Among the various eye-pieces, of different magnifying powers, is one furnished with a prism. This, necessarily bent at right angles, enables the observer to see the stars without touching the frame. The use of it has been found convenient; but habit and proper caution enable the astronomer to use either of the other glasses. Having given this cursory and general description of the instrument, as seen at first view, I shall proceed to an explanation of the plates, which show its various parts.

Particular Description of the Zenith Sector.

Plate X. Represents a general section of the instrument and stand. AB is one of the four great uprights of the external mahogany frame, and CB its top, having an opening in D, for admission of light. The uprights consist of two strong pieces, firmly screwed together; each upright having seven strong screws, as seen in the upright AB. The top may be considered as a sort of square table, screwed down on the upper part of the frame. Between each of the two uprights is a brace, diagonally fixed, for strengthening the stand, as may be seen in the plate; and four others go horizontally, from upright to upright, for purpose of still farther strengthening the whole. Across the bottom of the frame, and exactly in the middle of it, is a very strong mahogany plank, whereon rests the sector, having a stout straight edge bar of the same substance underneath. In the middle of this cross piece, as seen at E, is an apparatus of brass, furnished with an azimuth circle, having a hollow receptacle of bell-metal in the centre, in which rests, on a conical point, the interior mahogany frame FGHI. This brass work, which is strong and substantial, may be seen in Plate XIV. It is there represented in plano, with the bottom part of the interior stand

the

D

parallel to the meridional one, and at equal distances from it; they were placed there with a view of rendering an adjustment of the horizontal wire sufficiently easy. These are illuminated by means of the lamp which carries the concave reflector before spoken of. There is a hole, with a lens, in the side of the telescope, directly opposite to the lamp, having behind it a diaphragm of brass, coated with plaister of Paris, and inclined to the vertical axis of the tube, at an angle of 45 degrees. The quantity of light, suited to the circumstances of the observation, is regulated by coloured glasses, placed over the hole in the side of the tube.

The plummet, suspended at the wire, falls into a cylindrical cup, swinging by two pins on its edge, on the extremity of a brass frame annexed to the interior stand; which frame is capable of being raised or lowered at pleasure by a milled-headed screw; so that the wire can, at any time, be released from the weight of the plummet, by screwing up the vessel containing it.

There are two arches attached to the end of the tube, one on each side of it, and firmly united together by means of brass pillars; which arrangement effectually secures the divided arch from injury. The total extent of the arch is about 15°, having half of its subtense on each side zero. It is divided into every 5 minutes; the micrometer-screw measuring any supplementary quantity. Golden pins were let into the arch, by the advice of the Astronomer Royal, on which the divisions were laid off by Mr. BERGE, in a very masterly and accurate manner, as will be seen hereafter. A magnifier, whose focal distance is about half an inch, is placed under the bottom of the cross piece opposite to the arch, and is furnished with a horizontal adjustment for bringing it directly over the plumb-line.

Among the various eye-pieces, of different magnifying powers, is one furnished with a prism. This, necessarily bent at right angles, enables the observer to see the stars without touching the frame. The use of it has been found convenient; but habit and proper caution enable the astronomer to use either of the other glasses. Having given this cursory and general description of the instrument, as seen at first view, I shall proceed to an explanation of the plates, which show its various parts.

Particular Description of the Zenith Sector.

Plate X. Represents a general section of the instrument and stand. AB is one of the four great uprights of the external mahogany frame, and CB its top, having an opening in D, for admission of light. The uprights consist of two strong pieces, firmly screwed together; each upright having seven strong screws, as seen in the upright AB. The top may be considered as a sort of square table, screwed down on the upper part of the frame. Between each of the two uprights is a brace, diagonally fixed, for strengthening the stand, as may be seen in the plate; and four others go horizontally, from upright to upright, for the purpose of still farther strengthening the whole. Across the bottom of the frame, and exactly in the middle of it, is a very strong mahogany plank, whereon rests the sector, having a stout straight edge bar of the same substance underneath. In the middle of this cross piece, as seen at E, is an apparatus of brass, furnished with an azimuth circle, having a hollow receptacle of bell-metal in the centre, in which rests, on a conical point, the interior mahogany frame FGHI. This brass work, which is strong and substantial, may be seen in Plate XIV. It is there represented in plano, with the bottom part of the interior stand

D

placed above it. The means of making the interior stand vertical, are found in the work annexed to the azimuth circle. They consist of two screws, attached to two plates of brass, placed at right angles to, and also flat on each other. Ss (Plate XIV.), are the screws. A vernier on the divided circle may be seen at S; and at s, the method of clamping the bottom of the stand. On the opposite side is another provision for clamping this stand, when the face of the sector is changed from east to west, or vice versa.

KLMNOP (Plate X.) is a section of the telescope and axis, MR, MR, being two of the four braces for strengthening the axis, and steadying the telescope. K is the place of the eyetube; L the elliptical reflector for illuminating the wires at K; and ON a hollow cylinder of brass, independent of the tube. In the upper part of this cylinder, the object-glass is rivetted; the cylinder itself being fastened to the great eye-tube, in a perma

nent manner.

W, W, are two weights, hanging freely from the ends of two levers, the opposite ends being furnished with four frictionwheels. The points of support, between the weights and wheels, are at TT, being at the extremities of two upright solid pieces of metal, which are moved up or down by the screws beneath them. These counterpoising weights prevent any bending of the axis, between the pivots and those parts to which they apply. The apparatus for carrying the levers, is attached to the inside mahogany frame by screws, as represented in the section. See also Plate XIII.

The plummet and plumb-line are seen at aed; the point of suspension being a, and the plummet d; the plumb-line passing close by the arch, whose section is bc, and also near to the dot

or small circle e, described on the thin slice of mother-ofpearl, shown in the section of the axis at e.

A lamp is attached, or rather rests, on circular supports annexed to the side of the interior frame, and may be seen at XZ.: At the back of the lamp, placed in a recess, is a concave reflector at Z; and, in the front of it, a tube running out to X, having a double convex glass at P, for throwing the light on L, which first passes through a double concave glass in the side of the telescope, and then, from the reflector L, is thrown down on the wires near K. The concave speculum Z, has two adjustments for converging the reflected light on the little elliptical speculum b. This last-mentioned speculum throws off the said light at b, which passes into the axis at G, illuminating the mother-of-pearl. at e, and, finally, is transmitted out of the axis at p.

klmn, is a section of the long microscope, for conveying the image of the dot and wire, sent out of the axis at p, to the eye at k. This microscope is firmly attached to the side of the frame, by brass cylinders, kk, ll, mm, n, and has one plano-convex glass at q, a prismatic eye-glass at u, and a metallic reflector at the top, o. At the upper end of this long microscope, and directly behind the speculum, is a screw, by which the reflecting. metal is brought into one of its requisite positions. The other. adjustment of that metal is performed by two screws, which; apply to the sides, and give it lateral motion. The plano.convex. glass at q, is rivetted into the head of a long tube uq, which slides. up the microscope. The upper part of the microscope at o, is placed exactly opposite the end of the axis, in a very firm way,

The rod for giving motion to the plumb-line, is vwx; v being the top of it, w the place of the universal joint, which separates the two parts of the rod, and in the bottom of the rod