objects the brightest and most distinct, and which bears the greatest aperture, and the most convex and concave eye-glasses, without colouring or haziness. A circular object-glass is said to be truly centered when the centre of its circumference falls exactly in the axis of the glass; and to be ill centered when it falls out of the axis. To prove whether object-glasses be well centered, hold the glass at a due distance from the eye, and observe the two reflected images of a candle, varying the distance till the two images unite, which is the true centre point: then if this fall in the middle, or central point of the glass, it is known to be truly centered. As object-glasses are commonly included in cells that screw upon the end of the tube of a telescope, it may be proved whether they be well centered by fixing the tube and observing, while the cell is unscrewed, whether the cross-hairs keep fixed upon the same lines of an object seen through the telescope.

OBJECTIVE line, in perspective, is any line drawn on the geometrical plane, whose representation is sought for in a draught or picture: and the objective plane is any plane situated in the horizontal plane, the representation of which is required. See PER

SPECTIVE.

OBLATE, flattened, or shortened, as an oblate spheroid, having its axis shorter than its middle diameter, being formed by the rotation of an ellipse about the shorter axis. The oblateness of the earth refers to the diminution of the polar axis in respect of the equatorial. The ratio of these two axis has been determined in various ways; sometimes by the measures of different degrees of latitude, and sometimes by the length of pendulums, vibrating seconds in different latitudes. See EARTH, DEGREE, &c.

OBLIGATION, in law, a bond containing a penalty, with a condition annexed, either for payment of money, performance of covenants, or the like. This security is called a specialty. See BOND and DEED.

OBLIGOR, in law, he who enters into an obligation; as obligee is the person to whom it is entered into.

OBLIQUE, in geometry, something aslant, or that deviates from the perpendicular. Thus an oblique angle is either an acute, or obtuse one, i. e. any angle except a right one. See ANGLE.

OBLIQUE cases, in grammar, are all the cases except the nominative.

OBLIQUE line, that which, falling on an

other line, makes oblique angles with it, riz. one acute, and the other obtuse.

OBLIQUE planes, in dialling, are those which recline from the zenith, or incline towards the horizon.

The obliquity, or quantity of this inclination, or reclination, may be found by means of a quadrant.

OBLIQUE sailing, in navigation, is when a ship sails upon some rhumb between the four cardinal points, making an oblique angle with the meridian; in which case she continually changes both latitude and longitude. Oblique sailing is of three kinds, viz. plain sailing, Mercator's sailing, and great circle sailing. See NAVIGATION.

OBLIQUE sphere, is where the pole is elevated any number of degrees less than 90°; in which case the axis of the world, the equator, and parallels of declination, will cut the horizon obliquely.

OBLIQUITY of the ecliptic. See ECLIP.

TIC.

OBLIQUUS, in anatomy, oblique, a name given to several muscles, particularly in the head, eyes, and abdomen. See ANATOMY.

OBOLARIA, in botany, a genus of the Didynamia Angiospermia class and order. Natural order of Personatæ. Pediculares, Jussieu. Essential character: calyx twoleaved; corolla four-cleft, bell-shaped; stamina from the slits of the corolla'; capsule one-celled, two-valved, many-seeded. There is but one species, viz. O. virginica.

OBSERVATION, in astronomy and navigation, is the observing with an instrument some celestial phenomenon, as the altitude of the sun, moon, and stars, or their distances from each other. But by this term, mariners commonly mean only the taking the meridian altitudes, in order to find the latitude; and the finding the lati tude from such observed latitude, they call "working an observation."

OBSERVATORY, a place destined for observing the heavenly bodies: it is a building usually in form of a tower, erected on an eminence, and covered with a terrace for making astronomical observations. Most nations have had observatories, which have been noticed at large in La Lande's Astronomy of these, the following may be mentioned:

The Greenwich Observatory, or Royal Observatory of England. This was built and endowed in the year 1676, by order of King Charles the Second, at the instance of Sir Jonas Moore, and Sir ChristopherWren: the former of these gentlemen being

Surveyor General of the Ordnance, the office of Astronomer Royal was placed under that department, in which it has continued ever since.

This observatory was at first furnished with several very accurate instruments; particularly a noble sextant of seven feet radius, with telescopic sights. And the first Astronomer Royal, or the person to whom the province of observing was first committed, was Mr. John Flamsteed; a man who, as Dr. Halley expresses it, seemed born for the employment. During fourteen years he watched the motions of the planets with unwearied diligence, especially those of the moon, as was given him in charge; that a new theory of that planet being found, shewing all her irregularities, the longitude might thence be determined. In the year 1690, having provided himself with a mural arch of near seven feet radius, made by his assistant, Mr. Abraham Sharp, and fixed in the plane of the meridian, he began to verify his catalogue of the fixed stars, which had hitherto depended altogether on the distances measured with the sextant, after a new and very different manner, viz, by tak ing the meridian altitudes, and the moments of culmination, or in other words, the right ascension and declination. And he was so well pleased with this instrument that he discontinued almost entirely the use of the sextant. Thus, in the space of upwards of forty years, the Astronomer Royal collected an immense number of good observa. tions; which may be found in his "Historia Coelestis Britannica, published in 1725; the principal part of which is the Britannic Catalogue of the fixed stars,

Mr. Flamsteed, on his death in 1719, was succeeded by Dr. Halley, and he by Dr. Bradley in 1742, and this last by Mr. Bliss in 1762; but none of the observations of these gentlemen have yet been given to the public.

