QUADRANT. 



,JjV of a degree, or 5'i, was tlie length of one of tlie 

 vernier divilions, and ,'t of this, or 30", was ihc fmallefl 

 quantity indicated by this vernier ; bnt on the arc of 96 

 grand divifions there was, as in Bird's arc, 16 equal parts, 

 or fubdivifions, in every grand divifion, and the vernier 

 for this arc had 17 divifions on it, occupying the fpacc 

 of 16 fubdivifions in the arc, fo that the length of this 



— of ~ — = Co'.S? nearly, and one divifion 

 16 96 



vernier vi'as 



of it 



17 



:= 3'-52 nearly. 



Hence in the. firll vernier its number 1 1 is one hfs than its 

 equivalent arc contains on the limb of 90° ; but in the fecond, 

 the number 17 is one more than its equivalent arc contains in 

 the arc of 96 parts, on which account the reading of one ver- 

 nier is in the direction from right to left, but that of the 

 other, on the contrary, from left to riglit ; one meets, and 

 the other overtakes the dividing llrok.es of the divided limb ; 

 and for the fame reafon, in the common Hadley's quadrant, or 

 rather oftant, fometimes 19 fubdivifions, and fometimes 21 

 on the limb, aft againft 20 in the vernier, but then the 

 readings are not in the fame dtredion. The telefcope is 

 clamped to the arc in any fituation by the mechanifm for 

 (low motion in the ufual way, which probably was firll 

 adopted in one of the large quadrants, and its counterpoife 

 beyond the centre of the arc, gives it the advantage of re- 

 maining in any pofition. It has crofs hairs in the focus of 

 the eye-glafs. When the lines of the vernier are none 

 of them coincident with any one on the limb of either 

 quadi-antal arc, the portions lefs than what the verniers 

 profefs to indicate were ejlimated by the eye, by examining 

 the fituation of other pairs of dividing lines, to the right 

 and left of thofe neareit to a coincidence ; and to a want 

 of a micrometrical nut to the fcrew of flow motion may be 

 attributed the remark, that has been made, that the readings 

 thus taken, even with fo large radii as 96.85 and 95.8 

 inches, were not to be depended upon to 10". That the 

 motion of the telefcope might be quite eafy, and that the 

 centre of the quadrant might be relieved of its weight, the 

 following contrivance was introduced ; a b reprefents an 

 iron axis laid acrofs the top of the wall, having two brafs 

 plates fixed perpendicularly to its ends, with notches cut 

 in them for this axis to turn in, which axis points to the 

 centre of the quadrant at right angles to its plane : to that 

 end of it next to the quadrant, at a, an iron arm, c d, is fixed, 

 having two brafs plates, ce, d f, almoft perpendicular to it ; 

