QUADRANT. 



horizontal during the time the preceding adjuftment was 

 made, or, which is the fame thing, that the parallel vertical 

 wires were perpendicular to the horizon. This is proved in a 

 fimple manner thus ; diredt tiic tclefcope to a fine fmall dif- 

 tant mark, and make the adjullmciit forvifion, if neceflary ; 

 then if one of the vertical wires will continue to hifedt the 

 faid mark through the whole field of view while the tele- 

 fcope is elevated or depreiTed, the wires are right, but if not, 

 they muil be made fo by the proper fcrews for that purpofe, 

 near the focus of the oye-glafs. This preparation ought to 

 precede the lail adjuftment, and when once made, feldom re- 

 quires altering, except in cafe of accidental injury. 



It has alfo been aifumed in the preceding adjulimcnt, that 

 the maker of the inftrument placed the plane of the quad- 

 rant parallel to the axis of its motion, and alfo the line of 

 coUiniation of the telefcope parallel to the faid plane. Tiie 

 former may be known to be true thus; if, ulien the plumb- 

 line is adjulled, at its centre of fufpi-nrioii, juil to efcape 

 touching the limb, (which fltould always be the cafe,) the 

 quadrant's motion in a/.imuth will not alter it in this refpetl, 

 the plane it truly fixed ; but if not, the fcrews, tiiat fix the 

 q\iadrant to its axis, muft be reforted to for the alteration, 

 which is belt done by the maker. When there is no plumb- 

 line, a fmall fpirit-level, fixed at right angles to the plane 

 of the quadrant, will anfwer the fame purpofe ; for the red- 

 ing of the bubble during the quadrant's revolution in azi- 

 muth, will be a proof that the plane to which it is at right 

 angles is vertical. With refpeft to the parallel pofition of 

 the telefcope, as this is guided by the vernier Aiding on the 

 limb, it is the bufincfs of the maker to adjuft it properly, 

 which he will beft do by a comparifon with a good tranfit 

 initrument of the paflages of a high and of a low ftar in 

 each of the two inftruments ; but a fmall deviation of the 

 telefcope with refpeft to parallelifm, though to be avoided 

 if prafticable, will not fenfibly affeft; the meafurement of 

 altitudes, which is the fole bufinefs of this initrument. If, 

 however, this deviation is confiderable, the eye-end of the 

 telefcope mull be fet nearer to or farther from the limb, as 

 the cafe may be, by the maker himfelf. We have been the 

 more minute in defcribing thefe adjultments, not only be- 

 caufe they are indifpenfably neceffary in making good obfer- 

 vations, but becaufe they will apply, one or other of them, 

 by means of the plumb-line, or of the fpirit-level, to all 

 Other aftronomical quadrants that have a motion in azimuth. 



We have before us the drawing of a large and beautiful 

 quadrant made by Mr. Cary, for Leopold, the late grand 

 duke of Tufcany, with two telefcopes and a graduated azi- 

 muth circle, but the dcfcription we have given of Ramfden's 

 inftrument will equally apply to his, except as to the di- 

 menfioas. 



Portable AJlrononncal Quadrant by Troughton. — Though 

 we have defcribcd Ramfden's portable aftronomical quadrant 

 with much minutenefs, and have detailed the moft ufeful 

 methods of making Rich adjuftments as will apply to the 

 other portable quadrants that have a motion in azimuth, yet 

 we fhould do violence to our own feehngs, as well as to the 

 ingenuity- of an exifting artift of the firil- eminence, if we 

 withheld from the public eye the great improvements that he 

 has made in this inftrument fince the death of Ramfden. 

