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that this arrangement would be attended with so many inconveniences that I preferred to place them inside the room. The only serious inconvenience is the limitation of the zenithal are within which stars can be observed by reflexion, but it will be seen by the results that this is sufficient to exercise a very severe check on the zenith points as obtained solely by Bohnenberger's eyepiece.

The instrument, as used by Mr. Carrington, had four reading microscope-micrometers, two of which were placed horizontal and two vertical. This arrangement was, however, inconvenient, because the use of the upper microscope was seriously interfered with by the flame of the central light used for illumination of the field of the telescope and of the divisions of the limb of the circle under the microscopes. As soon, therefore, as the piers were erected, I caused them to be bored for four other microscopes placed at angles of 45°, or at equal spaces between the original ones; and, with the usual liberality of the Radcliffe Trustees, I was allowed to order from Messrs. Troughton and Simms four additional ones of precisely the same construction as the original. These latter were not received till the month of June, and, till they were ready for use, only the two horizontal microscopes were employed. In this place it may be proper to state that I took on myself the whole superintendence of the mounting of the instrument; that I adjusted the microscopes and collimators; and made the observations, which will be detailed presently, for the determination of the flexure of the telescope and of the value of the screw of the declination-micrometer. The actual erection of the piers and other similar work was performed under the direction of Mr. T. Grimsley, and the carpenter's work under that of Mr. H. Radbone.

Amongst other subsidiary matters may be mentioned the provision which was made for observation of stars by reflexion. For this purpose two stages with convenient steps were placed one on the north and the other on the south side of the western pier, supporting a connecting horizontal rail running at a convenient height between the telescope and the graduated circle for protection of the observer. The uprights which support this rail are so contrived as to form a protection for the circle from accidental injury, and also an additional guard from the sun's rays during observation. The mercury trough is carried by a frame of a very simple construction (running freely on rollers), consisting of a horizontal board and two vertical boards merely kept firm by braces at the open sides, and with brackets or ledges on their inner surfaces for supporting the mercury trough at any required height. This apparatus has been found very convenient in practice.

Between the piers is a thick slab of stone sunk to the depth of

9 inches for the support of a second mercury trough kept in a fixed position for the observation of the nadir point in the usual way by Bohnenberger's eyepiece: and this, when the nadir observation is completed, is covered by a massive platform of wood of nearly 2 inches in thickness, which is raised or depressed on hinges, and is, when down, level with the flooring, so that the observing-chair can traverse without interruption. The space is too limited to admit of a deep pit, as in the case of the Transit Circle at Greenwich. For the support of the observer, while observing the nadir point, a tall moveable stage is used, which is found sufficiently convenient.

For the illumination of the instrument and the clock-face gas is employed. For the central illumination of the field of the telescope and the microscopes it has been found convenient to introduce a piece of flexible tubing a little below the support of the gas-holder, which makes the adjustment of the light easy. A metal tube with large mouth, immediately above the flame, carries off the heated air into the chimney of the room. A fixed gas-jet on the western pier, and on a level with the reflector of the Bohnenberger's eyepiece when the telescope is vertical, serves for the nadir-point observation. The collimators are also illuminated by gas, of which the heating effect is hindered by thick disks of plate-glass. The clock is placed against the north wall of the room, opposite to the western pier.

As the Introduction to Mr. Carrington's Catalogue of Circumpolar Stars contains a very full and particular description of the instrument as it was used by him, it will be sufficient to refer to that work for minute details, as well as for a drawing made to scale, and to give merely its dimensions and all such circumstances as are connected with its use at this Observatory. The general design was copied from that of the great Transit Circle at Greenwich, but the proportion of the separate parts is not exactly retained; gun-metal is used instead of iron, and the use of tangent-screws (for coarse motions) is retained. The achromatic object-glass is of 5 inches aperture and of 66 inches focal length, and I am happy to endorse Mr. Carrington's statement of its excellence. The eyepiece (of power about 140) is moveable by a slide in the direction of the meridian, and this slide is carried by a second at right-angles, moveable by a coarse screw. As used by Mr. Carrington, a frame, moveable east and west by a micrometer-screw, carried nine transit-wires, at equatorial intervals of about 10 seconds and 5 seconds of time, and this was used unaltered during the year 1862. The intervals were, however, found to be inconveniently small, and, at the commencement of 1863, Mr. William Simms replaced them by a webbing of 13 transit-wires, of which those at

