ZENITH. 



ftituted an achromatic objeft-glafs, which lias modernized 

 the inftruraent, and put it nearly on a level with the new in- 

 Itruments which have lately been conftrufted and erefted in 

 the Royal Obfervatory by Troughton, on the beft princi- 

 ples. If the zenith-fedor had been made to reverfe in po- 

 iition in the fame fituation, i. e. without being carried acrofs 

 the room, its ufe would have been more convenient, and the 

 obfervations more certain, inafmuch as the fame dillance 

 would have been preferved between the axis of motion of the 

 telefcope, and the point aded upon by the micrometer-fcrew, 

 without any refulting allowance. Befides, the initrument 

 would have been lefs liable to accidents ; and reverfed ob- 

 fervations might probably have been made on the fame 

 evening. When Bird afterwards made a zenith-feftor for 

 the Oxford Obfervatory, he noticed thefe inconveniences, 

 and obviated them by making the tube of the telefcope 

 turn round in its own place, fo that the pofitions of the 

 axis can be reverfed by an azimuthal motion that carries the 

 plumb-line round at the fame time ; and when the plumb- 

 line will cover the dot at zero during this whole motion, the 

 telefcope is truly vertical. 



The adjuftment for collimation is effefted by an apparatus 

 that moves the wires in the eye-piece, as in the tranfit-inftru- 

 ment, and may be thus efFefted : Let the graduated arc face 

 the eaft, and view a ftar paffing in or near the zenith, the 

 proper time for doing which may be known from the flar's 

 right afcenfion, by means of a fidereal clock, or by conver- 

 fion of folar into fidereal time, and note the diftance from 

 zero when the plumb-line is quiet, which it will foon be if the 

 plummet is immerfed in a goblet of water, and mark down 

 this quantity as read partly by the divifions on the fcale, 

 and partly by the micrometer-head ; which call the eajlern 

 meafure with N. or S. annexed, accordingly as the ftar 

 pafles to the north or fouth fide of the zenith point : then 

 reverfe the pofition, and on a fucceeding night, which will 

 be 3' 56" fooner on every fucceffive night, meafure the 

 zenith-diftance of the fame ftar in like manner with the gra- 

 duated fcale facing the weft ; and call it the luejlern mea- 

 fure : then if the two quantities thus meafured be fimilar, 

 the collimation for zenith-diftance will be true ; but if other- 

 wife, one half of the difference of the two readings will be 

 the error of collimation, which may be either correfhed by 

 the proper apparatus at fucceffivo trials, or, which is perhaps 

 better, may be allowed for in each obfervation. When the 

 inftrument has been ufed for feveral obfervations, it will be 

 ftill better to take an average of all the obferved errors, as 

 afcertained by different ftars, and to apply it with its proper 

 fign in future fingle obfervations, lo long as the inftrument 

 remains in all refpefts unaltered. It is hardly neceflary to 

 obferve, that in ufing this inftrument, when a ftar is very 

 near the zenith of any place, the micrometer alone, without 

 reference to the graduated fcale or arc, will give the due 

 meafure. How the latitude of any place may be very ac- 

 curately determined from the meafured zenith-diftance of a 

 ftar, of known declination, will be feen prefently. 



Zenith-Se9or by Ramjden While the trigonometrical 



furvey of England was carrying on, it was found defirable 

 to have a portable zenith-fedor to aflift in meafurin^ an arc 

 of the meridian, and as Ramfden had one in an half-finiftied 

 ftate, that had been ordered by the duke of Richmond, the 

 parties were prevailed on to have this finiftied for the pur- 

 pofe, which was nearly done in Ramfden's life -time, in the 

 year 1801, and completed in April 1802 by his fucceflbr 

 Berge. It was firft tried at Greenwich, and then removed 

 to the Ifle of Wight, where the operations began, and from 

 whence they were continued northward from ftation to fta- 



tion, as defcribed in the «' Trigonometrical Survey " bv 

 captain Wilham Mudge, and publiftied from time to time 

 by Mr. Faden, of Charing-Crofa. The original account of 

 this complex inftrument has reference to fix large plates, 

 and is too long for us to copy. ( See vol. ii. p. 6. Jif fen of 

 part ii.) Fig. 2. of our Plate XXXIII. of AJlronomkal 

 Injtrumenis, gives a reduced perlpeftive view of this inftru- 

 ment, from an examination of which a better idea will be 

 formed of its general conftrnftion than by any detailed ac- 

 count we can give of its parts without the'additional plates. 

