QUADRANT. 



brought afterwards to the horizon, while tlie thread remains 

 (Iretched, and it will indicate the riling or felting amplitude, 

 as the cafe may be. 



PaOB. VI To Jind th: Afcenfwnal Difference. 



Reftify the bead as in the lad problem, and bring it to 

 the horizon, in which fituation the thread, extended to the 

 qiiadrantal arc, will (liew the afcenfional difference in de- 

 grees, which converted into time will (hew how much tlie 

 fun rifes before fix in fummer, and after (ix in winter, and 

 confequently will give the exaft length of the given day. 



Prob, VIT. — To find the Sun's Azimut!i. 



Retlify the bead for the given time, and obferve the fun's 

 altitude as explained in prob. 4. Then extend the thread to 

 the complement of that altitude, and the bead will indicate 

 the azimuth correfponding, and -vice verfd. 



Prob'. VIII. — To find the Hour of the Night by a Star. 



Put the bead on the thread to the diflance that will indi- 

 cate the ilar's declination, and look through the fight for 

 the liar till the plummet rells on the plane of the quadrant, 

 and in that fituation the bead will fliew, in the hour lines, 

 the ftar's dillance in time from the meridian of tlie place ; in 

 the next place fubtraft the fun's right afccnfion in time from 

 that of the flar, as given in fome catalogue, and to the re- 

 mainder add the obferved diftance from twelve o'clock in 

 fidereal time, and the fum will be the hour nearly, or the 

 approximate diftance of the fun from noon, which may be 

 corredled by applying the fun's variation of right afcenfion 

 fmce the preceding noon, which in every fix hours will be 

 about a minute. 



Sutton's quadrant, and CoUins's feftor on a quadrant, 

 are very fimilar, both in conftruftion and ufe, to the quadrant 

 we have here defcribed, and the dial on a card, by Fergufon, 

 is nearly related to it, particularly as it has been lately im- 

 proved by the Rev. W. Pcarfon. 



Of AJironomical Quadrants. — The quadrants which we 

 .have hitnerto defcribed may be confidered as by no means 

 perfeft, but as approximating only to an inftrument, tliat is 

 really ufeful in an obfervatory for determining the exaft 

 place of a heavenly body ; hence the quadrant which we pro- 

 pofe next to defcnbe, has obtained the name of ajiroiwmical, 

 from its fuperior pretenfions to accuracy in the meafurcment 

 of altitudes taken above the horizon, and therefore merits 

 our more particular attention. 



An aftronomical quadrant may be either portable or 

 fixed ; in the former cafe it is ufually mounted on a tripod, 

 with adjufting fcrews in the feet, and has a horizontal 

 motion as well as a vertical one, in order that it may take 

 altitudes in any azimuth, or be made to follow the body ob- 

 ferved in its apparent path ; but in the latter cafe it is fixed 

 againit a fteady wall, with its plane in, or very nearly in the 

 meridian, and is therefore denominated a mural quadrant. 



The firft aitronomical quadrant, of which we have any 

 account left us, is that which Ptolemy ufed ; it was the 

 fourth part of a circle placed faff againfl a flone pier, or 

 quadrangular log of wood, with zero of the arc in the ho- 

 rizontal line, and a pin of wood projetting from the central 

 point threw a fhadow on the limb when the fun fhone, 

 which fliadow was ufed by way of index : but it is obvious 

 that much accuracy was not to be expedled from fuch an 

 inftrument, however well conftruftcd or divided. We miglit 

 mention here the quadrants of Tycho Brahe and Hevelius, 

 but the former has been noticed under our article Circle, 

 and the latter was dcflroyed in the conflagration of the 

 owner's houfc in Dantzic. In more recent times, aftrono- 



mical quadrants have been made on accurate principles, and 

 with great care, ofpecially by Graham, Siffon, Bird, Ramf- 

 den, Gary, and Troughton, feveral of whofe inftruments 

 we will now defcribe, as far as any difference in their con- 

 ftrudlion renders diftinft accounts neceffary. We will pro- 

 ceed, as we have done on former occafions, chiefly in the 

 order of time, which, generally fpeaking, will be found to 

 be alfo in the order of fucceffive improvements. 



