QUADRANT. 



Adjujlmenfs. — I. To adjujl the axis of the pedejlal vertical. — 

 This itdjutlment may be pcrtormtd cither by the plumb-line 

 or by the level, botii which methods we will explain in fuc- 

 ceilioii. We have already faid that when the wire of the 

 plumb-line will continue to bifeijt both the upper and lower 

 dot, while the inflrument is turned quite round in azimuth, 

 its axis is vertical in all directions, but this could not be 

 effedlcd unlels the plumb-line palling through tlie two dots 

 were alfo parallel to the axis; this adjuftment, therefore, is 

 made partly by the fcrews at the feet of the tripod, and 

 partly by the fcrew that moves the piece bearing the upper 

 dot in a lateral direftion. In the firft place then turn the 

 quadrant in azimuth till its plane, or, which is the fame 

 thing, the telcfcope lies parallel to a line joining any two of 

 the three feet, and turn one of thefe two fcrews till the wire 

 bifedls the lower dot, and with the proper fcrew brir.£; the 

 upper dot to the fame wire ; then reverfe the telefcope by 

 turning i8o°in azimuth, and if both dots are again bifetled, 

 the axis is vertical in the diretlion that the telefcope has 

 pointed ; in the next place turn the telefcope the fpace of a 

 quadrant till it points in the fame direftion as the third foot 

 of the tripod, and make the wire bifeft the lower dot by the 

 fcrew of this foot, and it will be f'^und to bifeft the upper 

 .dot alfo, if the iirll adjuftment of the dot was properly 

 made, but if not, repeat the operation till both dots are 

 bifefted in all the reverfed fituations of the telefcope, and 

 then the axis will be vertical in every direftion. 



In making this adjuftment by the level alone, the procefs 

 muft be thus ; firft, the level mull be made parallel to the 

 rod on which it hangs, and fecondly, this rod muft be put 

 perfectly horizontal, and the level will then be horizontal alfo, 

 with the bubble in the middle. In order to make the level 

 parallel to the rod, place it parallel to a line joining two of the 

 feet fcrews, and bring the bubble to the middle by one of 

 the feet fcrews in queftion ; then take off and reverfe the 

 pofitiGnx)f the level, and if the bubble is found in the middle 

 now, the parallelifm is perfett, if not, one half of the error 

 muft be reftified by the fame foot-fcrew, and the other half 

 by the adjufting icrews at the end of the rod, by releafing 

 one and fcrewing up the other. A repetition or tvv'o of this 

 procefs will make the bubble ftand in the middle in both of 

 the reverfed fituations. In the next place, with the level thus 

 parallel to the rod of fufpenfion, turn the quadrant round its 

 axis an entire femicircle as nearly as can be eftimated, and 

 if the bubble will now reft in the middle, the rod is level, and 

 being at right angles with the axis of the quadrant's motion, 

 proves that this axis is vertical in every direftion ; but if the 

 bubble is found to run to one end of the tube, bring it one 

 halfway back by the rod's adjufting fcrevvs, releafing one 

 and fixing the other, as the cafe may be, and the other half 

 by the proper foot-fcrew. A repetition of this procefs will 

 foon fettle the bubble in the middle during a whole revolution 

 in azimuth, and then the adjuitment of the axis is perfeft, as 

 well as of the rod and level. 



2. The fecond adjuftment is that by which the line of 

 coUimation of the telefcope is made parallel to the horizonta,l 

 line that pafl'es from the centre of the quadrant to zero on 

 the limb, or quadrantal arc, at the fame time that zero 

 on the vernier coincides with zero on the limb. This im- 

 portant adjuftment may be made in feveral ways, fome of 

 which are tedious and otherwife obje6lionablc ; but we wiU 

 confine ourfelves to two which apply, one' to the vertical, 

 and the other to the horizontal line of the quadrant, which 

 two methods, when duly effected, will not only check each 

 other, but deteft the error of the total arc, if there is any, 

 at the fame time ; which is an acquifition of the utmoft im- 

 portance. Firft then, to adjuft by the vertical line, let the 



axis of the quadrant be firft made truly perpendicular in all 

 direftions by tlie adjuftmeiil we have already defcribed, and 

 fix on a ftar within a few degrees of the zenith, when exadtly 

 on the meridian, and mcafurc its altitude by the crofs-wire 

 in the field of view in the ufual way, and note down the 

 rcfult ; do the fame on a fucceffive night foon after, if pof- 

 fible on an evening of fimilar temperature, with the quad- 

 rant turned half round in azimuth, and note again the refult ; 

