n 



TRANSIT-INSTRUMENT, 



ftone, care being taken that the line which joins the two 

 conical points be in the direftion of the meridian, or fo 

 nearly fo, that the adjufting fcrew of one of the Y s will 

 bring it into that fituation. 



Mr. Thomas Jones, of Charing-Crofs, has made feveral 

 30 and 42-inch tranfit-inftruments of the portable fort, fup- 

 portedby oblong frames of caft-iron, which look very neat, 

 and anfwer the purpofe very well, a drawing of which con- 

 ftru<ftion we fhould have introduced into our plate, if it 

 would have admitted of fuch addition. Thefe inftruments 

 have all the properties of the inftrument we laft defcribed, 

 and have of courfe greater powers in their telefcopes, and are 

 alfo cheaper in proportion to their fize. He has alfo made 

 fome of them with telefcopes of only twenty inches, for the 

 fake of greater portabihty. 



Before any ufeful obfervation can be made with a tranfit- 

 inftrument of either the ftationary or portable conftruftion, 

 it is neceflary tliat all the adjuftments be nicely made, and 

 alfo that they be examined occafionally after a few obferva- 

 tions are finifhed, in order to prevent errors that cannot be 

 detefted, or at leaft appreciated, at a fubfequent period. 

 Thefe adjuftments are nearly the fame for all the common 

 tranfit-inllruments, and may be explained under eight heads, 

 as follow ; viz. 



1. To adjujlthe Spirit-Level. — When the level hangs on, or 

 IS made fall to the axis, put the telefcope in its place, and fee 

 to which end of the level the bubble runs, which will always 

 be the more elevated end ; bring it back to the middle by 

 the Y fcrew for vertical motion, or by the foot -fcrew under 

 the end of the axis, if it be a portable inftrument, and then 

 invert the axis, end for end ; then, if the bubble is again 

 found in the middle, the level is already parallel to the axis ; 

 but if not, adjuft one half of the eiTor by the adjuftiiig-fcrew 

 of the level, and the other half by the Y fcrew, or foot of 

 the fupport, as the cafe may be ; and let the operation of re- 

 verfing and adjufting by halves be repeated, until the bubble 

 will remain ftationary in eitherpofition of the axis, in which 

 cafe the level will be right. When the detached level is 

 ufed, that notch muft be made a little deeper, by fcraping 

 with a penknife, which has the bubble reftinr over it, in- 

 ftead of ufing an adjufting-fcrew, with which it is not 

 ufually provided ; and when the notches that reft on the 

 pivots arc once made right, they will fcldom require a fecQnd 

 reftification. In the hanging level there are fide-fcrews alfo, 

 which adjuft for parallelifm of the level, as it regards a line 

 joining the pivots of the telefcope's axis ; and this adjaft- 

 ment is known to be truly made, when the bubble does not 

 run to one end, when the level is moved fome degrees by a 

 rotatory motion round its pivots, or central pins of fuf- 

 penfion. 



2. To place t'l.e Axis of the Telefcope truly horizontal. — If 

 the fpirit-kveJ is made ufe of, which is generally the cafe in 

 inftruments of ordinary dimenfions, the fame operation 

 which we have juft defcribed will put the axis level, at the 

 fame time that it puts the level parallel to the axis ; for un- 

 lefs both thefe conditions are fulfilled, the adjuftment of the 

 level will be deranged by reverfion, and when this is not the 

 cafe, it is a proof that botli the level and the axis are truly 

 horizontal. Hence, when the level is previoudy adjufted, it 

 will be fufficient to bring the bubble to the middle of the 

 level by the Y fcrew, or foot -fcrew alone, as the conftruftiou 

 may require. 