wide distances had for the equatorial interval about 14 seconds, and the central wires intervals of 3 and 6 seconds. For declination there is no fixed wire, but only one moveable by a distinct micrometer. The screws have the same value, and the angular value of a revolution of each is about 32". The horizontal axis consists of a central tube of 9 inches in the side, and of two cones, to which the pivots are mechanically joined by bolts, and (excepting the pivots) it was made in a single casting of gun-metal. In the central cube is a reflector which can be differently inclined at pleasure by a rod acted on at the eye-end of the telescope, and the illumination of the field is provided by a gas flame outside the western pier. The length of the horizontal axis from the centre of one pivot to that of the other is 50 inches. The bearings on which the pivots rest are of the form of the letter Y, of great solidity, constructed of cast brass, and each capable of similar screwadjustment in level, while neither admits, when once the bed-plates are screwed down to the pier, of motion in azimuth. The instrument was brought into proper azimuthal position by tapping the eastern bed-plate, the south collimator being used for estimating the amount of the azimuthal deviation obtained by observations of circumpolar stars.

The horizontal axis carries two gun-metal circles (each of which was cast from the same pattern in a single flow, and carefully annealed) of 42 inches diameter. The east circle is used as a clamping and setting circle, being roughly divided to spaces of 5' on its outer or cylindrical rim. The west circle carries on its western face a band of gold inlet into its surface, on which divisions of 5' interval were made by Mr. Simms's dividing-engine; the divided rim being bevelled, as in the case of the Greenwich Circle, for the purpose of enabling the divisions under the microscopes to be illuminated by one fixed central light. It has been previously mentioned that the number of microscopes originally mounted was four, placed two and two horizontally and vertically, but that four additional, placed between the others at equal intervals, were mounted in June 1862. They are in conical arrangement, the micrometer ends of a diametral pair being 24 inches apart. Each is furnished with two parallel wires in its focus, and, for reading off, the division is placed midway between them. A great portion of the weight of the instrument is taken off the Ys by counterpoises, having the fulcra of their levers near the inner edges of the piers; these levers carrying also friction-rollers applied to grooves in the axis. The residual weight on each bearing has been about 15 lbs., the counterpoise weights being placed in the same positions as in the use of the instrument by Mr. Carrington.

The Collimating Telescopes, mounted on separate piers, as described

at page iii, are of 33 inches focal length and 2 inches aperture, and the systems of wires in their foci are precisely similar to those used at Greenwich; that is, each webbing consists of two nearly horizontal and two nearly vertical wires crossing each other so as to form a square, and with another nearly horizontal wire at a distance of about ten times the side of the square. The north collimator has a micrometer moving the whole system of wires in the vertical direction, and the south collimator a micrometer moving its system of wires in the horizontal direction.

SECTION II.-Reduction of the Observations of Right Ascension, made with the Transit Circle.

The observations were made generally by Mr. Quirling and Mr. Lucas, whose initials are Q and L.

The designations of the stars observed follow the same rules as in preceding years. For stars in the Nautical Almanac, the names there given are preferred to all others. For stars contained in Baily's Flamsteed, the Greek or Italic letter of Bayer there adopted is used, or in defect of that, Flamsteed's number, with the name of the constellation. For other stars, the British Association Catalogue is preferred to all other catalogues, and then, in order of preference, the numbers in the Catalogues of Groombridge, the Radcliffe, Piazzi, Carrington's Red Hill, Weisse's Bessel, Lalande, and Lacaille (as published by the British Association), Oeltzen's Argelander, and Rümker, are used.

Intervals of Wires.-As the system of wires in the year 1862 was identically the same as that used by Mr. Carrington, and as his values of the intervals had been determined with very great care, it was thought sufficient to redetermine the values of the intervals of the closer central wires (1, 2, 3, 4, 5, according to his notation), by means of the transits of Polaris made in the ordinary course of observation. By 14 observations of the intervals of transit between wires I, 5, and the central wire (3), and by 28 observations of the intervals between 2, 4, and the central wire, it was found by accurate computation that the equatorial intervals in time of each of these wires from the central wire were as follows:

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As this agreement was so close and satisfactory, I used, without farther scruple, the intervals of the wide wires (A, B, C, D in Mr. Carrington's notation) as determined by him, and finally decided upon the following intervals for the whole set from the mean of all :

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and these were used during the whole of the year 1862.

For the observations of the sun and planets, allowance has always been made for the motion in R. A. in the reduction of im-* perfect transits. For the reduction of imperfect transits of the moon the usual formula has been used; namely—

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Transit Telescope Micrometer.-It has been stated that the screw of this micrometer gives motion, east or west, to the whole system of the transit-wires. In its actual use it is employed for placing the central wire upon the north and south collimator-marks, and for measuring the angular space between the position of the line of collimation and the position of the central wire as set for observation, or the Error of Collimation.

This is always a small quantity, and therefore a very accurate value of the screw is not indispensable.

The following observations of transits of Polaris S. P. will, however, prove that the value found by Mr. Carrington, namely 32" 03, is accurate.

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