each circle with tubes, be made a complete astrolabe. The practical difficulty consists in keeping so many circles exactly in their proper relative positions. The distinction between the astrolabe of the ancients and the circular instruments of the moderns, is as follows:-First, the ancients endeavoured to form an astrolabe of two circles, so as to measure both latitude and longitude, or both right ascension and declination, by the same instrument; while the moderns, instruments were made to revolve, to find the star, or were furnished in most cases, measure only one of the two. Secondly, the ancient or ecliptic, according as declination or latitude was to be measured. with at least one revolving circle, moving round the pole of the equator The moderns for the most part fix their instruments in the meridian and wait for the star. But the equatorial, the altitude and azimuth meaning of the term. circle, and the theodolite, are strictly astrolabes, according to the ancient Hipparchus is the first we know of who can be reasonably supposed to have made use of an astrolabe. But, at the same time, there are reasons for supposing that Eratosthenes, a century before Hipparchus, made use of a circle fixed in the meridian, for measuring the obliquity of the ecliptic. He is also said to have erected armillary circles at was in all respects his follower, and therefore, probably, in describing Alexandria. Ptolemy does not mention Hipparchus expressly; but he his own instrument, he is only repeating that of his great predecessor. And Nicholas Cabasillas (an ecclesiastic of the 14th century, cited by Delambre) attributes to Hipparchus an instrument consisting of an |