lower the telescope to the horizon, in order to observe some point there in the centre of your glass, and that point is your meridian mark found by one observation only; the best time for this operation is three hours before or three hours after twelve at noon. 2. To point the telescope on a star, though not on the meridian, in full daylight. Having elevated the equatorial circle to the co-latitude of the place, and set the declination semi-circle to the star's declination, move the index of the hour circle till it shall point to the precise time at which the star is then distant from the meridian, found in tables of the right ascension of the stars, and the star will then ap. pear in the glass. Besides these uses peculiar to this instrument, it is also applicable to all the purposes to which the principal astronomical instruments, viz. a transit, a quadrant, and an equal altitude instrument, are applied. See Vince's "Practical Astronomy." OBSIDIAN, in mineralogy, a genus of the Pitch-stone family, found in nests in the pearl-stone of Hungary. It is common likewise in Iceland, Siberia, the Levant islands, and in South America, and has obtained the name of the Iceland agate. The principal colour is velvet-black, but it passes into greenish grey. It is often striped and spotted. The specific gravity is about 2.4: it melts into an opaque, grey mass. Specimens have been analysed, and found to contain It is on account of its great hardness and ⚫ opaque blackness, and of its capability of receiving a high polish, used as an ornament in dress. In Peru, before the conquest of the country by Spain, obsidian was used as a mirror, and in Europe it has been fashioned into reflectors for telescopes. OBTUSE, signifies blunt, dull, &c. in opposition to acute, sharp, &c.; thus we say, obtuse angle, obtuse angled triangle, &c. OCCIDENT, in geography, the westward quarter of the horizon, or that part of the horizon where the ecliptic, or the sun therein, descends into the lower hemisphere, in contradistinction to orient. OCCIPITAL, in anatomy, a term appli ed to the parts of the occiput, or back part of the skull. OCCULT, something secret, hidden, or invisible. The occult sciences are, magic, necromancy, cabbala, &c. OCCULT, in geometry, is used for a line that is scarcely perceivable, drawn with the point of the compasses, or a leaden pencil. These lines are used in several operations, as the raising of plans, designs of building, pieces of perspective, &c. They are to be effaced when the work is finished. OCCULTATION, in astronomy, the time a star or planet is hidden from our sight, by the interposition of the body of the moon, or of some other planet. OCCULTATION, Circle of perpetual, is a parallel in an oblique sphere, as far distant from the depressed pole, as the elevated pole is from the horizon. All the stars between this parallel and the depressed pole, never rise, but lie constantly hidden under the horizon of the place. OCCUPANCY, in law, is a right which one acquires to a thing by being the first to gain possession of it. But this right is now chiefly done away by the English law. Formerly, if a tenant for a term of another's life died, leaving the cestui que vie; that is, during the life of the person for whose life the estate was held; he who first entered should hold the land during the other man's life; and he was in law called an occupant, because his title was by his first occupation. But now this title is prevented by the statutes 29 Charles II. c. S, s. 12, and 14 George II. c. 20, s. 9, which make the estate personal assets devisable, and chargeable with the debts of the deceased, in the hands of the heir, who enters as special occupant. OCEAN, in geography, that vast collec tion of salt and navigable waters, in which the two continents, the first including Europe, Asia, and Africa, and the last America, are inclosed like islands. The ocean is distinguished into three grand divisions. 