QUADRANT. 



downwards, to prevent any inclination to bending by its 

 weight, and has an axis of motion of four inches and a half 

 long, that paHes through the thick pillar, that connefts the 

 two quadrantal portions of the double frame at the exaft 

 centre of motion, and of the divided arc. The counter- 

 poife is applied at the redundant end of the tclefcope, and 

 the vernier borne by its end, near the eye-piece, divides 

 the 5' fpaces of the vertical arc into 5", while a micromctri- 

 cal microfcope, attached to the metal of the telefcope, lub- 

 divides this lalt quantity miofingle ftronds. A nicely ground 

 fpirit-level hangs on the horizontal bar, with its proper ad- 

 jullment apparatus, which alone will afccrtain the poiition 

 of the quadrant to nfecond ; but, as if this were not fuf- 

 ficient, a plumb-line is made to defcend, free from dull, or 

 agitation by the wind, from an adjuitable point of fufpcn- 

 fion through the hollow column, where there is little or no 

 diilurbance from amotion in azimuth, and enfures the pofi- 

 tion ilill more certainly. The water vellel in which the 

 plummet is immerfed, to prevent vibration, is contained in 

 the hollow fhort cone before defcribed, as containing a tri- 

 angular frame within it, and the iituation of the wire is 

 examined in two direftions, at right angles to each other, by 

 microfcopes looking acrofs the bore of the column, at a con- 

 venient height for the obferver to fee without a change of 

 pofition of his body, when he has been juft obferving at the 

 telefcope. The mark ufcd for each wire to bifeft is a lumi- 

 nous difc, known by the name of Ram/din's ghoji, from its 

 being oidy the image of a luminous point without fubftance, 

 occalioned by a contrafted aperture of a tube, fixed at the 

 remote fide of the column into which light enters, and by 

 which it is direfted without parallax. Befides thefe appen- 

 dages, there is a fecondary or adjuiling telefcope, fuch as 

 the elder SifTon made for his fpirit-level, and fuch as we 

 have already defcribed, as furnilbing the means of adjufting 

 the horizontal line of a quadrant that moves in azimuth, and 

 of afcertaining the error of the whole quadrantal arc, by 

 comparifon with the adjuflment by a ftar near the zenith. 

 (See Portable AJlronom'ical Qiiadrant by Ramfden, before de- 

 fcribed ) Lattly, the radial bars that bear the quadrantal arc, 

 taper downwards from the centre, thereby giving ilrength to 

 the part moft liable to alter its figure by weight, as it is fup- 

 pofed Bird's mural quadrant has done in a fmall degree for 

 want of fuch precaution. This quadrant has an arc of ex- 

 cefs at each end, and is capable of all the adjuftments we 

 have above defcribed in an exquifite degree. 



Of Nautical Quadrants, meafur'ing by ReJleB'wn. 



Sir Jfaac Netuton's refleEling Quadrant. — A manufcript ac- 

 count of a quadrant, meafunng altitudes and diftances by 

 refleftion, of the hand-writing of fir Ifaac Newton, was 

 found among the papers of Dr. Halley after his death, which 

 quadrant, according to Stone, was aftually made in the year 

 1672, when Dr. Halley was preparing to go to the South 

 feas, to make an addition to his catalogue of fixed ftars ; 

 but the manufcript account was not produced, or even men- 

 tioned, when Hadley's inllrument was fliewn to the Royal 

 fociety, nor was made known till the year 1742. (See 

 Phil. Tranf. N° 465. ) Hence fome doubt has been enter- 

 tained whether fir Ifaac Newton or Hadley was the firll in- 

 ventor of the reflecting quadrant. The moft probable in- 

 ference is, that each invented his own, feeing that though 

 the principle is the fame in bothconflruftions, yet the mode 

 of applying it is different, as wdl be feen from a comparifon 

 of the figures, and from our defcription of each in fuccef- 

 fion. Sir Ifaac Newton's quadrant was preferved for feveral 

 years at the houfe of Mr. Heath, who was a mathematical 

 inftrument maker in the Strand, London, and it is probably 



12 



in exiflence at this time. Dr. Hooke is alio faid, by Dr. 

