TRANSIT-INSTRUMENT. 



Venus i? obferred. The fpirit4eTel, which hangs on two 

 vertical ->ds orer the axis, is capable of being reverfed in 

 pofition, and is ufed, in the firft place, for placing the axis 

 of the telefcope perfedly horizontal, and in the next, for 

 watchina- the permanence of fuch pofition by the fituatioa 

 of its bubble. As the ends of the brafs tube that contains 

 the level revolve on two oppofite pins, projecting firom the 

 rods of fufpenfion refpectively, the bubble will always oc- 

 cupy the upper part of the glafs tube in every ftate of the 

 telefcope's elevation ; but in the reverfed pofition of the 

 axis c d, the level will be underneath it, though the bubble 

 will retain its place. The infide of the tube is ground to 

 fuch a long radius, xhixjinglt fez<ituL of deviation from a true 

 bonzontal pofition may be read on an ivory fcale by the 

 end of the bubble. When, however, the telefcope is large, 

 and the axis confequently bulky, a fpLrit -level cannot be fo 

 well applied, as with telelcopes of ordinary fize ; and when 

 this is the cafe, a bafoa of pure mercury may be fubftituted 

 with oreat advantao^e : for when the pole-flar is on the me- 

 ridian, and ftiines brightly, its refleiied image, and alfo the 

 ftar itfelf, will both pafs together over the central ver- 

 tical fpider's line, at correfponding degrees of elevation and 

 depreiaon of the telefcope, provided the axis be truly hori- 

 zontal, and as little time as poffible be funered to elapfe 

 between the faperior and inferior obfervations : but to 

 render this operation eafy, flops may be clamped to the 

 graduated circle, to limit the fpace that is neceflary to be 

 defcribed by the object -end of the telefcope. There is in 

 the inftrument before us, moreover, a plumbJice apparatus 

 for levelling the axis, which is feen in_/fr. 2, and partially 

 on a larger fcale injf^. 3. At a is a pin, from which a fine 

 filver wire may be impended ; and at ? is a horizontal fcrew, 

 between two threads of which the wire reils, fo as to be 

 capable of adjuftment in a lateral direi^on : the long narrow 

 tube, which lies parallel with the main tube of the telefcope, 

 then receives the wire, and is put into a perpendicular di- 

 Ttddon, fo that the fofpended plummet may be immerfed 

 in a veflel of water ; the microfcope at_/", next the eye-piece, 

 now uppennofl, views the image of a tranfparent dot, as a 

 point to which the wire is referred, and to cover which it is now 

 brought by the fcrew ,• : at the oppofite end of the main tube, 

 near 3, is a fimilar microfcope with a point of fufpenfion and 

 adjufhng fcrew ; fo that when the objeS-end of the telefcope 

 is placed uppermoft, the fame adjuftment is required to 

 bring the ftretched wire over its luminous image ; and when 

 this is done, the wire will bife<3 the lower luminous image 

 alio, if the axis be perfeSly level, but not otherwife. To 

 produce this coincidence, it will ufually be neceflary to ad- 

 juft partly by the vertical fcrew of the proper Y, and partly 

 by the fcrew s of the plumb-line apparatus after each invcr- 

 fion of the tube. 



When the achromatic objeS-glafs does not produce a 

 round and well-defined point, as- the image of a lar^e flar, 

 in any obferration, it will be advantageous to dxminilh the 

 aperture till this appearance takes place, which will gene- 

 rally be the cafe when the central part of the objeS-glafs 

 only traidmits the light of fuch a ftar ; but for tnaU ftars, 

 it will always be better to aUow the whole ajKrture to be 

 open ; not only becaufe more hght is thus admitted, but 

 aHb becaufe the image of a fmall ftar is always a point free 

 from luminous irradiations, even in inferior achromatic tele- 

 fcopes. In the ten-feet telefcope which Mr. Troughton has 

 lately converted into a magnificent tranfil-infmunent at 

 Greenwich (ria. in i8l6), he has ingenioxilly contrived a 

 fpecies of irjx, that will contraS or enlarge the effedive part 

 of the aperture by the fimple turning of a nuDed nut, within 

 reach of the hand, while the eye is at its place for making 



the obfervation ; fo that the proper quantity of li^t can 

 at any time be proportioned to whet the nature of the ob- 

 fervation may require. There are lailly fcrews of adjult- 

 ment of the fpider's lines in the eye-piece both for horizontal 

 and vertical motion, and, in the beil inftruments, the eye-piece 

 itfelf has a Qiding horizontal motion, which enables the ob- 

 ferver to keep the celeftial body, though \n motion, at the centre 

 of the field of view during the whole paffage over the five hnes. 