On the demise of Mr. Bliss, in 1765, he was succeeded by Dr. Nevil Maskelyne, the present Astronomer Royal, whose valuable observations have been published, from time to time, under the direction of the Royal Society, in several folio volumes,

The Greenwich Observatory is found, by very accurate observations, to lie in 51° 28′ 40′′ north latitude, as settled by Dr. Maskelyne, from many of his own observations, as well as those of Dr. Bradley.

The Paris Observatory was built by Louis the Fourteenth, in the Fauxbourg St.

Jaques; being begun in 1664, and finished in 1672. It is a singular but magnificent building, of eighty feet in height, with a terrace at top; and here M. de la Hire, M. Cassini, &c. the King's Astronomers, have made their observations. Its latitude is 48° 50′ 14′′ north, and its longitude 9′ 20′′ east of Greenwich Observatory.

In the Observatory of Paris is a cave, or pit, 170 feet deep, with subterraneous pas sages, for experiments that are to be made out of the reach of the sun, especially such as relate to congelations, refrigerations, &c. In this cave there is an old thermometer of M. de la Hire, which stands at all times at the same height; thereby shewing that the temperature of the place remains always the same. From the top of the platform to the bottom of the cave is a perpendicular well or pit, used formerly for experiments on the fall of bodies; being also a kind of long telescopical tube, through which the stars are seen at mid-day.

Tycho Brahe's Observatory was in the little island Ween, or the Scarlet Island, between the coasts of Schonen and Zealand, in the Baltic Sea. This observatory was not well situated for some kinds of observations, particularly the risings and settings; as it lay too low, and was land-locked on all the points of the compass except three; and the land horizon being very rugged and uneven,

Pekin Observatory. Father Le Compte describes a very magnificient observatory, erected and furnished by the late Emperor of China, in his capital, at the intercession of some Jesuit missionaries, chiefly Father Verbest, whom he appointed his chief observer. The instruments here are exceedingly large; but the divisions are less accurate, and, in some respects, the contrivance is less commodious than in those of the Europeans. The chief are, an armillary zodiacal sphere of six Paris feet diameter, an azimuthal horizon six feet diameter, a large quadrant six feet radius, a sextant eight feet radius, and a celestial globe six diameter.

should be erected; one at Delhi, another at Agra, and the third at Benares.

Wanting the use of optical glasses, to magnify very distant, or very small objects, these people directed their attention to the increasing the size of their instruments, for obtaining the greater accuracy and number of the divisions and subdivisions in their instruments. Accordingly, the observatory contains several huge instruments of stone, very nicely erected and divided, consisting of circles, columns, gnomons, dials, quadrants, &c. some of them of 20 feet radius, the circle divided first into 360 equal parts, and sometimes each of these into 20 other equal parts, each answering to 3′, and of about two-tenths of an inch in extent. And although these wonderful instruments have been built upwards of 200 years, the graduations and divisions on the several arcs appear as well cut, and as accurately divided, as if they had been the performance of a modern artist. The execution, in the construction of these instruments, exhibits an extraordinary mathematical exactness in the fixing, bearing, fitting of the several parts, in the necessary and sufficient supports to the very large stones that compose them, and in the joining and fastening them into each other by means of lead and iron.

We have referred to this article from the EQUATORIAL, for some account of practical astronomy, and the instruments used in this branch of science.

By practical astronomy is implied the knowledge of observing the celestial bodies with respect to their position and time of the year, and of deducing from those observations certain conclusions useful in calculating the time when any proposed position of these bodies shall happen. For this purpose, it is necessary to have a room or place conveniently situated, suitably contrived, and furnished with proper astronomical instruments. It should have an uninterrupted view from the zenith down to, or even below, the horizon, at least towards the cardinal points; and for this purpose, that part of the roof which lies in the direction of the meridian, in parti. cular, should have moveable covers, which may easily be moved, by which means an instrument may be directed to any point of the heavens between the horizon and the zenith, as well to the northward as southward. This place, called an observatory, should contain the following instruments:

I. A Pendulum Clock, for showing equal

time. This should show time in hours, minutes, and seconds; the observer, by hearing the beats of the pendulum, may count them by his ear, while his eye is em ployed on the motion of the celestial object he is observing. Just before the object arrives at the position described, the observer should look on the clock and remark the time, suppose it 9 hours, 15 minutes, 25 seconds; then saying, 25, 26, 27, 28, &c. responsive to the beat of the pendulum, till he sees through the instrument the object arrived at the position expected; which suppose to happen when he says thirty-eight, he then writes down 9h 15′ 38" for the time of observation, annexing the particular day. If two per. sons are concerned in making the observation, one may read the time audibly while the other observes through the instrument, the observer repeating the last second read when the desired position happens.

