CATALOGUE. — PRACTICAL ASTUONOMY. 



353 



conoidal cavity, convex to the axis, and above in a cy- 

 linder, passing through an octagonal aperture in the upper 

 ftame. 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 telescope 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 in- 

 strument ; 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 vf ith 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 horizon- 

 tally is prevented by a fixed friction wheel on one side, and 

 a spring supporting a friction wheel on the other : four cy- 

 lindrical braces are employed to fix the telescope firmly 

 to the axis ; and the bending of the axis is still further ob- 

 viated by levers with counterpoises, acting by means of 

 friction wheels, close to the tube of the telescope, so as to 

 leave so much of the w^eight only to be supported by the 

 pivots, as is necessary to keep the instrument steady. The 

 telescope is moved by strings and puUies, and is retained in 

 any given situation by weights. A long spirit level is era- 

 ployed 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 ex- 

 tremely near to the chord of 7° 10', was laid off on each 

 side zero ; and this arc was verified by comparison with 

 another, obtained, by means of continual bisections, from 

 an arc of 60°. The micrometer screw carries a head divi- 

 ded into 59 parts, nearly corresponding to seconds ; the 

 half of the arc on one side zero was found to contain only a 

 single second mare than the other portion. 



The greatest error that could ever be observed from a 

 difference of temperature in different parts of the observatory, 

 was found to be little more than half a second for an arc of 

 five degrees. The observations of the zenith distances of 

 the various stars employed were completed in October 

 IS02 ; and the instrument was brought back to London 

 without having sustained the least perceptible injury. 

 VOL. 11. 



Equatorial Instruments. 



Short on an equatorial instruments Eh. tr. 



1749. XLVI. 241. 

 Nairne's equatorial, or portable observatory. 



Ph.tr. 1771. 107. 

 Si Ibersch lag's uranometer. A. Berl. 1778. H. 



3S. 

 Smeaton on observations out of the meridian 



with an equatorial micrometer. Ph. tr. 



1787.318. 

 Haupoin's equatorial instrument. Roz. XLII. 



286. 

 *ShuckburgIi on the equatorial instrument. 



Ph. tr. 1793. 67. 



Levels, Mechanical or Hydrostatical. 



On levels. Hooke Anim. on Hevelius. Wa- 

 ter levels. 



Perrault. Mach. A. I. 63. 



Picard. A. P. VII. i. 51, 235. 



Couplet. A. P. 1699. 127. H. 112. 



Veijus. Mach. A. II. 83. 



Lahire. A. P. 1704. 251. H. 99- 



Leu;;old. Th. Horizontost. Th. Hy'drot#chn. 

 Th. Suppl. 



Grandjean de Pouchy. Mach. A. VI. 113. 



Hadley's spirit level to be fixed to a quad- 

 rant. Ph. tr. 1733. XXXVIII. 167. 

 The spirit to vibrate through a stop cock till it settle. 



Soumille. A. P. 1737. H. 109. Mach. A. 



vii. 71. 



With an index enlarging the scale by means of a lever. 

 Leigh's water level fixed to Davis's quadrant. 



Ph.tr. 1738. XL. 413. 

 Leigh's mercurial level for a quadrant. Ph. 

 tr. 1738. XL. 417. 

 True to a'. 

 Gensanne. A. P. 1741. H. 164. Mach. A. 



VII. 109. 



Reflecting. 



Mathieu. A. P. 1746. H. IQl. 

 z z 



