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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. Bramin’s Observatory at Benares, in the East Indies, which is still one of the principal seminaries of the Bramins, or priests of the original Gentoos of Hindostan. This observatory at Benares, it is said, was built about 200 years since, by order of the Emperor Ackbar; for as this wise prince endeavoured to improve the arts, so he wished also to recover the sciences of Hindostan, and therefore ordered that three such places 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 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 EquatonLAL, 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 bo
dies, 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 convenient
ly 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 particular, 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 employed 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 supose to happen when he says thirty-eight, F. then writes down 9h 15' 38" for the time of observation, annexing the particular day. If two persons 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 astronomical 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 Fquatorial 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. T 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 H F 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 H I, 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, C E, 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, A B, 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 secfor a little to the westward of both the stars, move the telescope, C E, 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 es
timation into smaller parts. The length of the square axis, H I F, is eighteen inches, and of the part, H I, 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 objectglass, 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, h fo, the telescope being fixed to the limb ; for this purpose, let the telescope, k l a, 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 m n, if the cross hairs cover the same star supposed at b, the elevation of the axis, h fo, is exactly right; but if it be necessary to move the telescope into the position, u f c, in order to point to this star at c, the arch, mu, which measures the angle m fu, or 5 fc, will be known ; and then the axis, h fo, must be depressed half the quantity of this given angle, if the star passed below 5, 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 a op 5 c, to be an arch of the meridian, make the angle a fp, equal to half the angle a fe, and the line fp, 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 fu, found by the observation; because the difference of the two angles, afh, a fe, is double the difference of their halves, a fo, and a fp. Unless the star be very near the pole, allowance must be made for refractions. See QUADRANT. WI. 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 axis must be so supported, that what is called the line of collimation, or line of sight of the telescope, may move in the plane of the meridian. This instrument is made of various sizes, and of large dimensions, in our great observatories; but the following is one of a size sufficiently large and accurate for all the useful purposes. The axis, A B (fig. 2), to which the middle of the telescope is fixed, is about two feet and a half long, tapering gradually toward its ends, which terminate in cylinders well turned and smoothed. The telescope, CD, which is about four feet, and an inch and a half diameter, is connected with the axis by means of a strong cube or die, G, and in which the two cones, MQ, forming the axis, are fixed. This cube, G, serves as the principal part of the whole machine. It not only keeps together the two cones, but holds the two sockets, K H, of fifteen inches length, for the two telescopic tubes. Each of these sockets has a square base, and is fixed to the cube by four Screws. These sockets are cut down in the sides about eight inches, to admit more easily the tube of the telescope; but when the tube is inserted, it is kept in firm by screwing up the tightening screws at the end of the sockets at K and H. These two sockets are very useful in keeping the telescope in its greatest possible degree of steadiness. They also afford a better opportunity of balancing the telescope, and rectifying its vertical thread, than by any other means. In order to direct the telescope to the given height that a star would be observed at, there is fixed a semicircle A N, on one of the supporters, of about eight inches and a half diameter, and divided into degrees. The index is fixed on the axis, at the end of which is a vernier, which sub
divides the degrees into twelve parts of five minutes. The index is moveable on the axis, and may be closely applied to the divisions by means of a tightening screw. Two upright posts of wood or stone, YY, firmly fixed at a proper distance, are to sustain the supporters of this instrument. These supporters are two thick brass plates, R R, having well smoothed angular notches in their upper ends, to receive the cylindrical arms of the axis. Each of these notched plates is contrived to be moveable by a screw, which slides them upon the surfaces of two other plates immoveably fixed upon the two upright pillars; one plate moving in a horizontal, and the other in a vertical direction; or, which is more simple, these two modes are sometimes applied only on one side, as at V and P, the horizontal motion by the screw P, and the vertical by the screw V. These two motions serve to adjust the telescope to the plane of the horizon and meridian; to the plane of the horizon by the spiritlevel, EF (fig. 4,) hung by D C on the axis M. Q, in a parallel direction, and to the plane of the meridian in the following manner: Observe by the clock, when a circumpolar star seen through this instruments transits both above and below the pole ; and if the times of describing the eastern aud western parts of its circuit are equal, the telescope is then in the plane of the meridian: otherwise the screw, P, must be gently turned, that it may move the telescope so much that the time of the star’s revolution be bisected by both the upper and lower transits, taking care at the same time that the axis remains perfectly horizontal. When the telescope is thus adjusted, a mark must be set at a considerable distance (the greater the better) in the horizontal direction of the intersection of the cross wires, and in a place where it can be illuminated in the night-time by a lanterm hanging near it; which mark, being on a fixed object, will serve at all times afterwards to examine the position of the telescope by the axis of the instrument being first adjusted by means of the level. To adjust the Clock by the Sun’s Transit over the JMeridian. Note the times by the clock when the preceding and following edges of the sun’s limb touch the cross wires. The difference between the middle time and twelve hours shows how much the mean time, or time by the clock, is faster or slower than the apparent or solar time for that day; to which the equation of time being applied, will show the time of mean noon for that day, by which the clock may be adjusted. The Equal.Altitude Instrument, is an instrument that is used to observe a celestial object, when it has the same altitude on both the east and west sides of the meridian, or in the morning and afternoon. It principally consists of a telescope about thirty inches long, fixed to a sextantal or semicircular divided arch, the centre of which is fixed to a long vertical axis. The Equatorial or Portable Observatory, an instrument designed to answer a number of useful purposes in practical astronomy, independent of any particular observatory. It may be made use of in any steady room or place, and performs most of the useful problems in the science. The following is a description of one lately invented by Mr. Ramsden, from whom it has received the name of the Universal Equatorial. The principal parts of this instrument (fig. 3,) are, 1. The azimuth or horizontal circle, A, which represents the horizon of the place, and moves on an axis, B, called the vertical axis. 2. The equatorial or hour circle, C, representing the equator, placed at right angles to the polar axis, D, or the axis of the earth, upon which it moves. 3. The semicircle of declination, E, on which the telescope is placed, and moving on the axis of declination, or the axis of motion of the line of collimation, F. 4. The telescope, which is an achromatic refractor, with a triple object glass, whose focal distance is 17 inches, and aperture 2.45 inches, and furnished with six different eye-tubes; so that its magnifying powers extend from 44 to 168. The telescope in this equatorial may be brought parallel to the polar axis, as in the figure, so as to point to the pole star in any part of its diurnal revolution ; and thus it has been observed near noon, when the sun has shone very bright. 5. The apparatus for correcting the error in altitude occasioned by refraction, which is applied to the eye-end of the telescope, and consists of a slide, G, moving in a groove or dove-tail, and carrying the several eye-tubes of the telescope, on which slide there is an index corresponding to five small divisions engraved on the dovetail; a very small circle, called the refraction circle, H, moveable by a finger screw at the extremity of the eye-end of the telescope; which circle is divided into half minutes, one entire revolution
of it being equal to 3' 18", and by its motion raises the centre of the cross hairs on a circle of altitude; and likewise a quadrant, 1, of 1% inch radius, with divisions on each side, one expressing the degree of altitude of the object viewed, and the other expressing the minutes and seconds of error occasioned by refraction, corresponding to that degree of altitude; to this quadrant is joined a small round level, K, which is adjusted partly by the pinion that turns the whole of this apparatus, and partly by the index of the quadrant; for which purpose the refraction circle is set to the same minute, &c. which the index points to on the limb of the quadrant; and if the minute, &c. given by the quadrant exceed the 3' 18" contained in one entire revolution of the refraction circle, this must be set to the excess above one or more of its entire revolutions; then the centre of the cross hairs will appear to be raised on a circle of altitude to the additional height which the error of refraction will occasion at that altitude. This instrument stands on three feet, L, distant from each other 14.4 inches; and when all the parts are horizontal, is about 29 inches high : the weight of the equatorial and apparatus is only 5915. avoirdupoise, which are contained in a mahogany case. The principal adjustment in this instrument is, that of making the line of collimation to describe a portion of an hourcircle in the heavens; in order to which, the azimuth circle must be truly level, the line of collimation, or some corresponding line, represented by the small brass rod, M, parallel to it, must be perpendicular to the axis of its own proper motion; and this last axis must be perpendicular to the polar axis; on the brass rod, M, there is occasionally placed a hanging-level, N, the use of which will appear in the following adjustments: the azimuth circle may be made level, by turning the instrument till one of the le. vels is parallel to an imaginary line join. ing two of the feet screws; then adjust that level with these two feet screws; turn the circle half round, i.e. 180°; and if the bubble be not then right, correct half the error by the screw belonging to the level, and the half error by the two foot screws; repeat this till the bubble comes right; then turn the circle 90° from the two former positions, and set the bubble right, if it be wrong, by the foot screw at the end of the level ; when this is done, adjust the other level by its own screw, and the azimuth circle will be
