QUADRANT. 



ventor ; the latter was cffefted by femi-opaqiic glafs, intro- 

 duced at firft without, but afterwards, with butter cflcdt, 

 with telefcopic fights; at tlie fame time, eiilai iniig- the 

 vifual angles fiibtended by the fun, and yet dimn/ilhinjr the 

 intenfity of his light by a partial tranfmidioii through the 

 fmoakcd or coloured glais. 



Da-vis's Qiim/rant, or Bnch-Stajf. — In the year 1 590, 

 captain John Davis, a native of Sandridge, near Dartmouth, 

 prcvioufly to his failing into the South leas under the com- 

 mand of Mr. Cavendilh, contrived that inllrument, which 

 is reprefented in Jig. 2. of Plate I. of /IJlronomicnl Injlru- 

 ments, and which has been called the Engli/h quadrant, or 

 bach-Jlaff. This inftrument difpenfed with the ufe of the 

 plumb-lnic, and confequently was better adapted to nautical 

 purpofes than the old quadrant, or than the fore-lhitt, that 

 preceded it ; but wanted the telefcojiic fights, which have 

 given lubfequent inftruments the advantage over it. It was, 

 however, probably the firft quadrant in which the horizon 

 was ufed as one of the objefts in a back obfervation, and 

 from whicli the refledting inllruments afterwards borrowed 

 an ufeful principle, where altitudes are concerned ; though 

 it was in the ufe of the fore-Jlaff, (defcribed under our ar- 

 ticle CiniLK,) that the horizon was firlt made one of |he 

 extreme limits of an altitude, taken by a forward obfervation. 

 Captain Davis found that pear-tree anfwercd very well as 

 the material on which his inllrument was conltrufted, and 

 an ingenious arrangement of two divided arcs and three 

 vanes conltituted his plan, according to the following de- 

 fcription. The vane at A was called the horizon-vane ; 

 the one feen at B the fhade-vane, becaufe its (hadow fell on 

 the horizon-vane during the inftant of completing an ob- 

 fervation ; and the third, at C, was denominated the fight- 

 vane, by reafon of its being the vane to which the eye was 

 applied in taking an obfervation. The arc of fmaller radius, 

 D E, contained 60'^ and the other, F G, of larger radius, 

 contained only 30°, in continuation of the former, making 

 together the whole quadrant. The arc D E was divided 

 into whole degrees only, on account of the fmallnefs of its 

 radius ; but the arc F G had its degrees fubdivided'by con- 

 centric and diagonal lines, as feen in the figure. The man- 

 ner of afcertaining the altitude of a heavenly body, by the 

 joint ufe of thefe two arcs, is not obvious at firit light of 

 the inftrument, but may be thus explained. When the al- 

 titude of the fun is taken, the horizon-vane is fitted to the 

 extreme end or centre A of the quadrant, and the fhade- 

 vane B is put to within about 10^ or 15° of the fuppofed 

 co-altitude, but to a lefs quantity than the co-altitude, 

 while the fight-vane remains for adjuftment on the arc F G. 

 Things being in this ftate, the back of the obferver is 

 turned to the fun, and the quadrant is fo elevated, that the 

 ftiadow of the upper edge of the fiiade-vane B falls upon 

 the upper edge of the flit in the horizon-vane A, when 

 viewed through the fmall hole in the fight-vane. If now, 

 in this fituation, the horizon is feen through the laid flit, 

 the obfervation is exatt ; but if not, the fight-vane is moved 

 backwards or forwards on the arc F G, accordingly as the 

 flty or fea is feen, till the horizon appears m its place, while 

 the fhadnw of the fhade-vane refts on the required fituation 

 in the flit of the horizon-vane, and then the cibfervation is 

 finifhed ; and the fum of the two readings on the refpeflive 

 arcs, B and C, as read by the fiducial edges of the vanes, 

 is the co-altitude or zenith diftance of that limb of the fun, 

 upper or lower, which was obferved from the correfponding 

 limb of the fhadow. If a lens, of a focal length equal to 

 the radius of the fmaller arc, were ufed, the focal luminous 

 point occafioned thereby would be a better objedt to meafure 

 the plr.c^ of, than a fhadow with an edge not fufiicicntly 

 9 



defined. This inftrument, it fhould fecnr, was not capable 

 of taking the altitude of a liar or jilanct, nor of the moon, 

 unlefs hn- difc was large enough at the time to projeft a 

 fhadow. 