 Fig- 6. of the fame plate that contains Ramfden's inftru- 

 ment, is a reduced perfpeftive view of the improved aftro- 

 nomical quadrant of Troughton, which was made by him 

 and fent to Bilboa in Spain, about the time that aftronomical 

 circles began to be conftrufted. It has been afferted, under 

 our article Circle, that this inftrument, as conftrufted by 

 Troughton, ( and we may now add, by Thomas Jones, who 

 has learnt his mode of dividing,) is greatly fuperior to any 



quadrant that can be made, on account of properties whicli 

 the circle exclufivcly pofrcfTes ; but fo far as a quadrant's 

 accuracy can be depended on, Troughton's improved con- 

 ftruftion is to be preferred to all others. And, indeed, 

 when we confider that fimplicity, fteadinefs of performance, 

 and permanence of the adjuftments once made in this inftru- 

 ment, are properties which it pecuharly poffefles, along with 

 comparr.tive cheapnefs, it is probable that there will always 

 be purchafers, when fuch an inftrument is on fale, provided 

 the improver will confeut to make luch inftrument with 

 limited powers, when he can have rapid fale for thofe whicU 

 he now couftrudls, with all the advantages that the circle 

 affords. 



The radius of this quadrant is three feet, and the body is 

 made double, that is, of two quadrantal frames united into 

 one, by fmall pillars holding their planes parallel, and enfur- 

 ing the two properties, not often united in other men's in- 

 ftruments, of lightnefs and ftrength at the fame time. The 

 tripod, on which the quadrant is fupported, is a frame of 

 mahogany, braced in different direftions, fo as to refift any 

 ordinary prefl'ure, when the quadrant is put in motion, or 

 the telefcope ufed. The three feet-fcrews are furniftied with 

 each a Hooke's joint and long handle, fo that the obferver 

 may make an adjuftment with any of thefe -fcrews without 

 ftooping, and confequently without withdrawing his atten- 

 tion from the plumb-line apparatus, or fpirit-level, that in- 

 dicate the quantity of adjuftment that may be neceffary from 

 any individual fcrew. About the middle of this pedeftal, 

 or frame-work of the tripod, is a three-armed horizontal 

 bracing piece, on which the item of the quadrant refts ; and 

 this ftem is kept vertical by a focket of brafs made faft to 

 the centre of the table, that furmounts the pedeftal, in 

 which focket the vertical axis turns both fteadily and freely, 

 while it refts on the three-armed bracing piece below. The 

 azimuth or horizontal circle is centered on this axis, but fo 

 as to admit of a motion round it of about two degrees, for 

 the purpofe of putting the zero of the quadrant right when 

 the telefcope is in the plane of the meridian. This fmall 

 motion is produced by the tangent fcrew feen in front, and 

 the other tangent fcrew regulates the flow motion of the 

 telefcope and vernier, by taking hold of the folid vernier 

 plate, that reads at oppofite points, and that may be 

 clamped, when in ufe, to the azimuth circle, which is alfo 

 a folid circular plate of brafs, fubdivided into lo' fpaces, 

 and reading by the verniers alone to lo". 



The vernier or index-plate is of confiderable depth, and 

 hollow, terminating with a chamfered edge below, and con- 

 tains in it a triangular frame not feen, foldered to it, and 

 oppofed, for the fake of ftrength, by another fimilar frame 

 feen above its plane. From the frame within the hoUovr 

 index-plate arife three fmall pillars, which fupport the upper 

 triangular frame at the three corners, and palling through 

 it receive fo many milled nuts on their tapped ends, by 

 which means the whole are compaftly united. Thefe milled 

 nuts are ufeful for adjufting the plane of the quadrant paral- 

 lel to the vertical axis of the quadrant's motion in azimuth ; 

 and it is here where the quadrant is to be feparated from the 

 ftand for clofe package. Upon the upper triangular frame 

 is foldered faft a fhort but ftrong conical tube, that fupports 

 the long column that terminates with a fupplementary cone. 

 From nearly the lower extremity of this long column a 

 couple of braces afcend about twenty inches to the upper 

 part of the body of the quadrant, and, by being made faft to 

 it, complete the fteadinefs of the ftrufture in an admirable 

 manner. The telefcope, which is about forty-two inches 

 long, and achromatic, as well as furnilhed with an adjuft- 

 able fyftem of Spider's threads, tapers from the objeft-glafs 



down- 



