ly to 0°. The principal uses of this equatorial are, 1. To find your meridian by one observation only; for this purpose, elevate the equatorial circle to the colatitude of the place, and set the declination semicircle to the sun's declination for the day and hour of the day required; then move the azimuth and hour-circles both at the same time, either in the same or contrary direction, till you bring the centre of the cross hairs in the telescope exactly to cover the centre of the sun; when that is done, the index of the hour circle will give the apparent or solar time at the instant of observation; and thus the time is gained, though the sun be at a distance from the meridian; then turn the hour circle till the index points precisely at twelve o'clock, and 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 day light. Having elevated the equatorial circle to the co-latitude of the place, and set the declination semicircle 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 appear 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."

وو

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 capabili ty 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 applied 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. 3, 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 collection 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 gos, on account of the characteristic property which it possesses of acquiring a light brown colour after being heated. The mineral consists of

dish 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 blow-pipe, 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. Magnolia, Jussieu. Essential character: calyx fiveleaved; corolla five-petalled; berries oneseeded, 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, Jussieu. Essential character: calyx double, outer three-leaved; anthers connate, anfractuose; capsule five-celled, many-seeded. There is but one species, viz. O. lagopus, a large tree, with divaricating branches; the wood is white, tender, and sufficiently light to be used in stead of corks for nets; the bark is thick, fibrous, and ash-coloured; leaves frequently a foot and a half in diameter; flowers on the upper branchlets, on thick, straight peduncles; calyx greenish-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; petals 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 order. Natural order of Verticillata. Labiatæ, Jussieu. 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 under shrubs, 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

E

be inscribed in a circle. If the radius of a circle, circumscribing a regular octagon, ber, and the side of the octagon

=y, then y

== 2rr2r2. 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 perpendicular height of one of them, and then 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 distant an eighth part of a circle, or 45° from each other.

OCTAVE, in music, an harmonical interval, consisting of seven degrees, or les ser 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 Oppositifolia. Corymbiferæ, Jussieu. Esand order. Natural order of Composite sential character: calyxes many-flowered; corollets 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

difform; in the disk sessile, barren; fruit crowned with the calyx and pistil. There hemlock water dropwort, commonly are eleven species; of which O. crocata, 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 parsnips, 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 Calycanthemæ. Onagræ, Jussieu. Essential character: calyx four-cleft; petals four; capsule cylindrical, inferior; seeds naked. There are eleven species; of which O. biennis, broad-leaved tree primrose, has a fusiform, 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 the 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 ANATO

MY.

OESTRUS, in natural history, gad fly, a genus of insects, of the order Diptera. Mouth with a simple aperture, and not exserted feelers two, of two articulations orbicular at the tip, and seated each side in a depression of the mouth: antennæ of three articulations, the last subglobular, and furnished with a bristle on the fore-part, placed in two hollows on the front. The face of this singular genus is broad, depressed, vesicular, and glaucous, and has some sort of resemblance to the ape kind. They are extremely troublesome to horses, sheep, and cattle, depositing their eggs in different parts of the body, and producing very painful tumours, and sometimes death. The larvæ are without feet, short, thick, and annulate, and often furnished with small hooks. There are twelve species, named from the animals which they infest: thus we have O. bovis, O. equi, O. ovis, O. hominis, &c. The principal European species, is the O. bovis, or ox gad-fly, which is the size of a common bee, and is of a pale yellowish colour, with the thorax marked with four longitudinal dusky streaks, and the abdomen by a black bar across the middle the lip is covered with tawny orange-coloured hairs; the wings are pale-brown, and unspotted. The female of this species, when ready to deposit her egg, fastens on the back of a heifer, or cow, and piercing the skin with the tube situated at the tip of the abdomen, deposits an egg in the puncture, and then proceeds to another spot at some distance from the former, repeating the same operation, at intervals, on many parts of the animal's back. The pain which this operation occasions is extreme; and hence

cattle, as if foreseeing their cruel enemy, are observed to be seized with the most violent horror when apprehensive of the approaches of the female oestrus, flying instantly to the nearest pond or pool of water: it having been observed that this insect rarely attacks cattle when standing in water. The eggs are laid in August or September, and the larvæ remain till the following summer before they under. go the change to the pupa state. At this period they force themselves out of their respective cells, and falling to the ground, creep beneath the first convenient shelter, and laying in an inert state, become contracted into an oval form, but without casting the larva skin, which dries and hardens round them. When the included insect is ready for exclusion, it forces open the top of the pupa coat, and emerges in its perfect form, having remained within the chrysalis somewhat more than a month.

