TRANSIT-INSTRUMENT. 



aUp^ m^ aedfined fay each, may be alceruined by a 

 graduated ftaff, and the tables which we pubhlhed under 

 TzLEScaPE. 



5. To mail ihi L'tra of ColSrcattin perpaiduular to the Axb. 

 ^After having adjuited for dift'-cct vifio3 to fome diftant 

 knovvji objed, make the middle Une in tbe eye-p!?« bifeci 

 fome well-defined point in it, and, having reverfed the ends 

 of the axis, obferve if the fame line again bifeds the fame 

 point esattly ; if it does, the line of coUimation is right ; 

 but if not, the error cay arife either from the fituation of the 

 point obferred, or of the centre ef the objeck-glafs, as they 

 refpefijTely regard the vertical lice ic the eye-piece ; that is, 

 one at other of thofe line? may deviate a little from a true 

 perpendicular to the line of the axis cf n-.odon, or both mav 

 be erroneous in their refj>eftive degrees : therefore, when 

 reverfion has taken place, bring the vertical line one halfway 

 towards coincidence with the obferred point, bv the hori- 

 zontal Icrew at the end of the asis, which alters the azimuth, 

 and the other half by the fide-fcrews of the eye-piece, which 

 move the plate 01 the parallel Hues, releafing one fcrew and 

 tightening the other, fo as to leave the laid plate faa ; then 

 reverfe again, and repeat the operation, till the fame point is 

 bifeaed in both pohdons of the axis. 



6. To ilfummai i ice Wins, Heirs, sr Spider's Lirjs Before 



aa obfervation of a ftar or planet can be taken, the parallel 

 liaes mufl be rendered vifible to the eye of the obferver, and 

 the quantity of hght to be admitted will depend on the magni- 

 tude of the body to be obferved. A lantern containino- 2 lamp 

 muft be fixed oppofite that end of the axis which has its 

 aperture clofed by 2 lens, a::d fo much cf the hght muit be 

 excluded as is lupernvraus for the obfervation ; for when a 

 ficall fbr is the object, cjch light will render it invifible, 

 ud yet it is neceuary to have light e-ough to render the 

 paralle! lines viiible ; hence cbiervatioiis en very fmall ftars 

 require management cf the quantity, and lometimes of the 

 quality tooj>f the extraneous light; on which account 

 prifrts of different cobured glafs ha»e been interpofed be- 

 tween the lamp and the end of the axis, with Aiding mo- 

 tions, to produce various modifications in the illumination, 

 acpording to the exigencies of the coaent. 



7. To aSj-jji zh; bcrlz,orJai JVin or Hair for taiiag Aki- 

 tudts — When the altitude of an objeft ^5 to be accurately 

 taken, bring the bubble of the dounle vernier to the middle 

 cf the level, and turn the telefcope on its axis of motion, 

 till vernier i has i-^ zero at c in the circle, or at 90°, if the 

 circle is graduated for zenith diilacces, as is mcilly the cafe ; 

 thee notice what diitant point is cut by the horizontal 

 wire ; reverfe then the axis, and bifeft the fame point again, 

 tad in this fituation fee what quaatitv is read by the fame 

 i^THJer ; or, which will be iliU better,' half the fum cf the 

 readmgs o:^t oth vemiers-i and 2 may be taken at both times, 

 and the dinerence of the two avera^ readings nray be ccn- 

 fiderable ; then one half of this diference, - or — , is the 

 real error of each obfervation, accordinglv as the axis is 

 fituated m pofition.; and if this error is cot obviated by a 

 proper adiuitment, it muft be allowed for in the reading of 

 every o:>lrryadon. But this error may be owing partly to 

 tae unaijufted ilate of the vernier level, and pardv to the 

 fituation which the horizontal wire occupies in the eye-piece ; 

 therefore the fcrews which elevate and denrefs the hori- 

 aontal wire, which, however, are not in all inftru-nents, may 

 ecmacl one half of the error, and the fcrews of the vernier 

 level the other, in each Reverfed pofition till the error dif- 

 appears ; j. f. till there is no difference perceived in alti- 

 tudes of the fame ebjea taken in rrrerfcd pofitions cf the axis 

 cf the ttkfcojre'i motsoK. But it ij uiual wnh a:trsEcaer?, 



who take aldtodes in reverfed pofitiocg of a circle, to ai'cer- 

 tain the error ariiing from want of trje coDinxation in alti- 

 tude, from a repetition of obferrations, and to apply it to 

 each obfervation as a correftion, rather than attempt ar 

 exa£t adjufiment in this refpe£l, when the error is incon- 

 fiderable. 