1. The Atlantic Ocean, which divides Europe and Africa from America, which is generally about three thousand miles wide. 2. The Pacific Ocean, or South Sea, which divides America from Asia, and is generally about ten thousand miles over: and 3. The Indian Ocean, which separates the East Indies from Africa, which is three thousand miles over. The other seas, which are called oceans, are only parts or branches of these, and usually receive their names from the countries they border upon. For the saltness, tides, &c. of the ocean, see the articles SEA, TIDes, &c. OCHRES, in chemistry, combinations of earths with the oxide of iron: they are of various colours, and are principally employed as pigments. OCHROIT, in chemistry, an earth discovered by Klaproth: the colour of the mineral in which the earth is found, and which is denominated ochroites, is between red and brown. It is compact, and breaks splintering in irregular or angular pieces. It is perfectly opaque, and the powder is of a reddish grey. The specific gravity is about 4.6. The earth was called ochroit, from the Greek word wygos, on account of the characteristic property which it possesses of acquiring a light brown colour after being heated. The mineral consists of Ochroit earth is capable of combining with carbonic acid, during its precipitation from acids by carbonated alkalies, and strongly consolidating a portion of water. It is observed in "Nicholson's Journal," that the ochroit earth bears the nearest relation to ittria, and like that, it forms a connecting link between the earths and the metallic oxides. Like ittria, it has the pro perty of forming a reddish-coloured salt with sulphuric acid, and is precipitable by prussiate of potash; but it differs from ittria, in that it does not form sweet salts; that it is not soluble, or at least very sparingly, in carbonate of ammonia; and that, when ignited, it acquires a cinnamon-brown colour. It differs also from ittria, by not being soluble in borax, or phosphate of soda, when urged upon charcoal before the blowpipe, which salts easily effect a solution of ittria, and melt with it also into a pellucid pearl. See ITTRIA. OCHNA, in botany, a genus of the Polyandria Monogynia class and order. Natural order of Coadunatæ. Magnoliæ, Jussieu. Essential character: calyx five-leaved; corolla five-petalled; berries one-seeded, fastened to a large, roundish receptacle. There are three species. OCHROMA, in botany, a genus of the Monadelphia Pentandria class and order. Natural order of Columniferæ. Malvaceæ, Jussien. Essential character: calyx donble, outer three-leaved; anthers connate, anfractuose; capsule five-celled, many-seed ed. There is but one species, viz. O. lagopns, a large tree, with divaricating branches; the wood is white, tender, and sufficiently light to be used instead of corks for nets; the bark is thick, fibrous, and ash-coloured; leaves frequently a foot and half in diame ter; flowers on the upper branchlets, on thick, straight peduncles; calyx greenisha red; petals white, fleshy; capsule eight or ten inches long. It is a native of America. OCHROXYLUM, in botany, a genus of the Pentandria Trigynia class and order. Essential character: calyx five-cleft; pe. tals five; nectary an annular three-lobed gland; capsule three, approximating, onecelled, two-seeded. OCIMUM, in botany, basil, a genus of the Didynamia Gymnospermia class and or der. Natural order of Verticillatæ. Labiatæ, Jussien. Essential character: calyx with the upper lip orbiculate, the lower four-cleft; corolla resupine, with one lip four-cleft, the other undivided; filaments, the two outer putting forth a reflex process at the base. There are twenty-five species; these are either herbs or undershrubs, possessing a sweet scent; their flowers are in whorls, forming a loose spike, terminating and axillary. OCTAGON, in geometry, is a figure of eight sides and angles: and this, when all the sides and angles are equal, is called a regular octagon, or one which may be inscribed in a circle. If the radius of a circle, circumscribing a regular octagon, be and the side of the octagon =y; then = 1, y = √7-r√/2r2. OCTAGON, in fortification, denotes a place that has eight bastions. OCTAHEDRON, or OCTAEDRON, in geometry, one of the five regular bodies, consisting of eight equal and equilateral triangles. See the article BoDy. The square of the side of the octahedron is to the square of the diameter of the circumscribing sphere, as 1 to 2. If the diameter of the sphere be 2, the solidity of the octahedron inscribed in it will be 1.33333, nearly. The octahedron is two pyramids put together at their bases, therefore its solidity may be found by multiplying the quadrangular base of either of them, by one-third of the per pendicular height of one of them, and then difform; in the disk sessile, barren; fruit doubling the product. OCTANDRIA, in botany, the eighth class in Linnæus's system, consisting of plants with hermaphrodite flowers, which are