TRANSIT-INSTRUMENT. 



may bs done by its aid. Indeed a perfeft tranfit-inftrument 

 and a good fidereal clock are the indifpenfable furniture of 

 every fixed obfervatory, and as companions, that mutually 

 aflift each other, they ought never to be feparated. 



We might now proceed to illuftrate the ufe of the tranfit- 

 inftrument by real examples in praftice, if we had not antici- 

 pated this part of our article under the heads Ascension and 

 Chronometer ; in one or other of which articles we have 

 explained both the method of determining the right afcenfion 

 of any body, and alfo of afcertaining, without much calcu- 

 lation, the rate of a chronometer or ajlronomical dock. See 

 die refpeftive /!roWf;nx under Chronometer. 



The tranfit-inftruments which we have above defcribed 

 are neceflarily expenfive, even in the portable form ; and 

 encouragement has been held out, by the Adelphi Society, 

 for further improvements in the portable kind, fo that 

 they may become more common. And the frequent 

 enquiries that have been made for a cheap portable in- 

 ftrument, induced fir H. C. Englefield, bart. to contrive 

 a very fimple one, which was firil made by Mr. Thomas 

 Jones, mathematical inftrument-maker in London, and is 

 now fold at his houfe at Charing-Crofs. A defcription of 

 this inftrument, called a Jide trnnjtt-injlrument, written by 

 the contriver, is fold along with it, from which it appears, 

 that it is not to be confidered an original inftrument, but 

 an improvement on an old tranfit-inftrument formerly be- 

 longing to Mr. Aubert, and purchafed at his fale by the 

 late Mr. W. Walker. We propofe to copy the defcription 

 nearly as it has been publiflicd. " It if well known," fays 

 the ingenious author, " that the tranfit-inftrument, in its 

 ufual form, is liable to great injuries from blows, or other 

 violence affefting the perpendicularity of the telefcope to 

 the axis ; that it necefTarily takes up much room in package ; 

 that, unlefs of a very fmall fize, it i-. not eafily fixed in a 

 window or other opening in a common dwelling-houfe ; and 

 that it is quite impoffible, except in fixed obfervations, tomake 

 it fweep the entire arc between the fouihern and northern points 

 of the horizon. In windy weather its ufe is alfo very dif- 

 ficult and inconvenient. In the inftrument now offered, all 

 thefe inconveniences are avoided. It may be fixed almoft 

 any where ; in many places it may be made to defcribe the 

 entire femicircle of the meridian ; the obferver is put to no 

 dif&culties by change of place, as he always looks direftly 

 along the axis ; it is packed in one-eighth of the fpace re- 

 quifite to pack a common tranfit-inftrument of the fame 

 real fize ; its weight is not more than a fixth of the other ; 

 from its fimplicity it will be afforded at half the price ; and 

 its verifications and adjuftments are cafy and fimple. It has 

 alfo another advantage, that the mark by which it is placed 

 in the meridian may be either in the meridian or at right 

 angles to it ; or, if convenient, two marks may be erefted, 

 one to the fouth or north, the other caft or weil ; and if fo 

 afed, it will be always feen, by infpeftion only, whether the 

 mirror needs adjuftment or not. In many confined fitua- 

 tions, fuch as occur in cities, the power of having a mark 

 at right angles to the meridian may be eminently ufeful. 

 The general defcription of the inftrument is as follows : The 

 telefcope is included in a brafs cylinder, having a fmall cy- 

 bnder at each end, turned true in the ufual manner, and 

 rcfting in Ys of the ufual conftruftion. Thefe fmaller 

 cylinders are both pierced. In one is the eye-piece of the 

 telefcope, with its wires, &c. The other is open, for the 

 purpofe of feeing through it, if neceflary, the eaftern or 

 weftern mark ; and for adjufting by direft vifion the line of 

 coUimation of the inftrument. It alfo ferves for the illu- 

 mination of the wires. The objeft-glafs of the telefcope is 

 placed fo near this cylinder, as only to allow room for an 



unfilvered plane glafs mirror to be placed before it at an 

 angle of 45°. It is obvious, that as the telefcope revolves 

 on its axis in the Ys, every celeftial objcft at right angles 

 to it may fucceffively be feen by refleftion from the mirror ; 

