QUADRANT. 



glafs. This conftruflioii, witli the addition of fir Ifaac 

 Newton's fecoiid mirror, would coniUtute an union of two 

 inftruments, that would greatly refcmble the combination 

 adopted by Mr. Hadley, which follows next in our lid of 

 quadrants ; but which may not have been copied from Inch 

 union ; for Cole has got tlie vernier fcale, which is preferable 

 to the diagonal one adopted by Hadley, and therefore pro- 

 bably he followed Hadley, though the quadrant before us 

 has not recommended itfeif to general ufe, as being an, im- 

 provement. 



Hadley' s Qimdranl. — As we cannot help confidering Had- 

 Icy's qiiadrani, or more properly oclatit, as a near relative at 

 lead to his friend lir Ifaac Newton's, we will defcribe it 

 next in order, while the principle of its conflruftion is frefh 

 • in the reader's memory. The circuinltance of Mr. Had- 

 ley's being preiident of the Royal Society was favourable to 

 the early notice of his inftrument ; and the Intcreft that the 

 Britifli nation took, and muft ever take, from its infular 

 fituation, in nautical Improvements, coiilriijuted to Its early 

 adoption, at a time when fuch an inltrument was greatly 

 wanted. 



Mr. Hadley, wc learn, tried various modifications in the 

 conltruAlon, but that which has been approved from long 

 ufage is the one we (hall felcci for. defcription. Fig. I. in 

 Plate XXIII. of AJlronmnical Injlrumeiits, is the reprefenta- 

 tion of Hadlcy's oftant, as it is now condrufted with a ver- 

 nier, which it liad not at firft, and which is preferable to the 

 diagmal divifions at firft applied by Hadley, as well as by 

 fi/ Ifaac Newton. For the fake of lightnefs, united with 

 ftrength, the frame of the inftrument, when made of any 

 of the hard woods, is put together ufually as is repre- 

 fented in the figure, but when made all in brafs, and 

 particularly when the limb is extended to 120 half degrees, 

 reading as 120°, in which cafe it is called s. /extant, the bed 

 modern makers make it double ; that is, have two feparate 

 light frames, united by fhort pillars, danding at right an- 

 gles to their planes, which thus become parallel. This 

 condruftlon, we believe, was introduced by Mr. Troughton, 

 and allows a more iteady motion to all the moveable and 

 adjudable parts, by lengthening their axesof. motion, which 

 penetrate acrofs both parts of the double frame, and by that 

 contrivance have longer bearing?. But we are now pro- 

 pofing to defcribe a quadrant of Hadley'sown condru&lon. 

