ZENITH. 



feveral of Ramfden's contrivances. It was moreover necef- 

 fary to introduce various weights, pulleys, cords, fprings, 

 &c. to facilitate and keep fteady the motions of the different 

 afting parts, which, together with the reading microfcopes, 

 lamps, rods, and adjufting fcrews, give the inftrument the 

 appearance of great complexity. 



The manner of adjufting the inftrument for obfervation is 

 thus performed, according to Mr. Mudge's own words : 

 viz. " The feet of the external ftand (hould be firft cai-efuUy 

 brought into a horizontal plane ; and when they are fo, the 

 azimuth circle will be neceffarily parallel to it, having its 

 centre under the middle of the opening in the mahogany 

 frame fcrewed on the top of the ftand. This being done, 

 and the inftrument fet up, the plane of the arc (hould be 

 brought parallel to one of the fides of the ftand, in which 

 fituation the internal frame is to be clamped to the azimuth 

 circle, and the wire brought to its proper diftance from the 

 limb, by means of the adjufting fcrew attached to one of 

 the Aiders, which carries the concave receptacle and conical 

 point. The dot at zero (hould then be brought exaftly 

 under the plumb-line, as feen through the magnifier, and 

 the point on the micrometer-head, at which its index ftands, 

 noted. The inftrument is then to be turned half round ; 

 and if the fame dot on the arc ftill continues bifefted, it 

 will afford a proof of the internal ftand being upright in 

 one direftion. But if the dot (hould not continue bifefted 

 by the plumb-line, it muft be made to do fo, and the revo- 

 lutions, or parts of a revolution, counted ; half of which is 

 to be turned back on the micrometer-head. The fame dot, 

 zero, is then to be brought under the plumb-line (wire), by 



means of the other adjufting fcrew, beneath tlie aximuth 

 circle. If the ftand is pretty accurately fet up, on« opera- 

 tion is fufficient for bringing the interior frame upright 

 in one direftion, wz. either in that of the meridian, 

 or the one at right angles to it. The arc is then 

 to be turned round 90°, and the fame operation gone 

 through. This being properly done, the interior frame is 

 made perfeftly upright. The next ftep to be taken is that 

 of placing the long level on its axis above, and reftifying 

 that axis by means of the Y plate fcrews. If this be done 

 carefully, the bubble will remain between the pointers of 

 the level, whatever pofition the feftor may be placed in. 

 Having thus reftified the inftrument, by making the internal 

 frame upright, and the axis horizontal, the only remaining 

 point to engage attention is, placing the plumb-line at a 

 proper diftance from the arc : this is done by means of the 

 fcrew afting on the fpring juft under its point of fufpenfion. 

 If great care be ufed in going through thefe feveral adjuft- 

 ments, the inftrument may, at any future time, be accurately 

 adjufted for obfervation by turning the proper fcrew belong- 

 ing to the azimuth circle, and bringing the arc to its ufual 

 diftance from the wire." 



In order to (hew the accurate refults that may be obtained 

 from an inftrument of this defcription, and alfo the care 

 that is neceffary in ufing it, and in clearing the obfervations 

 of errors arifing from natural caufes, we will fubjoin a few 

 Tables that were found ufeful in the grand trigonometrical 

 operations, by means of which, in the years 1802 and 1806, 

 the meridian arcs were compared with the correfponding 

 terreftrial meafurements. 



Table (hewing the Runs of the Micrometer-Screw over every Five Minutes in the Fir-ft Degree on each Side of Zero. 