 Pratt, to have invented a quadrant that was, or might be, 

 ufed at fea with one refleftion only ; and, indeed, he is ailerted 

 to have been the firft man who propofed the ufc of a mirror 

 in a nautical inftrument. See Cikcle. 



Sir Ifaac Newton's quadrant, reprefented by Jig. 7. of 

 Plate I. of /IJlronomical Injlruments, confifts of an entire 

 fcdtoral plate of brafs, P (j R S, to the plane of which the 

 telefcope A 13 is fixed and lies parallel, and an index, which 

 is moveable about an axis of motion at A. The limb 

 D Q was accurately divided into half degrees, and, as is 

 faid, half minutes, on a fcale of four feet radius, and was fub- 

 divided by a diagonal fcale into -,^th of a minute. The 

 principle on which this femi-divilion is founded is this ; vi-z. 

 , " If a fixed ray of light be rcfledted at a plane refledlor, 

 and if the refledlor be made to revolve about an axis perpen- 

 dicular to the plane palling through the incident and refletled 

 rays, which may be called the plane of refleftion, the an- 

 gular velocity of the reflefted ray will be double to the angular 

 velocity of the rcfleftor." See Vince's Praftical Aitro- 

 nomy, p. 7, &c. 



Hence one fmall mirror, G, is made faft to the plane of the 

 feftoral plate, and perpendicular thereto, but inclined in an 

 angle of 45° to the axis, or length of the telefcope, and in 

 fuch a way, as to cover one half of the aperture, while an- 

 other fimilar mirror is borne by the index, in fuch a pofition, 

 that when the index is at zero, both the mirrors are not only 

 perpendicular to the plane of the quadrant, or rather oftant, 

 but are parallel to each other. Now, according to the 

 principle that we have jull mentioned, when the index is 

 moved forward, a ray of light from any fixed luminous body, 

 when caught by the index mirror, will be reflefted on the 

 fixed mirror, and an eye direfted through the fixed telefcope 

 tow.irds the faid body, will fee it divide into two bodies the 

 inftant that the index begins to move ; that is, the body 

 itfelf will be feen Jlationary through one half of the tele- 

 fcope's aperture, and its image will be feen in the other half 

 in motion, and this motion has double the velocity that the 

 index has, which bears the revolving refleftor ; and on this 

 account it is, that the divifions for half degrees and half mi- 

 nutes, are read as whole degrees and whole minutes, as well 

 as the -rVth read as ,1th by the diagonal fcale. From this 

 fliort explanation of the piinciple and ftrufture of fir Ifaac 

 Newton's reflefting quadrant, it is eafy to perceive that the 

 conilruftion is derived immediately from the principle in 

 the fimplefl, though in praftice not the beil manner ; for 

 the inftrument itfelf illuftrates the principle in the moft ob- 

 vious way ; but, from its magnitude and mode of being ufed, 

 it is very inconvenient to be fupported without a ftand, which 

 on board a fliip is inadmilTible. 



Perhaps it might be on this account that Dr. Halley did 

 not pay more attention to it than he appears to have done, 

 according to the information that we at this diftance of time 

 poflefs. However, it is evident, that the inftrument before 

 us is capable of meafuring either vertical, horizontal, or 

 oblique angles, in the way it profefTes. 



Calebs Quadrant by Jingle RtfleSion. — Fig. 2. of Plate 

 XXIII. of yljlronomical Injlruments, is the figure of an in- 

 ftrument, which, like Dr. Hooka's contrivance, meafured 

 altitudes by fingle refleftion, as we conceive, the drawing 

 having fallen into our hands without the defcription. An 

 arc compofed of an entire quadrant, and au index with a 

 fingle mirror fixed to it at its centre of m.otion, conftitute 

 its leading features, while a fight-vane, at the remote end 

 of the prolonged index, inftead of a telefcope, has a plane 

 hole, through v.-hich the fun is viewed after refleftion from 

 the ir.dex-glafs, his rays having firft palfed through a coloured 



glafs. 



