QUADRANT. 



apparatus. At tiie top ot the vertical tube, or ftem, is a 

 fmall horizontal circle with a clamping apparatus for flow 

 horizontal motioTi, by means of wiiich the whole quadrant, 

 with its attached tclefcopc, turns gradually rotind in azi- 

 muth. Wlien obfervations are made in or near the zenith, 

 the plumb-line falls in the way of the telefcope, and is obliged 

 to be removed, on which account the large quadrant made 

 by Ramfdcn for the duke of Marlborough, and now placed 

 at Blenheim, has the plumb-lme fufpcndcd at the poilerior 

 face of the inflrument. This inconvenience is, however, 

 remedied by the addition of a fpirit-level fufpended from 

 an adjuitable horizontal brafs rod, under the uppermolt 

 radial bar of the quadrant, and this level not only fupplies 

 the place of the plumb-line when taken off, but at all times 

 ferves as a check on its adjullment, and, when furnifhed with 

 a graduated fcale, may very well be made its fubltitute. 

 The conftruftion of Ramfden's portable quadrant was pro- 

 bably borrowed from Bird's, as well as his method of 

 dividing the larger inflruments that exceeded the reach of 

 his dividing engine ; but the inftrument before us has no other 

 horizontal or azimuth circle, except what is ufed plain, for 

 the purpofe of giving flow motion. Neither is there a 

 fecond horizontal telefcope fixed permanently in that fituation 

 by which the telefcope of obfervation may have its coUima- 

 tion determined, and the zero of its vernier adjuftcd. 



The vernier of this quadrant reads with two fets of divi- 

 fions, hke that of the mural quadrant by Bird, but the 

 radius being fmall, the fubdivifions are made into four parts 



each ; the inner arc is divided into 90'', and the fubdivifions 

 into 15' each, but the outer one into 96 parts, or grand 

 divifions, and thefe again each into four parts, or fub- 

 divifions ; and thofe two arcs operate as a check on each 

 other. The micrometer lub-dividcs the 15' into 3" ; but it 

 is necelTary to convert the reading of the arc of 96 grand 

 divifions into a correfponding quantity, exprefled in degrees, 

 minutes, and feconds, which may be done by dire£t pro- 

 portion, or, which is more feafible, by a table that we have 

 fubjoined, calculated for this purpofe. The readings of the 

 arc of 96 are put down with titles Jivijions, fiib-JiviJions, and 

 vernier, where 16 (and fomctimes 32 1 on the vernier are 

 equal to one fub-divifion, andybar fub-divifions to one divifion ; 

 for example, fuppofe the reading to be thus, 21 div. 2 fubv 

 1 1 ver. we fhall have from the tabic 



When, however, the coincidence of the vernier is not 

 perfeft, the quantity brought up by the micrometer-fcrew 

 mull be added as a fratlional portion of 52. "8, which is the 

 value of one ftep of the vernier, when 16 is the number of 

 fteps, but if 32 are inferted, then 26".4 would be the value 

 of one ftep forwards from zero. 



A Table for the Reduftion of the Grand Divifions, Sub-divifions, and Vernier, of the Arc of 96, into Degrees, 



Minutes, and Seconds. 



Adjiijlmenls. — 