8. Tj tomJc: the Lhu of CoUimation move in tbe Plane of tie 

 MfriJiau — In order to fix a tranfit-inftnm^eat well in the 

 meridian of any given place, it is neceuary that the pole- 

 ftar be expofed to its view ; for as the polar point is fituated 

 in the centre of its ciumal circle, this circle muil be bifetted 

 by the middle wire, or fpider's line, that pafles through the 

 zenith, before it will move in the plane of the true meridian. 

 Formerly it was ufual to place the inftmment, by double 

 altitudes of the fun, or by a knowledge of the folar tim? 

 gamed by obfervation of his altitude on a given day, nea- 

 in the meridian of the place, and then to obferve which -; 

 the two femicircles, to the right and left of the approxi- 

 mate meridian hne, were paiTed through in the (horter time, 

 by means of taking both the upper and lower tranCts of 

 the pole-ftar fucceflively ; after which an aliovrance was 

 made for the de«"iation from the meridian, and the obler- 

 vations were repeated till both femicircles were performed 

 in the fame time. But bv this method of finding the centr- 

 of the circle furrounding tiie pole, the accuracy of the 

 refult depended on the rat? of the clock that was ufed, 

 during the whole time that the fuccefllve obferrations wer= 

 employed ; confequectly fome uncertainty remained as ; j 

 the comparative times intervening betvreen the fuccefll. ^ 

 tranfits, and the true polar point was, therefore, not eiiiiy 

 afcertained : but this difficulty has recently been obviated, 

 by fixing on two flars nearly at oppofite fides of the circ'e, 

 fo that their difference of right afc«ifion may be very nearly 

 1 2^ ; for, when the two femicircles are alike, the time thai 

 elapfes between the fuperior tranfit of one, and the inferior 

 one of the other, will not be altered by a change of fituation; 

 the diierence of the times will remain the fame, whichever 

 of the two ftars makes the upper tranfit ; but if the feirj- 

 dniles are unequal, the ir.tervals of tine will differ accorc- 

 ingly ; fo that if one dar precede the other two minutes at 

 its upper trardit, and only one at its lower, half of this 

 quantity will be the time correfpocding to the deviation in 

 pofidoD from the true meridian, which mav be reSiaed by 

 repeated obfervatioES of the fhort intervals, according to 

 which the two flars foEow one another, in tranfiting nrft 

 above and then below the pole refpe\3ively ; and all tiaat 

 the clock has to do, according :o this method, is to count 

 the fecmtds in each cf thofe ihort intervals. When an in- 

 ftruroent that is adjufted to have its hne of colKmation pa& 

 through the zenith by means of the horizontal poudoa of 

 its axis of modon, is thus made to bifeft a polar circle, it 

 will alfo pafs through the pole, and will coofequendy — .ove 

 in the plane of tbe true meridixi of the place. Various 

 other nithods of placing a common traaat-inltrument truly 

 in the meridian have been recommended, but this is cot 

 only the eafieil, bat the beft ; and if the right aicenfions of 

 Dr. Maikelyne's cr Mr. Pond's ftars come ri^ht, when ob- 

 ferved after the final verification, it may lately be coc- 

 cluded thai the pofidon of the inftrumect is ia the &ae 

 meridian. 



When a tranfit-intlrument is well conftrufted, and pro- 

 perly adjufte<J in all refpefts, it is one of the moft ufeful 

 inilrunsents ia a.lronomy, for without it the right afcenScBS 

 of the heavenly bodies would sot be accurately takes by 

 direct obfervation, seither could the time, either fola* cr 

 fidereal, be afctrtimed with that degree of preciJica which 

 9 may 



