3041 



ZENITH SECTOR. 



ZENITH SECTOR. 



1012 



telescope. If this reading is not that -which was intended by the 

 maker, the difference is called the error of collimation, and is applied 



as a correction, additive or subtractive, to all the observations, according 

 as they are on one or the other side of the zenith. 



Picard enters into no details with respect to his observations, but 

 gives at each place a zenith distance, which is the mean of a considerable 

 number. He only observed one star, and that to the north, namely, 

 the Knee of Cassiopeia (5), giving as a reason. " that a star nearer to 

 the zenith would have been more difficult to observe, and that if the 

 star had been between the two zeniths, the error of the instrument 

 (the division corresponding to the zenith, or error of collimation), 

 which might have been imperfectly determined, would have been 

 doubled in the apparent distance between the two zeniths, because 

 then the sum of the two observations must have been taken ; whereas 

 when a star is always observed on the same side of the zenith, there is 

 imly the difference to be taken, which must be correct, provided the 

 instrument is well centred and well divided."* 



In lb'7-t Hooke published ' An Attempt to prove the Motion of the 

 Earth from Observations.' in which he describes the instrument he 

 contrived for observing the distance of 7 Draconis from the zenith of 

 Uresham College, and the apparatus for measuring the variations which 

 might occur. This consisted of an object-glass of 35 feet loom 

 length, fixed at the top of the house, and referred by two plumb-lines 

 hanging from a bar in the object-cell and passing through apertures 

 in the floors, to a system of wires below. Before each observation 

 Hooke set certain marks in his wire-cell to the plumb-lines, then 

 fixed the wire cell, removed the plumb-lines, and bisected the star ; 

 after the observation, he verified the position of the wire-cell on 

 replacing the plumb-lines. There is a great deal to admire in this 

 simple and ingenious contrivance, but his mensm'fttur for noting the 

 m;ill variations seems clumsy and inexact. " Inconvenient weather 

 and great indisposition in his health " limited Hooke's observations to 

 four in number, from which he erroneously concluded that there was 

 an annual parallax of the earth's orbit, and therefore that Copernicus's 



* We have given the above extract from Picard because it shows, we think, 

 that he did not reverse his sector at each place ; if he had, he must have got a 

 double zenith distance free from all fixed errors but those of centre and division, 

 and his reasoning is not correct. The assumption which he tacitly makes, that 

 the error of collimation continued to be the name during his observations at 

 different places, is not allowable. It Is evident that with such an instrument 

 the exact adjustment to the meridian would be difficult, and hence probably his 

 choice of a star, which, being distant from the zenith, required less nicety in 

 :hu adjustment. 



ARTS AXD SCI. DIV. VOL. VIII. 



theory was true. With very little alteration, such as a nicer reference 

 of the plumb-lines to the cell of the eye-piece, and a screw micrometer 

 for a mensurator, Hooke's apparatus would still be applicable; and 

 if his idea of using a deep dry well for the telescope-tube were 

 adopted, we conceive that most accurate determinations might now be 

 made. 



Several observers about this time discovered a motion in the stars 

 which they could not account for. Picard, Roemer, and Flamsteed all 

 found that the position of Polaris varied at different times of the year, 

 and Flamsteed fancied that the change was due to parallax. In or 

 about 1 725, a gentleman of the name of Molyneux, then resident at 

 Kew, employed Graham to make him a parallactic telescope, so called 

 from its object, namely, to discover the change of place in the stars 

 occasioned by the earth's change of position in her orbit. This is 

 described in Smith's ' Optics,' book iv., cap. 7, and more fully in Mr, 

 Molyneux's own words in Bradley's ' Miscellaneous Works and Cor- 

 respondence,' p. 93. It consisted of a telescope 25 feet long, with a 

 short cross axis at the upper end, just at the place of the object-glass. 

 This axis was so adjusted on its supports that the telescope moved in 

 the meridian. A plumb-line hung on one end of the axis and passed 

 over a dot in a plate fixed on the tube near the eye-end. The tele- 

 scope was drawn forwards in the meridian by a string and weight 

 passing over a fixed pulley, and pushed back by an antagonist-screw, 

 which had a divided head and index. To make the observation the 

 plumb-line is first made to bisect the dot by the screw, and the division 

 read off ; then the star is bisected also by moving the screw, when the 

 division is again read off. The revolutions and parts through which 

 the screw is moved measures the angular distance of the star from a 

 line in the heavens, which continues fixed if there is no change in the 

 relation of the object-glass and wires to the axis and dot. 



