SECTOR, ZENITH, the name of an instrument for measuring small angles near the zenith. A very fine instrument of this kind, constructed by Ramsden and Berge, has been described by Major Mudge in the Phil. Transactions for 1803, p. 383. The follow ing abstract of this description has been given by Dr. Young.
" The external frame of the instrument is of maho gany, constituting a truncated pyramid, on a base of six feet square, tapering to a vertex of three. The internal frame, which immediately supports the sec tor, revolves on a vertical axis, terminating below in a cone, which rests in a conoidal cavity, convex to the axis, and above in a cylinder, passing through an oc tagonal aperture in the upper frame. As it turns, its motion is indicated by an azimuth circle attached to the lower part of the external frame, and it may be brought into the direction of the meridian by a teles cope fixed in the plane of the arch. The telescope of the sector is eight feet long, and its aperture four inches; the axis is like that of a transit instrument, the plumb-line passes through two perforations in it, and is adjusted by means of a screw with a jointed handle, and a long bent microscope with specula, so as to bisect a point marked on a plate of mother of pearl, precisely in the axis of the instrument; this plate is properly illuminated by the same lamp that serves for the micrometer wires of the telescope, its light being reflected downwards upon the wires from an oblique surface covered with plaster of Paris.
The pivots of the sector's axis are of bell metal, they rest in Y's, firmly attached to the frame; their sliding horizontally is prevented by a fixed friction wheel on one side, and a spring supporting a friction-wheel on the other; four cylindrical braces are employed to fix the telescope firmly to the axis; and the bending of the axis is still farther obviated by levers with coun terpoises, acting by means of friction-wheels, close to the tube of the telescope, so as to leave so much of the weight only to be supported by the pivots as is necessary to keep the instrument steady. The teles
cope is moved by strings and pullies, and is retained in any given situation by weights. A long spirit le vel is employed for bringing the axis into a position truly horizontal.
The arch is divided into portions of five minutes each, marked by points, on golden pins, let in at each division. A fine line was struck when the telescope was properly supported on the pivots; the instrument being then removed, the diameter of the circle, of which this arc was a part, was ascertained, and one sixteenth of this, being taken as extremely near to the chord of 7° 10', was laid off on each side zero; and this arc was verified by comparison with another, ob tained, by means of continual bisections, from an arc of 60°. The micrometer screw carries a head divid ed into 59 parts, nearly corresponding to seconds; the half of the arc on one side zero was found to con tain only a single second more than the other por tion.
The greatest error that could be observed from a difference of temperature in different parts of the ob servatory, was found to be little more than half a se cond for an arc of five degrees. The observations of the zenith distances of the various stars employed, were completed in October 1802; and the instrument was brought back to London without having sustain ed the least perceptible injury."