This adjuftment may alfo be made, in the larger inftru- 

 ments particularly, by means of a plumb-line, cither applied 

 to a frame, ful'pended by the pivots of the axis, that will 

 'reverfe in pofition according to Ramfden's method ; orhang- 

 ■■. -Vol XXXVI. 



ing on the tube of the telefcope parallel to the line of col- 

 hmation, as we have defcribed Troughton's in fgs. 2 and 3. : 

 in either cafe, a dot is bifefted by the plumb-hne near the 

 point of fufpeniion, and another near the lower end of the 

 line, in both the reverfed pofitions of the axis, when the ad- 

 juftment is truly made by the proper fcrews, as above direfted. 

 But the moft accurate, as well as probably the moft conve- 

 nient method of levelling the axis of a large inftrument, is by 

 refieSioti, by the aid of a bafon of pure quickfilver, as we have 

 already intimated ; and the pole-ftar, being flow in its appa- 

 rent motion, particularly at the time of its greateft elongation, 

 is the moft proper obje<9; by which to make the adjuftment 

 in queftion ; for when the axis is level, and the ftar is made 

 to cover any one of the fpider's hnes, its image reflefted from 

 the furface of the mercury will cover the fame line, pro- 

 vided the deprefiion of the telefcope is effefted inftantane- 

 oufly ; and if this is not the cafe, the proper fcrews muft be 

 ufed for effefting this purpofe without any reverfion of the 

 axis ; for by this experiment, the line of fight of the tele- 

 fcope is proved to have a motion truly vertical, which is 

 what the horizontal pofition of the axis is intended to produce. 

 Hence, if the exaft coincidence of the places of the ftar and 

 of its reflefted image takes place in an inftrument both be- 

 fore and after the reverfion of its axis, this is a proof that 

 both the pivots and Ys are refpeitively fimilar. This me- 

 thod of adjufting the axis of the large tranfit-inftrument at 

 Greenwich has been lately adopted, after a trial had been 

 made by the aftronomer-royal, of the application of a plumb- 

 line, fufpended in a detached ftate frofn a point of confiderable 

 elevation. 



3. To produce dijltnd Vifion. — Before the fubfequent ad- 

 juftment! can be made, it is neceflary that the telefcope 

 ftiould be put into a ftate of perfeft vifion ; Wz. that the 

 objeft fixed upon ihouLd be feen well defined. This may be 

 accomphftied by Aiding either the objeft-glafs or the e^-e- 

 piece within the tube, till the defired effeft is produced ; 

 but when wires, hairs, or lines of any kind are made faft in 

 the common focus of thefe glaftes, a motion given to the eye- 

 piece will make thofe lines difappcar, and a motion given to 

 the objeft-glafs is in danger of deranging its beft pofition : 

 therefore, when a meridian mark is necefiarily at fo ftiort a 

 diftancc, that the fame adjuftment «f the eye-piece will not 

 fuit it that has been made for a celeftial objedt, by which the 

 parallel lines have been fixed in the focus, an auxiliary objeft- 

 glafs, or glafs of long focus to be added to the proper objeft- 

 glafs, is provided to remedy thefe inconveniences, whenever 

 the meridian mark is the objeft viewed. The method of de- 

 termining the focus of fuch additional glafs, as fliall (horten 

 the compound focus of tlie objeft-glafs agreeably to what is 

 requifite for a given diftance, is explained under our article 

 Telescope. 



4. To put the Wires, Hairs, or Spider's Lines perpendicular to 

 the Axis. — The parallel hnes in the eye-piece of the telefcope 

 are ufually attached to a circular piece of brafs that turns 

 tight within the interior end of the tube, and if, when the 

 telefcope changes its elevation, a point in the meridian, or 

 other mark, travels along one of thefe lines, fo as to be bi- 

 fefted by it through the whole field of view, the faid line will 

 be perpendicular to the axis of motion ; or if a long plumb- 

 line be fufpended at a diftance, and a %'ertical line in the eye- 

 piece is found to coincide with it while the telefcope alters 

 its angle of elevation, this will not only be a proof of the 

 fame thing, but will moreover, fliew that the axis is level ; 

 and when this is not the cafe, the piece holding the lines 

 muft be turned till they are found by trial to be truly verti« 

 cal. The fimilarity of the intervals between the lines, and 



9' alfo 