truly level. The hanging level must then be fixed to the brass rod by two hooks of equal length, and made truly parallel to it: for this purpose make the polar axis perpendicular, or nearly perpendicular, to the horizon; then adjust the level by the pinion of the declination semicircle; reverse the level, and if it be wrong, correct half the error by a small steel screw that lies under one end of the level, and the other half error by the pinion of the declination semicircle ; repeat this till the bubble be right in both positions. In order to make the brass rod on which the level is suspended at right angles to the axis of motion of the telescope, or line of collimation, make the polar axis horizontal, or nearly so; set the declination semicircle to 0°, turn the hour circle till the bubble comes right; then turn the declination circle to 90° ; adjust the bubble by raising or depressing the polar axis (first by hand, till it be nearly right, af. terwards tighten with an ivory key the socket which runs on the arch with the polar axis, and then apply the same ivory key to the adjusting screw at the end of the said arch, till the bubble come quite right); then turn the declination circle to the opposite 90° ; if the level be not then right, correct half the error by the aforesaid adjusting screw at the end of the arch, and the other half error by the two screws which raise or depress the end of the brass rod. The polar axis remaining nearly horizontal as before, and the declination semicircle at 0°, adjust the bubble by the hour circle; then turn the declination semicircle to 90°, and adjust the bubble by raising or depressing the polar axis; then turn the hour circle twelve hours; and if the bubble be wrong, correct half the error by the polar axis, and the other half error by the two pair of capstan screws at the feet of the two supports, on one side of the axis of motion of the telescope; and thus this axis will be at right angles to the polar axis. The next adjustment is to make the centre of cross hairs remain on the same object, while you turn the eye-tube quite round by the pinion of the refraction apparatus: for this adjustment, set the index on the slide to the first division of the dove-tail ; and set the division marked 18" on the refraction circle to its index; then look through the telescope, and with the pinion turn the eye-tube quite round; and if the centre of the hairs does not remain on the same spot during that revolution, it must be corrected by the four small screws, two and two at a time (which you will find upon unscrewing the nearest end of
the eye-tube that contains the first eyeglass); repeat this correction till the centre of the hairs remain on the spot you are looking at during an entire revolution. In order to make the line of collimation parallel to the brass rod on which the level hangs, set the polar axis horizontal, and the declination circle to 90°, adjust the level by the polar axis; look through the telescope on some distant horizontal object, covered by the centre of the cross hairs; then invert the telescope, which is done by turning the hour circle half round; and if the centre of the cross hairs does not cover the same object as before, correct half the error by the uppermost and lowermost of the four small screws at the eye-end of the large tube of the telescope; this correction will give a second object, now covered by the centre of the hairs, which must be adopted instead of the first object: then invert the telescope as before; and if the second object be not covered by the centre of the hairs, correct half the error by the same two screws which were used before: this correction will give a third object, now covered by the centre of the hairs, which must be adopted instead of the second object; repeat this operation till no error remains; then set the hour circle exactly to twelve hours (the declination circle remaining at 90° as before); and if the centre of the cross hairs does not cover the last object fixed on, set it to that object by the two remaining small screws at the eye-end of the large tube, and then the line of collimation will be parallel to the brass rod. For rectifying the nonius of the declination and equatorial circles, lower the telescope as many degrees, minutes, and seconds, below 0° or Æ on the declination semicircle, as are equal to the complement of the latitude; then elevate the polar axis till the bubble be horizontal, and thus the equatorial circle will be elevated to the co-latitude of the place; set this circle to six hours; adjust the level by the pinion of the declination circle; then turn the equatorial circle exactly twelve hours from the last position; and if the level be not right, correct one half of the error by the equatorial circle, and the other half by the declination circle ; then turn the equatorial circle back again exactly twelve hours from the last position ; and if the level be still wrong, repeat the correction as before till it be right, when turned to either position: that being done, set the nonius of the equatorial circle exactly to six hours, and the nonius of the declination circle exact