QUADRANT. 



fuperfcded the ufe of both the quadrants by its fuperior 

 accuracy. The quadrantal arc of Graham's quadrant has 

 not lately undergone a fimilar trial ; but Troughton thinks 

 it probable that the iron inltrument has prcfcrved its figure 

 better than the brafs one, and that confequently the addi- 

 tion of the brafs arc on the face of the iron one lias had no 

 undue influence in altering either its temporary or permanent 

 dimenfions. At firft, tiie telcfcope of Bird's quadrant was 

 not braced ; but after a trial, it was found to bend a little 

 by a counterpoife of 80 pounds, and was afterwards braced 

 by frame-work furrounding it, that prevented its yielding. 

 A trial was made by Dr. Bradley of the total arc, in Ja- 

 nuary 1753, after it had remained three years on the weft 

 fide of the pillar ; and thougli its weight is eight hundred 

 pounds, the error was found to be not more than two 

 fcconds. He did the fame again in July, in the year 1759, 

 when it had remained fix years and a half in its new fitua- 

 tion, on the eall fide of the pillar, and found the arc ex- 

 aftly 90°. Thefe examinations, having been made in the 

 extremes of cold and heat, fhew that change of temperature 

 did not aflFeft the total arc ; and that, therefore, the 7" 

 diminution of the arc, detected by Troughton, is occa- 

 fioned by a change of figure in the fpace of 48 years. The 

 difference of the readings of the two arcs has never yet 

 amounted to 4", wliich is a (huiding proof of the accuracy 

 and fliill of Bird as a divider. Near the eye-piece of the 

 telefcope is a good micrometer, that not only gives flow 



very neatly ; and the reading, as was the cafe with Bird's 

 quadrant at firtl, is performed partly by a vernier, and 

 partly by a micromcter-fcrew. We have feledted this in- 

 ilrument, as a fpecimen of Siffou's conftruftion, principally 

 on account of a power that it podefTes, wliich is very im- 

 portant, but which probably was not contemplated, and 

 confequently not intended by the maker himfclf : the cx- 

 tenfion of the divided limb, to feveral degrees beyond the 

 north pole, affords the means of ufingthis inllrument in the 

 manner tliat Troughton's large mural circle is now ufed, at 

 Greenwich Obfervatory, by the prefent aflronomer royal ; 

 namely, to meafure the polar dillanccs of the liars dlrettly 

 from the true polar point, without any reference to the lati- 

 tude of the obfervatory, which method is one of the greateft 

 modern improvements in making aftrononiical obfervations. 

 It is much to be defired that this mural arc fhould be di- 

 vided again by Troughton, or fomc other fuperior divider, 

 and that it have microfcopic micrometers applied to it, 

 which may be placed by adjuftment over any optional part 

 of the divifions, and at any affumed dillance from each 

 other ; for though it would not then, as it does not now, 

 poflefs the valuable property of oppofite readings, and of a 

 motion in altitude to reverfe the oppofite arcs, by rcafon of 

 its being lefs than a femicircle, yet being compaftly made, 

 and of large radius, it is capable of receiving divifions fu- 

 perior to thofe on the quadrants which we have juft de- 

 fcribed ; and as tlie fituation and flrufture of the obfjrva- 



motion in taking obfervations, but meafures the number of tory are excellent, the appointment of a regular obferver 



feconds that the reading lines of the vernier is fliort of coin- 

 cidence in any obfervation : and formerly the quantity in- 

 dicated by the vernier had thefe feconds added, to obtain 

 the whole meafure ; but, by Smeatoii's advice, the "vernier 

 is now difrcgarded, and the addendum, obtained by feveral 

 turns of the micrometer-fcrew alone, is ufed to complete 

 the fimple reading of the divifions and fubdivifions, or 5' 