Observations were made at Kew for some time with this instrument 

 by Molyneux, Graham, and Bradley ; and in 1727 Bradley had a zenith- 

 sector constructed by Graham with which he made his celebrated 

 discoveries of aberration and nutation. There is a short description of 

 this instrument by Maskelyne in the first volume of his ' Greenwich 

 Observations.' p. 9, which Higaud has reprinted, with some memoranda 

 by Bradley, hi the ' Miscellaneous Works,' &c. ; but Bradley himself 

 gave no description in his Memoir on Aberration (' Phil. Trans.' vol. 

 xxxv., p. 637), and in his following Memoir on Nutation ('Phil. Trans.', 

 vol. xlv., p. 1) satisfied himself by referring to the description of a 

 sector on a similar construction. (' Ddgre' du Mdridien entre Paris et 

 Amiens," 1740.) This last-mentioned sector was made by Graham for 

 the measurement of the degree in Lapland, and afterwards employed 

 in the remeasurement of Picard's arc. We have now a full and minute 

 description of Bradley's instrument, with numerous plates, in a work 

 entitled ' Operations for the Verification and Extension of the Abbd de 

 la Caille's Arc of the Meridian,' by Thomas Maclear, Esq., pp. 67-81, 

 published by order of the Lords of the Admiralty. This account is by 

 the astronomer-royal, who directed the alterations required to fit the 

 sector for the field. The principal parts of this instrument, as originally 

 made by Graham, are a telescope with a short sector attached to tlie 

 eye-end, and a short cross or transit-axis to the object-end, which causes 

 it to move in the meridian when properly adjusted. A plumb-line 

 passes over a fine dot at the extremity of the upper axis, and beats on 

 the divided sector below, that is, it almost touches the dot above and 

 arc below, but still hangs perfectly free. To prevent any disturbance 

 from the wind, the plumb-line is screened by a tube, and the bob hangs 

 in water that it may sooner come to rest. To make the telescope 

 describe the meridian correctly, as well as to get a proper fixing for the 

 clamp and micrometer-screw, another arc is fixed to the wall, and the 

 telescope carries a frame with rollers at its eye-end, and is thus kept in 

 contact with the fixed arc before and behind. A clamping apparatus, 

 which slides along the fixed arc, and can be attached to it by screws in 

 any position, carries a fine screw with a micrometer head, which pushes 

 the telescope by acting on a piece of hardened steel, while the teles- 

 cope resists either by gravity or by a counterpoise weight. There are 

 numerous parts and contrivances for different adjustments, which will 

 be easily understood from Mr. Airy's account. In making the observa- 

 tion, the telescope is first to be set, or nearly so, to the star, the 

 bisection of the upper dot is verified, and then a division below is 

 bisected by carrying the screw one way, jorioard, for instance, and the 

 micrometer head is to be read off. When the star is in the centre of 

 the field, it is bisected by carrying the screw still forward, and the 

 micrometer is again read off. Finally, the screw is still to be carried 

 fortrard till the next division is bisected, and the micrometer read off. 

 A simple proportion will give the quantity, which is to be added to the 

 first reading, or subtracted from the second reading, in order to get the 

 reading corresponding to the star.* Bradley's sector as originally made 

 was not reversible, and was therefore only tit for measuring differences 

 or variations. When, after its removal to Greenwich, actual zenith 

 distances were required, the instrument was shifted across the room, 

 from the east to the west side, where a second fixed arc, &c. were ready 

 to receive it. We need not say that this was .very objectionable, as 



* In some of the books referred to, it will be seen that the dot bisected 

 before observing the star is directed to be bi-ectcd again, and the mean of ilr- 

 readings taken. But a screw which carries weight never reads the same when 

 moved forwards and backwards, and it is alwuyn sufest to cuiry the screw the 

 same way in the same operation , 



3 x 