 The beft conflruftion of a pcrtahle tranfit-inftrument which 

 we have yet feen, is that reprefented by_^V/. 6 and 7, which 

 exhibit all the parts that are neceflary for defcription, and 

 which was one of the numerous inventions which we have 

 had occafion to notice as the offspring of Troughton's in- 

 genuity. The telefcope of this inftrument is 20 inches 

 long, and magnifies from 20 to 35 times, according f 

 the eye-pieces that are ufed ; two of which are ufuaUy 

 the prilmadc or diagonal kind, to be ufed in high altitudes ; 

 the aperture is 1.75 inch, and the power is competent to 

 fee the pole-ftar by dear day-light. The conftrudion of 

 the tube and axis is fimilar to that of the ftarionary inftru- 

 ment, except that the graduated circle and double vernier, 

 with the level, he beyond the Y of the fupport, and the iris 

 is wanting : otherwife it has all the adjuftments for leveUing 

 the axis, and bringing the line of collimarion to a meridian 

 mark that the larger inflmment poffeffes, as well as thofe 

 that reftify the pofition of the fpider's lines in the focus of 

 the negative eye-piece. The bafe of the ftand is a thick 

 ring or rim of brafs, that receives three equi-diftant fcrews 

 for feet, befides the four fcrews that fix the two vertical 

 frames thereto, which conttitute the fupports of the axis ; 

 one of which is fees entire in f.g. 7. Thefe fupports are 

 kept perpendicular by the interior bracing-bars, of which 

 two are difcoverable ^J>g- 6, attached by thumb-fcrews at 

 both ends to the ring and upright frames refpectively. The 

 circular figure of the bafe b not only firm, but preierves its 

 (hape in all degrees of temperature ; and when tie parts are 

 detached by loofening the thumb-fcrews, they will all pack 

 into a bos that is of a convenient fize for carriage. The 

 diameter of the circular bafe, and the confequent length of 

 the axis, is a foot within, and the haght of the fnpp<wts ' 

 thirteen inches. The graduated circle being of fix inches dia- 

 meter, admits of readings bv each of the two oppofite verniers 

 to the accuracy of one minute, which is fufficient for finding 

 the meridian altitude of any celeftial body, of which the de- 

 clination is known when the latitude is given ; or for deter- 

 mining the latitude, when unknown, to the accuracy of one 

 minute. If the circle were made a little larger, and three 

 verniers fubftituted for two, a longer level might be ufed, 

 and the readings might be accurate I0 20" or 25" ; but as the 

 inftrument was never meant to be ufed as an altitude-inftru- 

 ment, the inventor confidered only what dimenfions are re- 

 quifite for conftituting an ufefol tranfit-inftrjment in a port- 

 «i)le form. The level of this inftrjment is entirely detached, 

 and equal in length to the axis itfelf ; its Ciape is delineated 

 in the upper part of Jig. 9, and the notches of its end-pieces 

 ftand upon the pivots of the axis, fo that the reverfion of 

 the ends is performed without the leaft impediment. It is 

 however neceffary to remove the level from the axis, when 

 the horizontal adjuftment is finifhed, to avoid its being dif- 

 placed and broken by an alteration in the elevation of the te- 

 lefcope. There are ufually three ftuds of brafs included, 

 with the darkening-glaffes, lantern, and other appendages ; 

 two of which ftuds have conical holes, to receive the points 

 of the fcrews, or feet of the circular bafe : and for this pnr- 

 pofe, all the ftuds moft be made faft to the flab or pillar 

 which fupports the inftmment, by plafter of Paris or putty 

 infertcd into as many holes in the plane of the marble or 



ftoae, 