Elton's Ouailrant. — An index bearing a fpirit-level, with 

 a vernier f.ale near the fight-vane, was added to the quad- 

 rant of Davis fome time afterwards, by one Elton, the ufe 

 of whicii was to take altitudes luithout an horizon ; but the 

 fimilarity of the two inftruments renders a more particular 

 defcription of this addition fuperfluous. An infpeftion of 

 Jig. 8. Plate I. will fufJiciently explain the difference. 



Gunter's Quadrant. — Among the numerous and ufeful 

 contrivances of the ingenious profefior of aflronomy in 

 Grefham college, was a portable quadrant, which now 

 claims our attention, and which was contrived in, or a little 

 before, the year i6l8. The obje£t of the inventor was not 

 to conftruct an inftrument capable of mcafuring altitudes 

 more accurately than that of captain Davis, which we liave 

 juft defcribed, but to make a quadrant fo comprehenfive in 

 its ufes, that, like the logarithmic fcale, which he divided, 

 it might fhew by infpeftion rcfults, which had previoufly 

 required long" and tedious calculations ; and in this point of 

 view it is ftill to be confidcred. Befides the quadrantal arc 

 for meafuring altitudes, this inftrument has various curves 

 ftereographically projefted on it, fuch as the equator, the 

 tropics, the ecliptic, and the horizon, on a fuppofition 

 that the eye is fituated in one of the poles, all which are 

 reprefented '\Vifg. 3. of Plate XXIII. of AJlronomical Injlru- 

 merits. The projeftion, according to Bion, is thus effefted; 

 when the quadrantal arc B C has been graduated, from 

 its centre A, with a convenient radius AT, defcribe the 

 arc T D to reprefent one of the tropics ; and let the line 

 A T be taken as the tangent of 56° 46', or half the fun's 

 greateft declination (fuppofed here to be 23° 32'), added to 

 the radius or tangent of 45° ; then to find the point E in 

 this line for the equinoftial, there will be this proportion, 

 as the tangent of 56"^ 46' : 1000 :: radius : 655 ; and there- 

 fore, if ^-V^,V parts of the line AT be taken, it will be the 

 proper radius of the arc EF, or equinoftial. 



To find the centre of the occult arc E D, which repre- 

 fents the ecliptic, let the meridional line AD be fo divided 

 by the point G, that if AF be taken as radius, AG may 

 be the tangent of 23^32', the fun's greateft declination ; 



in which cafe A G will be 



of the line A F, and the 



occult arc E D defcribed from the point G will be one- 

 fourth of the ecliptic, which may be divided into figns and 

 degrees thus : as radius is to the tangent of any degree's 

 diftance from the nearell equinoftial point, fo is the co-fine 

 of the fun's greateft declination, to the tangent of that 

 degree's right afcenfion ; for example, fuppofing the right 

 afcenfion of the firft point, of y to be 27'' 54', draw 

 a line from A, the centre of the quadrantal arc, to this 

 degree and minute on the faid arc, and "Viote where it 

 intcrfefts the occult arc of the ecliptic, and this point will 

 be the beginning of the fign t( ; a"d in like manner any 

 other part may be inferted. 



The line ET, or fine of declination, may be divided 

 thus ; taking AE for the radius of the equinoftial, or tan- 

 gent of 45°, let the tangents of 46°, 47°, 48°, &c. up to 

 68^ 30', be fucceflively taken and laid down on the line 

 E T, and the points of excefs above the tangental point of 

 45° will be the dividing lines of the fcale for I", 2°, 3*^, &c. 

 up to 23° 32', or greateft declination. 



When the fcale of declination is finidied, the quadrantal 

 arc may be taken as the meafure of right afcenfions, and 

 then the place of a ftar or any other heavenly body r^ay 

 be inferted on the plane of the quadrant thus ; let a line be 



drawn 



