ZENITH SECTOR. 



ZENITH SECTOR. 



1044 



such an operation, Uko what earn you may, is always liable to derangv 

 the relation of the parts of tho iuatrumcnt, and o to lead to untrue 

 conclusions. Even while it was at Wanstead, and considered to be 

 immoraaUe. a alight change acemi to have taken place, which ban been 

 investigated by Dr. Kuwh. in the' Kedu. i.)Wrvatiom made 



by Bradley at Kew and Wanstead,' Oxford. 1832. In i . :.i. uniting it, 

 the astronomer-royal ha* given the property of reveniuu very satis- 

 factorily, and Mr. Maclcar s observations with it are of extraordinary 

 accuracy. 



A lector wai used in the measurement of the meridian in France, by 

 Cunini de Thury and La Caille, which greatly reeembled the sector of 

 1'icarJ, except that the arc wag of much greater extent, being 6'2J and 

 framed with three radii. The telescope waa fixed at the back of the 

 imrtrument, ao u to bo at onco nioro firmly fastened and out of the 

 way of the plumb-line ; and, lastly its micrometer-screw waa applied to 

 the wires of the telescope (sometimes called the inferior micrometer), 

 instead of moving the whole sector. In this mode of observing, the 

 instrument in set approximately to tho star, [and the nearest dot 

 accurately bisected. When tho star in in the centre of the field, it if 

 bisected by the micrometer-acrew which carries a wire in the focus of 

 the object-glass, and thus the excess or defect from the nearest dot in 

 ascertained. This improvement in the application of tho micrometer- 

 screw U due to the Chevalier Louville, and in this respect we prefer 

 the French to the Kugluh construction. See ' La Mcridienne de 

 rObservatoire Royal de Paris verifiee,' par M. Cassiui de Thury, 

 Paris, 1740. The sector is figured at page 31, and described at page 

 Ixxi. 



Bouguer and La Condamine, in their measure of the arc of Peru, 

 were compelled to fabricate their own zenith sectors, and adopted a 

 very elegant mode of graduation, the merit of which is given by La 

 Condamiue to their colleague Oodin. Tho telescope and arc being 

 prepared, a star is selected which has pretty nearly the same zenith 

 distance at both extremities of the arc of the meridian. Now calculate 

 approximately the value of the chord of the double zenith distance of 

 the star, and find what fractional part it U of the radius. Suppose it is 

 nearly f, of the radius, then take a beam compass, mark two dots on 

 the arc, and step seventeen times with the same opening along the 

 radius, and so fix the dot over which the plumb-line is to pass. The 

 instrument is now graduated, and. is used as follows: After being 

 adjusted to the meridian, the plumb-line is made to pass over tho 

 'lot and one of the lower dote, after which the star is bisected 

 by the interior or Louville's micrometer. On a following day the 

 instrument is reversed, and the plumb-line being brought over the 

 upper dot and the other lower dot, the star is again bisected by tho 

 micrometer. It is plain that the double zenith distance of the star, 

 corrected for refraction, aberration, &c , is measured by tho arc sub- 

 tended at the central dot by the two dots below + the sum or 

 difference of the micrometer readings. But the arc is, by construction, 

 that the sine of which is ,',, which is found from the tables; and the 

 value of the micrometer readings being also known, the zenith distance 

 of the star is known. The operation may be repeated at the other end 

 of the arc with the same star, and using a different submultiple of the 

 radius. See ' Mesure des trois premiers degres du meridioii,' 

 de La Coudauiine, 1'arix, 1751, pp. 105 et soq. ; ' Figure do la 

 par M. Uouguer, Paris, 171'.', pp. 170 et seq. 



We iH.-crt here the method employed by Maupertuis, La Caille, and 

 others to ascertain the value of the total arc of the sector. A line of 

 considerable lengt: I'nlly measured from a well-defined spot 



and a signal erected ; then a perpendicular was measured t'n >m the 

 signal, of such a length as very nearly subtended at the spot the arc to 

 be verified, and here a second signal was placed. The sector was then laid 

 horizontally on a bed prepared for it, the centre being exactly over the 

 defined spot, and the telescope pointed to the first signal; when thin 

 wan done satisfactorily, a fine line was stretched over the centre and 

 tho lirst dot of the divided aro. Now shifting the sector round, the 

 second signal was bisected, and it was Been what division was bi- 

 sected by the line which continued to pass over the centre. The 

 true angle is evidently that marked on the ground, and is calcu- 

 lated from the given length of the perpendicular and the distance. 

 The value of the arc read off on the sector is compared with ti 

 the error of the total arc detected, which is afterwards used for 

 correcting all the angle* observed. At present tho value of tho total 

 arc of a sector would be determined by a comparison with tho mural 

 circle. 



In 1775 Bird erected a zenith sector at the Observatory of Oxford, 

 which is in most respects similar to Graham's, but it is fixed to an 

 upright pillar which revolves freely, so that the instrument is rever- 

 sible. From come cause or other, though observations have been made 

 with this instrument, they have not been considered satisfactory. It 

 appears to us to be an excellent instrument, and one capable of doing 

 good work, though one chief UM of zenith sectors, namely, ascertaining 

 the index error of the meridian declination instrument, has been sup- 

 plied to modern circles by observations by reflexion. 