 Captain Mudge fays, " that Mr. Ramfden has here ob- 

 viated the inconveniences attendant on the ufe of former 

 feCtors ; and has alfo diminillied, in a very confiderable de- 

 gree, the errors unavoidably refulting from their imperfeift 

 conftruaion. The principles, he adds, on which he has 

 founded the feveral improvements, confift in the means of 

 uniting the fedorial tube to its axis, fo as to enfure the 

 permanency of the length of its radiuis when erefted for 

 obfervations ; more accurate methods of adjufti/.g the in- 

 ftrument vertically ; and an eafy way of placing the face 

 of its arc in the plane of the meridian." The frame ex- 

 hibited in our figure confifts of two parts ; the exter- 

 nal ftand of mahogany, which fupports the apparatus to 

 which the feftorial tube is attached ; and an inner frame, 

 containing that apparatus with the tube itfelf. The ftand, 

 or outer frame, is in the ftiape of an obtruncated pyramid, 

 having a bafe fix feet fquare, and its vertex three. It 

 unites ftrength with fimphcity of conftrudlion. The inner 

 frame, within which the feftor is fufpended, is fupported at 

 top in every lateral direftion, while its lower extremity is 

 terminated by a cone refting in a metallic concavity, on 

 which it turns in azimuth ; and it can be kept in any po- 

 fition by a clamping apparatus ailing with an azimuth 

 circle, made faft to the bottom of the external frame. 

 The telefcope of the feftor has an objeft-glafs nearly 

 eight feet long, with an aperture of four inches, near which 

 is made faft the tranfverfe axis of motion, fimilar to that 

 of a tranfit-inftrument. The wires of the eye-piece of the 

 telefcope are illuminated by reflefted light, entering the axis 

 in the ufual way ; and a plumb-line, with the ghoft apparatus 

 for adjuftment to zero, is made a leading feature in the con- 

 ftruftion. The arc is divided into 15°, which was the 

 concluding work done by Berge, and each degree is fub- 

 divided into 5' fpaces, as in Graham's inftrument. A 

 fecond telefcope, of 29 inches focal length, is attached to 

 the long tube, and moves in the plane of the divided arc to 

 any given elevation, but partakes of the azimuthal motion 

 when the long vertical telefcope is turned round : with this 

 29-inch telefcope horizontal angles are meafured, by the 

 help of the azimuth circle, which therefore is divided for 

 this purpofe. Befides thefe eflViitial parts, there are various 

 appendages and bracing parts, rendered neceflary by the 

 fize of the inftrument, particularly a m.icrofcopic tube reach- 

 ing up to the upper dot of the p!umb-hne, and bent at 

 both ends into a horizontal pofition for convenience of the 

 obferver. This contrivance required refletlors, both of the 

 light, and of the image of the piece of mother-of-pearl 

 that is bifefted by the wire, conftituting the plumb-line. 

 The micrometer meafures minutes and feconds in the ufual 

 way, and the plummet is immerted in a fmall veflel of water 

 to prevent vibration. We mention thefe particulars ^efl<;r<7//)', 

 not only becaufe their particular ufes and modes of applica- 

 tion have been previoufly defcribed, when we explained 

 other inftruments, fuch as Circles, EauATOBiALS, Tran- 

 &n-lnjlruments, &c. ; but becaufe this inftrument has fur- 

 niftied hints to others who have copied in part, or wholly, 



feveral 