 fpaces ; in other words, the fcrew, and not the vernier, 

 fubdivides the lad 5' fpace in the obferved arc, and gives 

 what is due to the- obfervation. The adjullments of this 

 inftrumcnt, which was made of eight feet radius, on pur- 

 pofe to take the place of Graham's, are made m all refpefts 

 as they are made in its predeceffor ; and tlie fector, which 

 ferves for adjufting Graham's line of collimation by liars 

 near the zenith, is ufed for the fame purpole in Bird's 

 quadrant. If Any of our read(?rs widi to fee the original 

 plan of all the fcparate parts of Bird's quadrant, and to 

 read his defcription of them, we beg leave to refer them to 

 his quarto pamphlet, entitled " The Method of conftrufting 

 Mural Quadrants," publifhed by order of the commiffioners 

 of the Board of Longitude, in the year 1768. In this 

 pamplilet, however, it is not mentioned that the vernier 

 of the arc of 90° fubdivides the fpace of 5' into ten parts 

 of 30" each, by having ten divifions thereon, reading with 

 eleven fubdivifions of the limb ; but we know that this was 

 the original reading of the vernier, 



Jeremiah S'ljfon^s Mural Arc. — Perhaps it would have 

 been more confonant to the order of time, if we had fpoken 

 of the younger Siifon, who was Bird's contemporary, and 

 at one time his employer, next after Graham ; but the hif- 

 tories of the two mural quadrants at Greenwich being con- 

 neftcd, demanded that thefe inftruments fliould follow each 

 other in immediate fucceflion. In the year 1768, Jeremiah 

 Siffon, the fon of Jonathan, made the large mural arc at 

 the king's private obfervatory in Richmond gardens, which 

 extends 45-, or more, beyond a quadrant, and confequently 

 reaches beyond the north pole ; which circumftance gives 

 it the advantage of being put in the meridian, by an ob- 

 fervation of the pole-ftar. The divifions of this arc are into 

 degrees and its fubdivifions, but the ftrokes are not cut 



Vol. XXIX. 



to co-operate with the altronomer royal would be highly 

 conducive to the interefts of .iftronomy. We hope that this 

 hint may be taken up in a quarter where the power exifts 

 of realifing our wilhes in this refpeft. Allrononiica.1 clocks, 

 and various other auxiliary inlruments, are already in the 

 obfervatory ; fo that the principal expence of inllrumeats, 

 as well as of an appropriate and elegant building, is already 

 incurred. 



Portable AJlronomical Qiiadrant by Ramfden. — It fre- 

 quently happens that fuperior artifts vary the conftruftion of 

 their inftruments to fuit the views of the purchafer, or the 

 fcale on which they are to be conftrufted. We will feleft 

 out of the quadrants made by the late ingenious Mr. Ramf- 

 den, that which he made for Dr. Shepherd for the ob- 

 fervatory of Chrift-college, Cambridge, and which pro- 

 feflbr Vince has defcribed at confiderable length in his 

 " Treatife on Praftical Allronomy." The figure exhibit- 

 ing this quadrant is the 5th of Plate YSKlYi. oi AJlrono- 

 mical Injlrumiuts, which requires but little explanation. 

 The tripod on which the quadrant is mounted has fcrews of 

 adjuftment to fet the ftem, on which the horizontal motion is 

 performed, perpendicular, which is proved to be fo in all 

 direftions when the plumb-line bifefts both the fuperior and 

 inferior dots during the whole revolution round a horizontal 

 circle. The vifible ftem is a brafs tube, and through it 

 afcends a folid ftecl vertical axis, which fitting clofely at the 

 fuperior and inferior ends, has not the leaft ftiake, and pre- 

 ferves the pofition once given it fo long as the feet fcrews 

 are unmoleftcd. The telefcope is of the achromatic con- 

 ftruftion, and has the ufual apparatus for flow motion, the 

 fcrew of which is made a micrometer to fubdivide the fmall 

 refiduum of the angle that the vernier alone will not indi- 

 cate, when the coincidence is not perfeft. The telefcope 

 lies on a bar that carries the counterpoife, and in which is 

 the centre of its motion. It has a fyftem of wires in the 

 focus of the eye-glafs, which are adjuftable by fcretvs, both 

 upwards and fideways, as well as in a circular direction, fo 

 tliat the adjuftments for collimation, and for zero in the alti- 

 tude circle, may be effefted thereby. The point of fuf- 

 penfion of the plumb-line is alfo adjuftable by a proper fcrew 



T apparatus. ' 