 A B C is a frame of fome hard wood, fuch as ebony, which 

 may be of any convenient radius, from eighteen inches down- 

 wards to three, or lefs, if required for thg pocket, and A D Is 

 the index bearing the vernier at D, together with the ufual 

 clamping apparatus for flow motion. In making the contact 

 in any obfervation. This apparatus, together with the powers 

 of the vernier, and mode ot ufing It, have been explained under 

 oar article Circle, (fee alfo VermiiR,) and the.eiore may be 

 referred to by the reader unacquainted with their ufes. Whea 

 the radius is very fmall, the vernier fubdivides half degrees, 

 and has thirty divifions on It, but has twenty or fifteen, ac- 

 cordingly as the degree of the hmb is fubdlvlded Into thirds 

 ox fourths of a degree. The peculiar excellence of tills in- 

 ftrument, either in the form of an octant or fextant, is, that 

 all forts of angles can be meafured with It on board a flilp, 

 even while the lliip is tofled by the waves ; and alio, that it 

 requires no other auxiliary means, than the natural horizon, 

 which at fea Is always, or moftly prefent, when a heavenly 

 body can be feen., Tiie plumb-line and fpirit-level are 

 equally ' ipenfed with : to which may be added, that, if 

 any accidental injai'y be received, a circuniftance not impro- 

 bable in the hands of fallors, the adjudments are fo finiple, 

 that, generally fpeaklng, the derangements may be eafily 

 reftified. In the belt Inllruments, a fraall telefcope is 



fcrewed into the fight-vane, which not only prevents parallax, 

 by limiting the line of fight between two parallel wires, but 

 afiidsthe fight greatly in obtaining exadt contafts. There 

 is ufually an arc of excefs at each end of the limb, one of 

 which is ufeful in adjufting the index-error by the iun or 

 moon, and the other is ferviceable when angular dlilances are 

 meafured beyond a quadrantal arc ; indeed it would be well 

 if the arc were always extended to meafure 120"^, or more, 

 for then a fextant would be competent to meafure all forts 

 of angles that the mariner can require, to find his latitude, 

 time, and longitude. 



When the odlant has not the tangent fcrew of flow mo- 

 tion for adjudmeiit in making the contaft, the index is 

 nicely moved by hand, and then fixed .by a fcrew behind it 

 for this purpoie, and in either cafe the examination and 

 noting down of the altitude, or horizontal angle taken, 

 may be read at any time, for hours afterwards, which is 

 another important advantage that this iudrument poffeflcs 

 in common with the fextant ; for where more obfervcrs than 

 two are not prefent; one obierver may thus manage to take 

 both an altitude and a dldance in a lunar obfervation with 

 two feparate Indruments in quick fucccflion, while another 

 obierver is taking the altitude of the fecond objeft ; or 

 with the help of Margett's horary tables, the fecond alti- 

 tude may be had by infpeftion, when the hour, latitude, 

 and declination of the body are known, in which cafe one 

 obfer-uer can take a lunar obfervation with tolerable accuracy. 

 The limb of the inftrument is bed of metal, fuch as brafs, 

 filver, or platlna, when made and divided in the bed way 

 by a fuperior dividing engine, fuch as Troughton's ; but in 

 ordinary indruments a piece of ivory is frequently let into 

 the wood, and fometimes the divifions are made even on 

 the wood Itfelf, which is liable to be affe£led by moidure. 

 The index is mod frequently entirely of brafs, and wider 

 as it afcends to the centre of motion, to prevent lateral 

 bending, wliich would dedroy the accuracy of the readings. 

 When an obfervation is made by the larger fort, the right 

 hand ftiould be applied to the lower extremity of the index 

 to give it fteady motion, while the left holds tiie lower 

 end of the remote radial bar ; and the plane of the inftru- 

 ment muft be kept in the line that joins the two objefts, of 

 which the horizon is one, when an altitude is taken, and 

 then the inftrument is held vertical. In bringing down the 

 image of an objeft to the horizon by a_/orfobferv,-,tion, the 

 body of the obferver muft gradually incline towards the 

 horizon, and a little vibrating motion will aflift in deter- 

 mining the exaft plane of contaft ; but in taking a hori- 

 zontal angle, the obferver will handle the inftrument as beft 

 fuits his convenience. The mirror at A is placed over the 

 centre of motion of the index, in a direftion pointing to 

 zero on the vernier, and perpendicular to the flat face of 

 the index : this index-glafs, being completely filvered, re- 

 fleijts the light it receives dire£fly at right angles on the 

 glafs E, when the zero of the vernier is at the zero of the 

 limb ; but as the limb proceeds forwards, this angle alters, 

 and, as we have fald, is duuhle of the meafure of the real 

 angle to be meafured and indicated. This glafs E is alfo 

 fixed perpendicular, and has icrews of adjuftment for per- 

 pendicularity above Its focket of brafs, and a tall-piece 

 with a fixiug-fcrew behind the frame, for fixing the paral- 

 lelifm ; the want of which is called the, index-error : one 

 half only of this glafs, which is called the fore-horizon glafs, 

 is filvered, and the other half remains unfilvered, in order 

 that both the dlreft rays tranfmltted through the unfilvered 

 part, and the refledled rays coming from the filvered part, 

 may meet at the eye, on which account the middle of this 

 glafs, where the line of feparation erodes, is the part to be 



viewed 