 to thefe are rivetted two flcnder flips of deal, whofe 

 remote ends meet at g, near the eye-piece of the tele- 

 fcope, and arc held together by a brafs cap. Through 



- a fmall plate fixed to one fide of a collar, embracing this 

 lower end of the telefcope, there pades a fcrew-pin at g, 

 parallel to the telefcope ; which pin, being fcrewed into 

 the cap at the end of the faid flips of deal, holds up the 

 telefcope againft the centre-work, while the flips are braced 

 by other crofs flips of the fame light wood. The counter- 



- poife i is fupported by the rod h i at b, the retiring end of 

 the axis ab ; and a pair of brafs rollers h I, afting againft 

 the limb of the inftrument, give freedom to the motion of 

 the telefcope thus counterpoifed, and complete the con- 

 ftruftion. The quadrant being thus put together, fome 

 ftrong but Imall plates of brafs are made fall to its pofteriof 

 face, and bent fo as to fall into as many hold-fails in the 

 wall, into which they are refpetlively fcrewed ; but the 

 weight of the whole is fupported chiefly by two pins or 

 bolts inferted into the holes A and B, made in pieces of 

 metal attached to fuch angular points of the iron bars, as 



bell fupport the centre of gravity of the whole : the pin 

 A is made faft into the wall, but allows a motion round it, 

 and the pin B fixes the quadrant after its extreme radii are 

 adjufted, one horizontal, and the other vertical : this pofition 

 was given by means of a phimb-line of fine filver-wirc, that 

 at firfl was fo fufpended as to bifeft both the centre of the 

 quadrantal arc and the point 90' on the limb, but which 

 was transferred afterwards to an adjuftable point of fuf- 

 penfion out of the centre, v.ith a correfponding dot made 

 on the arc of excefs of the limb. The plane of the quad- 

 rant was made vertical, as compared with the plumb-line, 

 by the fcrew? of the hold-fafts ; and the tclclcope was 

 adjufted parallel to this plane by comparifon with a tranfit 

 telefcope viewing togeth-r both high and low ftarj in fuc- 

 ceflion ; but the line of coUimation of the telefcope could 

 not be fixed properly, as it regarded the true horizontal line 

 of the quadrant pafling through its zero, without the aid of 

 Graham's feclor. This fedlor was, therefore, f« adjufted 

 to a ftar near the zenith, that it m^■afured the fame zenith- 

 diftance, with its plane turned to the eaft, as it did when 

 turned to the weft, and had its error afcertained in this 

 way, and afterwards an altitude, taken with the quadrant, 

 was made to correfpond witli fuch correfted altitude of a 

 ftar taken with the reftifud feftor in reverfed pofitions, 

 which property the mural qnadrant does not poliefs. The 

 method of performing this adjuftment for coUimation will 

 be underftood from our dncdions hereafter given, when 

 treating oi Ramfdcn^ s portable aflronomical Quadrant. 



Bird's Mura! Quadrant.— After the deicription we have 

 given of Graham's mural quadrant, we fliall have no occa- 

 fion to dwell long on the ftrufture of Bird's, which was 

 made of brafs entirely, after a fimilar model, but divided in 

 both its arcs, of 90°, and of 96 grand parts, with mere 

 fl<ill than Graham proved himfelf mafter of in this part of 

 his labours. Smith and Stone defcribe Graham's quadrant 

 as fixed to the eaft fide of the pier at Greenwich, and look- 

 ing to the fouth, which was the fituation for taking meridian 

 altitudes of the greattft number of ftars ; but fince the year 

 1753, it has been placed on the weft fide, looking towards 

 the north, and Bird's then took its original place. 



This quadrant, wiiich was procured in the time of Dr. 

 Bradley, was firft placed on the weft fide of the pier, in 

 1750 ; and the obfervations of hirafelf and of Dr. Mafkelyne, 

 taken by it fince it was placed on the eaft fide, have contri- 

 buted largely to complete the beft catalogues of the heavenly 

 bodies, which otherwife muft have been very defective. 

 While, however, Mr. Pond, the prefent aftronomer royal, 

 was engaged in making his well-known table of declinations, 

 with one of Troughton's circles, at Weilbury, he found 

 reafon to fufpeft the accuracy of the total arc of this 

 quadrant, from a comparifon of his own determinations with 

 thofe previoufly made with Bird's quadrant ; and a fubfe- 

 quent examination of the arc by Troughton juftified hi. 

 fufpicion. With an apparatus exprefsly contrived for the 

 occafion, this celebrated mathematical inftrument-maker 

 meafured the total length of the quadrantal arc, and found 

 it too fmall by 7", exclufive of another fimilar error of 2'', 

 occafioned by the wear of the axis of motion ; though, on 

 a rigid trial of the intermediate divifions, he did not deteft 

 more than one fecond of eiTor, or rather of inequality, among 

 the neighbouring divifions in any part of the arc. This trial 

 was made in the prefence of the prefent aftronomer royal, in 

 the" year 1807 ; and the refult, while it ferves to correft 

 paft obfervations, by the addition of l" to every fucceflive 

 10° of altitude, would have tended to correft all future ob- 

 fervations by a like addition, liad not the large tranfit-circle, 

 lately made and fixed in the fame obfervatory by Troughten, 



fuperfeded 



