Q U A 



QUA 



Mr. Wales, in captain Cook's Voyage, applied it to 

 mcafuring the quantity etlipfed in an eclipl'c of the fun ; in 

 which operation it anfwers the purpofe oi a micrometer, to 

 a great degree of certainty. See an account of the im- 

 provements fuggefted in the conftruftion of thefe inftru- 

 nicnts, and aUo of the various ufes to which they may be 

 applied, in Magellan'* Defcription des OAants & Sextants 

 Anglois, &c. 4to. 



Other quadrants have been contrived fince, by Ionic in- 

 genious artirts, all of which have their merit ; but the par- 

 ticulars of their coullrutlion are too many for this place ; 

 and perhaps, on the whole, nothing preferable to Mr. 

 Hadley's invention has yet been found. 



Quadrant, HorodMcal, is a pretty commodious inllru- 

 ment ; thus called from its ufc in telling the hour of the 

 day. 



Its conllruftion is fo funplc and eafy, and its application 

 fo ready, that we (hall defcribe both, for the ufe of fome 

 who may want other conveniences. 



Quadrant, Conjlnitllon and Ufe of the HorodWical. 

 From the centre of the quadrant, C, [Plale XIX. /IJlru- 

 nomy. Jig. 8.) whofe limb A B is divided into 90% defcribe 

 feven concentric circles at intervals, at plcafurc ; and to thefe 

 add the figns of the zodiac in the order they are reprefented 

 in the fcheme. 



2. Applying a ruler to the centre C, and the limb A B, 

 mark upon the feveral parallels the degrees correfponding to 

 the altitude of the fun when therein, for the given hours ; 

 conneft the points belonging to the fame hour with a curve 

 line, to which add the number of the hour. To the radius 

 C A fit a couple of fights, and to the centre of the quadrant 

 C, tie a thread with a plummet ; and, upon a thread, a 

 bead to Aide. 



If, now, the bead be brought to the parallel in which 

 the fun is, and the quadrant be direftcd to the fun till a 

 vifual ray pafs through the fights, the bead will fhew the 

 hour. 



For the plummet, in this fituation, cuts all the parallels 

 in the degrees correfp<niding to the fun's altitude. Since, 

 then, the bead is in the parallel which the fun then defcribes, 

 2nd thi-ough the degrees of altitude to which the fun is ele- 

 vated every hour there pafs hour-lines, the bead mutl fhew 

 the prefent hour. Some perfons, who are not very nice, 

 reprefent the hour-lines by arcs of circles, or even by 

 ftraight lines ; and that without any fenfible error. 



QuAUDANT, Sintcal, is an inftrument of ufe in naviga- 

 tion. It is reprefented Plate II. Navigation, Jig. 4, and 

 confills of feveral concentric quadrantal arcs, divided into 

 ei^rht equal parts by radii, with parallel right lines crofling 

 each other at right angles. 



Now any of the arcs, e. gr. B C, may be accounted a 

 quadrant, of any of the great circles of the fphere, chiefly 

 of the horizon and meridian : if, then, B C be taken for a 

 quadrant, e. gr. of the horizon, either of the fides, e. gr. 

 A B, may reprefent the meridian ; and the other, A C, will 

 reprefent a parallel, or line of eaft and weft ; and all the 

 other lines parallel to A B will aifo be meridians ; and all 

 thofe parallel to A C will be eaft and weft parallels, or eaft 

 and weft lines. 



Again, the eight fpaces into which the arcs are divided . 

 by the radii, reprefent the eight points of the compafs in a 

 quarter of the horizon ; each containing 11" 15'. 



The arc B C is likewife divided into 90", and each degree 

 is fubdividtd into 12', diagonal wife. 



To the centre is fixed a thread, as A L ; which being 

 laid over any degree of tke quadrant, ferves to divide the 

 horizon. 



If the finical quadrant be taken for a fourth part of the 

 meridian, one fide thereof, A B, may be taken for the 

 common radius of the meridian and the equator ; and then 

 the other, A C, will be half the axis of the world. The 

 degrees of the circumference, B C, will reprefent degreen 

 of latitude, and the parallels to the fide A B, aftumed from 

 every point of latitude to the axis A C, will be radii of the 

 parallels of latitude, as likewife tlie fine-complements of 

 thofe latitudes. 



Suppofe, then, it be required to find the degrees of 

 longitude contained in 83 of the lefler leagues, in the pa- 

 rallel of 48°. Lay the thread over 48° of latitude, on the 

 circumference, and count thence the 83 leagues, 011 A B, 

 beginning at A ; thefe will terminate at H, allowing every 

 fmall interval four leagues, and the interval between the 

 broad lines tv.'cuty leagues. Then tracing out the parallel 

 li G, from the point H to the thread; the part A G of 

 the thread (liews that 125 greater, or equinoctial leagues, 

 make 6'' 15', allowing twenty leagues to a degree, and three 

 minutes for one league ; and therefore that 83 lefler leagues 

 A H, which make the difference of longitude of the courfe, 

 and are equal to the radius of the parallel G I, make 6° 15' 

 of the faid parallel. 



If the (hip fail on an oblique courfe, fuch courfe, beGdes 

 the north and fouth greater leagues, gives lefTcr leagnaes 

 eafterly and wefterly ; to be reduced to degrees of longitude 

 of the equator. But thefe leagues being made neither on 

 the parallel of departure, nor on that of arrival, but in all 

 the intermediate ones, we muft find a mean proportional 

 parallel between them. 



To find this, we have on the inftrument a fcale of crofs 

 latitudes. Suppofe, then, it were required to find a mean 

 parallel between the parallels of 40' and 60''. With your 

 compaftes take the middle between the 40th and 60th de- 

 gree on the fcale : this middle point will terminate againft 

 the 51ft degree, which is the mean parallel required. 



Quadrant, U/e of tie Slnkal. There are formed tri- 

 angles upon this inftrument fimilar to thofe made by a fhip's 

 way, v^nth the meridians and parallels ; the fides of which 

 triangles are meafured by the equal intervals between the 

 concentric quadrants, and the lines N. and S.E. and W. 



The lines and arcs are diftinguifhed, every fifth, by a 

 broader line ; fo that if each interval be taken for one 

 league, there w'U be five between one broad line and another ; 

 and if every interval be taken for four leagues, then there 

 will be twenty leagues, which make a fea degree, from one 

 broad line to the other. 



Now, fuppofe a fhip to have failed 150 leagues nortli- 

 eaft, one fourth north ; which is the third point, and makes 

 an angle of 33° 45' with the north part of the meridian. 

 Here are given two things ; -viz. the courfe, and the dif- 

 tance failed ; by which a triangle may be found on the in- 

 ftrument, fimilar to that made by the fliip's courfe, and her 

 longitude and latitude ; and hence Tjcaj the unknown parts 

 of the triangle be found. 



Thus, fuppofing the centre A to reprefent the place of 

 departure ; count, by means of the concentric arcs, along 

 the point the fhip failed on, as AD, 150 leagues from A 

 to D ; then is the point D the place the fliip is arrived at, 

 which note. This done, let D E be parallel to the fide 

 A C ; and then there will be formed a right-angled triangle 

 A E D, fimilar to that of the fhip's courfe, difference of 

 longitude, and latitude: the fide AE gives 125 leagues 

 for the difference of latitude northwards ; which makes 6° 

 15', reckoning twenty leagues to a degree, &c. and the 

 fide D E gives S3 leffer leagues anfwering to the parallels ; 



which 