We shall give an account of the O. equi, from the Transactions of the Lynnæan Society, drawn up with great accuracy by Mr Clarke. "When the female has been impregnated, and the eggs are sufficiently mature, she seeks among the horses a subject for her purpose; and approaching it on the wing, she holds her body nearly upright in the air, and her tail, which is lengthened for the purpose, curved inwards and upwards: in this way she approaches the part where she designs to deposit her egg; and, suspending herself for a few seconds before it, suddenly darts upon it, and leaves her egg adhering to the hair: she hardly ap pears to settle, but merely touches the hair with the egg held out on the projected point of the abdomen. The egg is made to adhere by means of a glutinous liquid secreted with it. She then leaves the horse at a small distance, and prepares a second egg, and, poising herself before the part, deposits it in the same way. The liquor dries, and the egg becomes firmly glued to the hair: this is repeated by various flies, till four or five hundred eggs are sometimes placed on one horse. The horses, when they become used to this fly, and find that it does them no injury, as the Tabani and Conopes, by suck. ing their blood, hardly regard it, and do not appear at all aware of its insiduous object. The skin of the horse is always thrown into a tremulous motion, on the touch of this insect, which merely arises from the very great irritability of the skin and cutaneous muscles at this season

of the year, occasioned by the continual teasing of the flies, till at length these muscles act involuntarily on the slightest touch of any body whatever.

"The inside of the knee is the part on which these flies are most fond of depositing their eggs, and the next to this on the side and back part of the shoulder, and less frequently, on the extreme ends of the mane. But it is a fact worthy of attention, that the fly does not place them promiscuously about the body, but constantly on those parts which are most liable to be licked with the tongue; and the ova, therefore, are always scrupulously placed within its reach. "The eggs thus deposited I at first supposed were loosened from the hairs by the moisture of the tongue, aided by its roughness, and were conveyed to the stomach, where they were hatched; but on more minute search I do not find this to be the case, or at least only by accident; for, when they have remained on the hairs four or five days they become ripe, after which time the slightest application of warmth and moisture is sufficient to bring forth, in an instant, the latent larva. At this time, if the tongue of the horse touches the egg, its operculum is thrown open, and a small active worm is produced, which readily adheres to the moist surface of the tongue, and is from thence conveyed with the food to the stomach. If the egg itself be taken up by accident, it may pass on to the intestinal canal before it hatches; in which case its existence to the full growth is more precarious, and certainly not so agreeable, as it is exposed to the bitter ness of the bile.

“

"I have often, with a pair of scissors, clipped off some hairs with eggs on them from the horse, and on placing them in the hand, moistened with saliva, they have hatched in a few seconds. At other times, when not perfectly ripe, the larva would not appear, though held in the hand under the same circumstances for several hours; a sufficient proof that the eggs themselves are not conveyed to the stomach. It is fortunate for the animal infested by these insects, that their numbers are limited by the hazards they are exposed to. I should suspect near a hundred are lost for one that arrives at the perfect state of a fly. The eggs, in the first place, when ripe, often hatch of themselves, and the larva, without a nidus, crawls about till it dies; others are washed off by water, or are hatched by

the sun and moisture thus supplied together. When in the mouth of the animal, they have the dreadful ordeal of the teeth and mastication to pass through. On their arrival at the stomach, they may pass mixed with the mass of food into the intestines; and when full grown, in dropping from the animal to the ground, a dirty road or water may receive them. If on the commons, they are in danger of being crushed to death, or of being picked up by the birds who so constantly attend the footsteps of the cattle for food. Such are the contingencies by which nature has wisely prevented the too great increase of their numbers, and the total destruction of the animals they feed on.

"I have once seen the larva of this oestrus in the stomach of an ass; indeed their is little reason to doubt their existence in the stomachs of all this tribe of animals. These larvæ attach themselves to every part of the stomach, but are generally most numerous about the pylorus, and are sometimes, though much less frequently, found in the intestines. Their numbers in the stomach are very various, often not more than half a dozen, at other times more than a hundred; and, if some accounts might be relied on, even a much greater number than this. They hang most commonly in clusters, being fixed by the small end to the inner membrane of the stomach, which they adhere to by means of two small hooks, or tentacula. When they are removed from the stomach they will attach themselves to any loose membrane, and even to the skin of the hand. The body of the larva is composed of eleven segments, all of which, except the two last, are surrounded with a double row of horny bristles, directed towards the truncated end, and are of a reddish colour, except the points, which are black. The larvæ evidently receive their food at the small end, by a longitudinal aperture, which is situated between two hooks, or tentacula. Their food is probably the chyle, which, being nearly pure aliment, may go wholly to the composition of their bodies, without any excrementitious residue, though on dissection the intestine is found to contain a yellow or greenish matter, which is derived from the colour of the food, and shows that the chyle, as they receive it, is not perfectly pure. They attain their full growth about the latter end of May, and are coming from the horse from this time to the latter end of June, or sometimes later. On dropping to the ground