TRANSIT-INSTRUMENT. 



.very nearly in contaft with the ftifpended line ; and the con- 

 taft or bifeftion may then more conveniently be cfFefted by 

 turning the vane a little round the joint E : in this fituation, 

 if the Itar be at the moment on the true meridian by calcu- 

 lation, the direftion of fight will be in the meridian line, 

 and bodies pafTing to the north, may be feen pafiing the 

 plumb-line B F at any altitude below the pole. Sufpend 

 row the plumb-line A D from D, and turn the horizontal 

 part on its joint, till this line bifefts the vane at A, and 

 move the vane at B gently round its joint E, till it is alfo 

 interfered by the other plumb-line D A, and the two lines 

 will be both fufpended in the meridian line ; confequently, 

 an eye looking through the vane at B, will fee any body to 

 the fouth traniit the Hne D A, provided it be below the 

 ecliptic. 



The condition to be attended to in the conftruftion 

 of a modern tranfit-inflrument is, that while the telefcope, 

 through which the objeft is viewed, undergoes a change of 

 elevation, its line of collimation fliall move exaftly in the 

 plane of the meridian circle of the place of obfervation, 

 after it has been once adjuited accurately into a line that 

 lies truly parallel with the north and fouth line. To effeft 

 this purpofe, it was found neceffary that the axis of motion 

 fliould lie exaftly horizontal, in the direftion of a hne that 

 joins the eail and well points ; that it fhould not bend by 

 tlie fuperincumbent weight in any polition whatever ; and 

 that the line of collimation (hould always cut the line of 

 tlie axis at right angles, without the leail perceptible devia- 

 tion ; for it is only while thefe conditions are fulfilled, that 

 the line of collimation will afcend from the horizon in a 

 truly vertical line, during an increafe of altitude ; and that 

 this vertical line will continue alfo in the meridian. The 

 firft perfon, we believe, who ufed an aftronomical inftrument 

 pofTefling the elTential properties which we have defcribed, 

 was Dr. Halley at Greenwich Obfervatory, foon after the 

 year 17 19, which was the year in which he fucceeded Flam- 

 iteed as aftronomer-royal. According to Dr. Smith, the 

 tube of Halley's telefcope was five feet and a half long, 

 and its axis of motion about an ell : it had alfo crofs-hairs 

 with the requifite adjnftments in the eye-piece, witli Y fup- 

 ports for the pivots of the axis, likewife adjuflable, and 

 the axis was levelled by a fpirit-level, fo that but little room 

 remained for fubfequent improvement, except in what re- 

 lates to the achromatic objeft-glafs, and the method of illu- 

 minating the hairs, and of limiting the quantity of light 

 neceffary for particular obfervations. In the older inftru- 

 ments, the reflefted light of a lamp or candle was received 

 at the aperture of the telefcope, as fhevvn in Jig. ij. 

 Plate XXIX. of AJlronomical Injlruments ; but this mode 

 of illuminating the hairs was found inconvenient, on account 

 of the change of elev.ition of the telefcope in fucceffive ob- 

 fervations, which required as many new adjullments of the 

 lamp; and it was not till about the year 1787, that Dr. 

 Ulher of Dublin contrived the new method of tranfmitting 

 light through one end of the horizontal axis to a diagonal 

 perforated refleftor, in the body of the tube, which brings 

 It to the hairs after reflection, whatever tlie altitude 

 may be to which the telefcope is pointed. In our defcrip- 

 tion of the different tranfit-inftruments, it will not be necef- 

 fary to repeat what we have already faid of the theory and 

 conllruftion of either objeft-glaffes or eye-pieces, under pur 

 article Telescope, to which therefore our readers are re- 

 ferred on thefe effential points. 



Tranfit-inftruments, as they are now conitrufted, may be 

 confidered either as they are Jlationary or portable; the 

 former of which was the original conllruftion, and con- 

 tinues to be ufed in fixed obfervatories, for the purpofe of 



determining, in conjunftion with a good aftronomical clock, 

 the right afcenfions of the heavenly bodies ; but the latter 

 may be ufed in any place, for afcertaining the rate of a 

 clock or chronometer, and when nicely brought into the 

 meridian, for determining alfo the right afcenfions with con- 

 fiderable accuracy. 