 and of courfe, if the axis be placed due eaft and weft, the 

 tranfits of all celeftial bodies over the meridian will be ob- 

 ferved with the utmoft accuracy and convenience. The 

 aperture in the axis beyond the objeft -glafs is not only of 

 ufe for the adjuftment of the inftrument to an eaftern or 

 wefteiTi mark, or for the illumination of the wires, but af- 

 fords a means of feeing the mark at the fame time with the 

 body whofe meridian paffage is to be obferved, and of 

 thereby being certain of the true adjuftments of the inftru- 

 ment at the very moment of obfervation ; which is impoffible 

 in any other conftruftion of the tranfit-inftrument, and feenis 

 to be a very material advantage. I am convinced that 

 tranfit-inftruments of the largeft fize might, with very great 

 advantage, be conftrufted on this principle. It is true, that 

 where very much light is wanted, as in obfervations of ftars 

 in the day-time, the lofs by refleftion will be fome difad- 

 vantage ; but the lofs of light from an unfilvered mirror is 

 very fmall : and the convenient and fimple form of the in- 

 ftrument ; by its lightnefs lefs fubjeft to flexure ; by it« 

 pofition much lefs liable to errors from unequal change of 

 temperature ; and fo extremely commodious in its ufe ; pre- 

 fent advantages of a very important nature, and fuch as 

 might introduce it into the moft extenfive obfervatories with 

 profit. The advantages may perhaps be even greater than 

 in fmall tranfit-inftruments. When in the prefent conftruc- 

 tion the telefcope is long, as its whole weight refts on the 

 moft difadvantageous paint of the axis, this is of neceffity 

 made very large towards its centre, to avoid flexure ; and the 

 whole inftrument is fo heavy, that an additional apparatus of 

 counterpoife muft be added to the pivots, left they fhould 

 wear away the Y s, and the reverfing the inftrument becomes 

 a work of fome difficulty and danger : whereas in the con- 

 ftruftion propofed, the finall comparative weight of the 

 objeft-glaifes and eye-glaffes lies very near the pivots, and the 

 middle of the tube is the lighteft part of it. The operation 

 of reverfing is performed with great eafe, both from the 

 form and lightnefs of the inftrument ; and it may be added, 

 that the comparative facility of obfervations is of greattr 

 advantage the larger the inftrument. Having fo far ex- 

 plained the general principles of this tranfit-telefcope, which 

 was executed very much to my fatisfaftion by Mr. Thomas 

 Jones, it will be proper to defcribe more particularly its 

 conftruftion. The tranfit-ftand is reprcfented by either the 

 upper or lower part oi Jig. 10. 



" The firft is conftrufted for being placed or fixed on a 

 vertical furface ; the fecond for a horizontal one, A B ; in 

 both are the Ys, or fupports for receiving the axis of the 

 telefcope. The end A, intended to be always next the eye, 

 is furnilhed with both the horizontal and vertical adjuft- 

 ments, fuch as are ufual to tranfit-inilruments. The tele- 

 fcope's axis is reprefented by the lower pai-t of Jig. 9. The 

 eye-end being at C, the objeft-end at D turns round in the 

 Yfi ; A B upon its cylinders n, n : the fcrew-head r, at the 

 objeft-end D, is for the purpofe of adjufting the paralllel 

 glafs. The telefcope is adjufted to diftinft vifion by means 

 of the head or knob at P. The wires of the telefcope are 

 adjufted by means of the four capftan head-fcrews at the 

 eye-end 1 1. The circle R is divided on the furface next 

 to the eye. The eye-tube has a Aiding motion for viewing 

 the wires diftinftly : the ftiort piece of tube at the objeft- 

 end turns round on the telefcope's tube, and ferves as a 

 cover for the objeft-glafs. The upper part oi Jig. 9. is the 

 riding level, and is placed upon the axis in the Y s, and ad- 

 S z juited 