Mural Qiiadrant by Graham. — Before we proceed to 

 defcribe the mural (juadrant, contrived and made by 

 Graham, and fixed at the weft fide of the ftoue pillar in the 

 middle room at the Royal Obfervatory at Greenwich, at the 

 expence of king George I., and for the ufe of that eminent 

 aftronomer Dr. Halley, it may be proper to mention that 

 Flamfteed, and his afliftant vSharp, had previoufly ufed an 

 arc of a circle fixed againft a ftone pier in the meridian, 

 which they had thcmfelves conftru6ted, and which was 

 removed at Flamfteed's deatli. We muft, therefore, con- 

 fider their inftrument as having been the prototype of 

 Graham's mural quadrant, or arc, as it has been alfo called 

 fometimes. In fig. 4. of Plate XXIII. of Aflronomical In- 

 Jlrumenls^ is given a reprefentation of the mural quadrant of 

 Graham's conftrutlion, which will equally reprefent that 

 of Bird, conftrutted after the fame model, and which is the 

 fame that both Dr. Smith, and Stone, the editor of Bion's 

 work, have given in their refpeftive accounts. 



The body of this quadrant is compofed chiefly of bars 

 of iron united together, as fecn in the figure ; fome, to form 

 the plane of the quadrant, placed flat-ways ; and others, to 

 give ifrength and ftability, fixed edge -ways. Thefe bars are 

 all of the fame dimenfions, namely 2.9 inches wide, and 

 0.175 thick, and are united together by right-angled fliort 

 bent bars in various places, both at the interfeftions made 

 by the fides of the fmall fquares, and at other fituations, 

 fo that while great firmnefs is obtained, great weight is 

 avoided. The quadrantal arc is compofed of two bars, 

 one of iron, united to the iron frame, and the other of brafs, 

 on which the divifions are made, as defcribed under our 

 article Graduation' ; this brafs bar is pinned faft to tlie 

 iron one, and being more llender than the iron, accommodates 

 itfelf thereto, and, as time has proved, does not alter the 

 ftiape of tlje arc, as we have lately been afl"ured by Mr. 

 Troughton. The breadth of the limb is 2.2 inches, the 

 brafs limb being more remote than the anterior edge of the 

 ■ iron arc by 1.2 inch, and the furface was planed, oi- rather 

 fcraped, by a tool fixed to a radial bar, that revolved on a 

 vertical axis of motion, placed in the centre of the arc, and 

 refting with its fuperior end in a fixed beam above the plane 

 of the quadrant, when this plane was lying in a horizontal 

 pofition, it being imprafticable to put fo large a body in 

 any ordinary lathe. The original divifions of Graham were 

 inferted on two feparate arcs, one graduated into 90° and 

 its fubdivifions, and the other divided into 96 parts and its 

 fubdivifions, as we have before explained in the article juil 

 referred to ; but the divifions, being laid down by -rough 

 dividing, are not now made ufe of, but a quadrantal arc of 

 '96, with its fubdivifions, put in by Bird in 1753 between 

 the two arcs of Graham, is that which all obfervations taken 

 by Graham's quadrant are now referred to, and the read- 

 ings ai-e transformed into degrees, minutes, and feconds, by 

 an appropriate table. The readings were at firft obtained 

 by a double vernier-piece carried by the telefcope, that 

 revolves round the centre of the arc, one fide of which 

 vernier-piece read with the arc 90°, and the other with the 

 arc of 96 parts, or grand divifions ; the degree was fub- 

 divided into 12 parts, or 5* fpaces, and 10 parts on its 

 vernier equalled li out of the faid 12 parts, fo that 