 if thefe readings prove to be at equal diltanccs from the 

 point 90°, one on the quadrantal arc, and the other on the 

 arc of excefs beyond 90'', the horizontal wire is truly placed 

 in the eye-piece, but if not, half of the difference of the 

 readings muft be corrected by the proper fcrew for raifing 

 or lowering the faid wire. This may be done by directing 

 the telefcope to a diftant mark till the crofs-wire bifects it, 

 then by moving the fcrew of flow motion of the vernier the 

 half quantity required, and by bringing back (up or down) 

 the crofs-wire thus dilplaced to its original mark again. This 

 operation repeated will place the crofs-wire in fuch fituatioH, 

 that zero on the vernier will be in its proper place with re- 

 fpeft to the point 90° ; or the half difference thus afcertained 

 may remain, without altering the crofs-wire, as an error of 

 adjuflment to be conllantly applied with the fign -)- or — , 

 as the cafe may be, in all fubfequent obfervations. Again, 

 to adjuft by the horizontal line palling through zero of the 

 quadrantal arc, it will be necefiary to have a fecond telefcope 

 turning on pivots in adjuftable Ys attached to the back of 

 the quadrant, on the fame level with the faid horizontal line 

 of the quadrant. This telefcope may be called the adjufling 

 telefcope, and may be alfo ufed to 'watch a diftant mark, 

 before and after an altitude is taken, in order to deteft any 

 deviation in the pofitioii of the vertical axis, that may hap- 

 pen during the operation of meafuring. Let the adjufting 

 telefcope bifeft a fine diftant mark with its crofs-wire, and 

 turn the tube of the telefcope round one halfway on its 

 pivots, as it lies in a horizontal pofition, and if the wire now 

 bifefts the fame mark it is truly fixed, if not, look out for 

 a new mark a little higher or lower, as the cafe may be, and 

 make it cut that in the reverfed pofitions of the crofs-wire, 

 by means of the proper fcrew for this purpofe ; now this 

 adjufting telefcope will be adjufted for coUimation : in the 

 next place, put zero on the vernier to zero on the limb, and 

 direft the telefcope of obfervation to the fame diftant mark, 

 by which the adjufting telefcope had its wire adjufted, and 

 let tliis mark be bifefted by both telefcopes, the level and 

 plumb-line at the fame time fhewing that the vertical axis is 

 perpendicular ; now turn the quadrant half round in azimuth, 

 and reverfe the adjufting telefcope fo as to view the fame 

 diftant mark again, and if it is foiind to bifeft it as before, 

 the horizontal line of the quadrant is right, and alfo the 

 quadrantal arc without error, fuppofing the telefcope of 

 obfervation to have its adjuftment for coUimation as fixed by 

 the point 90°, above defcribed ; but if this adjuftment of 

 the point zero on the limb be Jirfl made, half the apparent 

 error muft be reftified by the icrew at the eye-piece, by 

 means of reverfed pofitions and new marks ; and then after- 

 wards the adjuftment by a Ihu- near the zenith will deteft the 

 error of the whole arc. If, however, no error in the total 

 arc exifts, then the adjuftment for colhmation may be made 

 either from the horizontal or from the vertical meafurement, 

 as may be moft convenient ; one of which is more praftica- 

 ble by day, and the other by night. When this delicate 

 and very effential adjuftment is finally fettled, the objeft- 

 glafs of the telefcope fhould not be diiturbed, and therefore 

 it would be advifeable to have its interior furface weU cleaned 

 previoufly. 



It was taken for granted that the crofs-wire was perfeftly 

 T 2 horizontal 