The zenith sector of Koinsden, which was used in the trigonometrical 

 survey of Oreat Britain, and in tin- Holstein arc, is described and 

 figured in great detail in the ' Phi). Trans.' for 1803, and in the second 

 volume of the 'Account of the Ordnance Trigonometrical Survey of 

 England and Wales.' It was burned in the fire which consumed a 



considerable part of the Tower in 1841. As this instrument very 

 fully detailed in the account referred to, it U useless to enter into par- 

 ticulars here. Ramsden viewed the upper dot by a lung microscope, 

 which saved considerable trouble, as well as avoided the clu 

 deranging the bisection by mounting to read it The ' Astronomical 

 Observations with Ramsdcn'a Zenith Sector,' reduced by Lieutenant 

 Tolland, K.I-:., were published in ISl'J by order of the Board of 

 Ordnance. 



After the destruction of Ramsden's sector, ( y applied to 



the Astronomer Royal for his advice as to the best form of insti um.-nt 

 for determining latitude in <hr jiilil. The construction giv<-n l.y Mr. 

 Airy, and executed by Mr. Siinms, differs in many respects fr< 

 which preceded it. The description, with explanat"- 

 found in the ' Monthly Notices of the Royal Astronomical S 

 vul. v., p. 1SS. The vertical axis, which U cast in on. 

 strongly framed, carries at its back three levels, one above the 

 which being read off at the moment the star is bisected, deternn 

 position of the axis with respect to the zenith. The telescope 

 with the eye- and object-end is cast in another solidly braced pie. 

 is held at its middle on a centre in front of the vertical axis. This 



frame moves freely for a few degrees on each side the . 

 The divided arcs are graduated on the vertical axis near its t 

 bottom, and there ore four micrometer microscopes, one at each -ide 

 of the object and eye-end, the tubes of which are bored in tli 

 telescope-frame. There is a wire-micrometer in the focus of th 

 scope. A stop to the axis enables the observer to turn the insti 

 exactly half round by touch, and almost instantaneously. The <>< 

 tions are made thus : The instrument being pretty nearly in the 

 meridian, and the axis vertical, the telescope is set nearly tor i i 

 ami the microscopes are read off. Before the star reaches the centre of 

 the field, the observer bisects it with the micrometer-wire, noting the 

 time, while the assistant reads off both ends of each level. The whole 

 instrument ia then turned half round, and the star U again ol>- 

 the bisection being now performed by the tangent-screw of the tele- 

 scope-frame, the time is again noted, the assistant reads off the levels 

 as before, and, finally, the arcs above and below are read off by the 

 micrometer microscopes. In this way the double zenith distance 

 of a star, free from all error of collimation or of the vertical 

 axis, may be obtained in a few minutes. This instrument performs 

 very satisfactorily. The telescope is one of 3J feet focal length, and the 

 instrument bears the same relation to a mural circle that the ordinary 

 sector does to a quadrant. 



When Troughton first proposed the mural circle as the best form for 

 a meridian declination instrument, great doubt was thrown on the 

 practicability of observing by reflexion with sufficient nicety, and in 

 that case, as the mural circle does not reverse, a supplementary instru- 

 ment was wanted to show the position of the zenith or h 

 Partly on this account, but chiefly to settle the Constanta of aberration 

 and precession with the greatest precision, Troughton planned a 



nsisting of a telescope of 25 feet focal length, without any 

 sector, and in whieh tin- variations of / nith distance of y I >r.ioi>nis and 

 close zenithal stars were to be measured by a micron . The 



telescope rests on its lower end, continued be\ .ml the focus, on a 

 piece which has adjustments for vertically, and a collar oel> 

 object-glass is pressed by a spring into a Y bearing. The wins at the 

 focus are moved by a micrometer-screw, and the st.tr and wires are 



irough a diagonal four-glass eye-piece. The plumb-lin.- 

 within the tube, and is viewed above and below by micro 

 microscopes. Instead of adjusting the plumb line < obser- 



vation, it is bisected by the micrometers after the observation, and 

 a correction applied which is deduced from .the upper and 

 ings. .Mr. Airy having had some reason to suspect that the win- 

 twisted on reversing the instrument, gave a double suspension to the 

 plumb-line, and made the instrument reversible on a star in the 

 same night, by using a stop as in the ordnance wet or. Th, 

 observations with the zenith tube are printed yearly in the Greenwich 



The zenith sector has not been much used upon the continent since 

 the great surveys made in the middle of last centin 

 the figure of the earth. In the French arc from Dunkirk 1 

 mentera, the latitudes were observed by the repeating circle, and in 

 some of the stations there is reas< -t that error has been 



committed. Mora recently, the transit in the piimo vertical ha 

 employed in ml Russia for ascertaining d 



t>ide, and as it would seem with great success. [TRANSIT.] A 

 prime vertical transit was constructed by Kill to, ; 

 observatory of Pulkowa, of which a most favourable account ha 

 given by Professor Struve. While admitting the i this 



kind of instrument for telescopes of moderate size, we do not M 



d, far less surpass, the zenith sector when made reversible 

 and of the proper magnitude. 



Some years ago Mr. Babbage proposed a construction for a /milli 

 sector (' Memoirs of tho Astronomical Society,' vol. ii., p. In]) which 

 might perhaps be applied in the following manner: Conceive a 

 lurallel ruler to be placed upright, one of the bars being made into a 

 vertical axis with the necessary adjustments, and the other carrying a 

 telescope. It is clear that if the bands were equal the telescope 

 continue parallel to itself whether the ruler be open or shut. But if 