furnished with eight stamina or male organs of generation. There are four orders belonging to this class of plants which derive their names from the number of female organs possessed by the plants of each respective division. OCTANT, or Octile, in astronomy, that aspect of two planets, wherein they are dis. tant an eighth part of a circle, or 45° from each other. OCTAVE, in music, an harmonical interval, consisting of seven degrees, or lesser intervals. See MUSIC. OCTOBER, in chronology, the tenth month of the Julian year, consisting of thirty-one days: it obtained the name of October from its being the eighth month in the calendar of Romulus. See the articles MONTH and YEAR. ODE, in poetry, a song, or a composition proper to be sung. Among the ancients odes signified no more than songs; but with us they are very different things. The ancient odes were generally composed in honour of their gods, as many of those of Pindar and Horace. These had originally but one stanza, or strophe, but afterwards they were divided into three parts, the strophe, the antistrophe, and the epode. The priests going round the altar singing the praises of the gods, called the first entrance, when they turned to the left, the strophe; the second, turning to the right, they called antistrophe, or returning; and, lastly, standing before the altar, they sung the remainder, which they called the epode. OECUMENICAL, signifies the same with general, or universal; as oecumenical council, bishop, &c. OEDERA, in botany, a genus of the Syngenesia Polygamia Segregata class and order. Natural order of Compositæ Oppositifolia. Corymbiferiæ, Jussieu. Essential character: calyxes many flowered; corrollets tubular, hermaphrodite with one or two female ligulate florets; receptacle chaffy; down of several chaffs. There are two species, viz. O. prolifera, and O. aliena, both natives of the Cape of Good Hope. OENANTHE, in botany, dropwort, a genus of the Pentandria Digynia class and order. Natural order of Umbellatæ, or Umbelliferæ. Essential character: florets crowned with the calyx and pistil. There are eleven species; of which O. crocata, hemlock water dropwort, commonly grows four or five feet high, with strong jointed stalks, which being broken emit a yellowish fetid juice; the leaves are similar to those of hemlock, but of a lighter green colour; the roots divide into four or five larger taper ones, having some resemblance to parsneps, for which they have been taken. It grows naturally in several parts of Europe, on the banks of ditches, rivers, and lakes. OENOTHERA, in botany, tree primrose, a genus of the Octandria Monogynia class and order. Natural order of Calycanthema. Onagræ, Jussieu. Essential character: ca. lyx four-cleft; petals four; capsule cylin drical, inferior; seeds naked. There are eleven species; of which O. biennis, broadleaved tree primrose, has a fusiforin, fibrous root; from this, the first year, arise many obtuse leaves, spreading flat upon the ground; from among these, the second year, come out the stems, three or four feet in height, upright, of a pale green colour; flowers solitary, each being separated by a leaflet, or bracte; they usually open between six and seven o'clock in the evening; for this reason the plant is called evening, or night primrose; the mode of their expanding is curious; the petals are held together at top by the hooks at the end of the calyx; the segments of which first separate at bottom, discovering the corolla, a long time before it acquires sufficient expansive force to unhook the calyx at top; when it has accomplished this, it expands almost instantaneously to a certain point, it then makes a stop, taking time to spread out quite flat; it may be half an hour from the first bursting of the calyx at bottom to the final expansion of the corolla, which commonly becomes flaccid in the course of the next day, according to the heat or coolness of the weather; the uppermost flowers appear first in June; the stalks keep continually advancing in height, and there is a constant succession of flowers till late in autumn. It is a native of North America. OESOPHAGUS, the gula, or gullet, is a membranaceous canal, reaching from the fauces to the stomach, and conveying into it the food taken in at the mouth. Its figure is somewhat like that of a funnel, and its upper part is called by anatomists the pharynx. See ANATOMY. OESTRUS, in natural history, gad-fly, a genus of insects of the order Diptera. |