Fig. I. Plate XXXII. o{ jljlronomical Injlruments, re^re- 

 fents a tranfit-inftrument of the moft approved ftationary 

 conftruftion, which may be made of any dimenfions that 

 the aperture and focal length of the objeft-glafs will allow ; 

 but is ufually made with a tube of from 30 to 120 inches 

 long, and of proportional thicknefs, according to the degree 

 of accuracy that is required. A and B are either a pair of 

 folid ftones firmly fixed in the ground, or a pair of pillars of 

 firm mafonry, fufficiently high to allow the eye of the ob- 

 ferver to reach the eye-piece when the angle of elevation is 

 about 45°. Fig. 2. exhibits a lateral view of the fame, ex- 

 cept that only one of the pillars is feen, in the direftion of its 

 breadth ; wh'ihjg. 3. is an enlarged reprefentation of the 

 eye-end of the telefcope ; and Jgs. 4. and 5. are appendages 

 attached to the fuperior end of the pillars ; all which we 

 fliall defcribe in due fucceffion. The tube of the telefcope 

 a b '\s ufually of brafs, of which the eye-end is at a, and the 

 objeft-end at i : in the inftrument before us it is five feet 

 long ; e and d, which appear to be two cones, are united to 

 the central part of the tube, and form the axis of motion 

 about three feet long ; they are of brafs alfo, and hollow, 

 but are rendered very ftrong by circular pieces of metal, of 

 different diameters, that are forced into the hollow fpace one 

 after another at equal intervals ; fo that this axis has all the 

 ftrength without the weight of folid metal. The pivots at 

 the extreme ends of the axis are of bell-metal, and turned in 

 the lathe to precifely the fame dimenfions, fo that they may 

 at any time have their pofitions reverfed without affefting 

 the horizontal line. The pieces, ftiewn hy Jgs. 4 and 5, are 

 made faft to the upper extremities of the pillars, and receive 

 the pivots of the axis, each having an angular notch, called a 

 Y, which are adjuilable by proper fcrews, one in a vertical, 

 and the other in a horizontal direftion. The fituation of 

 each of thefe fcrews is feen by tlie handle inferted on 

 the axis of the icrew to which it belongs ; and the man- 

 ner in which the moving parts are made to fiide by the 

 aftion of the fcrews, between the parallel cheeks of the 

 fmall frames, requires no particular defcription. When the 

 weight of the telefcope and of its axis is confiderable, fome 

 part of this weight is fupported by a loaded lever, as feen in 

 Jg. I, which,by aftingon the ponderous matter at fomediftance 

 from the pivots, relieves them from a portion of the ftrefs 

 that would otherwife be laid on them by the whole weight. 

 Hence the pivots have their friftion diminifhed ; and confe- 

 quently their dimenfions, as well as thofe of the Ys, remain 

 unaltered. When the weight is fmall, fuch appendage may 

 be difpenfed with, provided the Ys and pivots be reftified 

 occafionally, when they are a little altered by attrition. 

 At a fmall diftance from one end of the axis a graduated 

 circle is made faft, which moving with the axis, fliews the 

 degree and minute of elevation, that the telefcope has in any 

 polition, on two oppofite adjuilable verniers, which carry a 

 fpirit-level, and are clamped in a horizontal pofition, as feen 

 in Jg. 2. The extreme end of the axis, which is remote 

 from the graduated circle, has a lens fcrewed into it, through 

 which the light of a lamp is made to pafs in its way to the 

 di.agonal refleftor, contained between the two conical por- 

 tions of the axis, within the tube ; and by the light thus 

 tranfmitted and reflefted, the fpider's lines, fubftituted for 

 hairs or wires, are illuminated ; without which illumination 

 they would be invifible by night, except when the Moon or 



Venus 