II. An Achromatic Refracting Telescope, or a reflecting one of two feet at least in length, for observing particular phenomena. See TELESCOPE.

III. A Micrometer for measuring small angular distances. See MICROMEter.

IV. A Quadrant, for a description of which, and its several uses, we refer to the article QUADRANT. We may, however, observe, that besides Hadley's quadrant, which is described there, we have the mural quadrant, which is reckoned one of the most useful and valuable of all the astronomical instruments, and is generally fixed to the side of a stone or brick wall, and the plane of it is erected exactly in the plane of the meridian. There is also a portable astro. nomical quadrant, which is in high estimation, on account of its being capable of being carried to any part of the world, and put up for the purposes of observation by almost any common workman.

V. Astronomical or Equatorial Sector. This is an instrument for finding the dif ference in right ascension and declination between two objects, the distance of which is too great to be observed by the micrometer. Let A B (Plate Observatory, fig. 1.) represent an arch of a circle containing ten or twelve degrees well divided, having a strong plate, CD, for its radius, fixed to the middle of the arch at D. Let this radius be applied to the side of an axis, HF I, and be moveable about a joint fixed to it at F, so that the plane of the sector may be always parallel to the axis,

HI, which being parallel to the axis of the earth, the plane of the sector will always be parallel to the plane of some hour circle. Let a telescope, CE, be moveable about the centre, C, of the arch, A B, from one end of it to the other, by turning a screw at G, and let the line of sight be parallel to the plane of the sector. Now, by turning the whole instrument about the axis, HI, till the plane of it be successively directed, first to one of the stars, and then to another, it is easy to move the sector about the joint, F, into such a position, that the arch, AB, when fixed, shall take in both the stars in their passage, by the plane of it, provided the difference of their declinations does not exceed the arch, A B. Then, having fixed the plane of the sector a little to the westward of both the stars, move the telescope, CE, by the screw, G, and observe by a clock the time of each transit over the cross hairs, and also the degrees and minutes upon the arch, A B, cut by the index at each transit; then in the difference of the arches, the difference of the declinations, and by the difference of the times, we have the difference of the right ascensions of the stars. The dimensions of this instrument are these; the length of the telescope, or the radius of the sector, is two feet and a half: the breadth of the radius, near the end, C, is an inch and a half, and at the end, D, two inches: the breadth of the limb, A B, is one inch and a half, and its length six inches, containing ten degrees, divided into quarters, and numbered from either end to the other.

The telescope carries a nonius, or subdividing plate, whose length being equal to sixteen quarters of a degree, is divided into fifteen equal parts, which, in effect, divides the limb into minutes, and, by estimation, into smaller parts. The length of the square axis, HIF, ís eighteen inches, and of the part, HI, twelve inches; and its thickness is about a quarter of an inch. The diameters of the circles are each five inches; the thickness of the plates, and the other measures, may be taken at the direction of a workman. This instrument may be rectified for making observations in this manner: By placing the intersection of the cross hairs at the same distance from the plane of the sector as the centre of the object-glass; the plane described by the line of sight, during the circular motion of the telescope upon the limb, will be sufficiently true, or free from conical curvity,

which may be examined by suspending a long plumb line at a convenient distance from the instrument, and by fixing the plane of the sector in a vertical position; and then by observing, while the telescope is moved by the screw along the limb, whether the cross hairs appear to move along the plumb-line. The axis, h fo, may be elevated nearly parallel to the axis of the earth, by means of a small common quadrant, and its error may be corrected by making the line of sight follow the circular motion of any of the circumpolar stars, while the whole instrument is moved about its axis, hf o, the telescope being fixed to the limb; for this purpose let the telescope, kla, be directed to the star a, when it passes over the highest point of its diurnal circle, and let the division, cut by the nonius, be noted; then, after twelve hours, when the star comes to the lowest point of its circle, having turned the instrument half round its axis, to bring the telescope into the position, mn; if the cross hairs cover the same star supposed at b, the elevation of the axis, hfo, is exactly right; but if it be necessary to move the telescope into the position, uf c, in order to point to this star at c, the arch, mu, which measures the angle m f u, or bf c, will be known; and then the axis, hfo, must be depressed half the quantity of this given angle if the star passed below b, or must be raised so much higher if above it; and then the trial must be repeated till the true elevation of the axis be obtained.

By making the like observations upon the same star on each side the pole in the six o'clock hour circle, the error of the axis, toward the east or west, may also be found and corrected, till the cross hairs follow the star quite round the pole; for supposing ao pbc, to be an arch of the meridian, make the angle, af p, equal to half the angle, a ƒ c, and the line, ƒp, will point to the pole; and if the angle, of p, which is the error of the axis, will be equal to half the angle, bf c, or m f u, found by the observation; because the difference of the two angles, a fh, af c, is double the difference of their halves, af o, and a ƒ p. Unless the star be very near the pole, allowance must be made for refractions. See QUADRANT.

VI. Transit and Equal Altitude Instruments. The transit instrument is used for observing objects as they pass over the meridian. It consists of a telescope fixed at right angles to a horizontal axis; which