THE MURAL CIRCLE.] 



ASTRONOMY. 



1025 



the Observatory at Cambridge, Massachusetts, of which 

 the accompanying engraving is a representation. With 

 tliis instrument one observer can, at the same time, 

 determine the right ascension and declination of a star 

 with great precision. The telescope T (Fig. 212) has 

 Kg. 212. 



an object-glass of four and one-eighth inches of an 

 aperture, and of four feet focal length. The length of 

 the axes between the shoulders of the pivot is ninety-six 

 inches ; the pivots are of steel, and two-aiid-a-half inches 

 in diameter, and the same in length. The eye-piece is pro- 

 vided with two micrometers, one having a vertical, and 

 the other a horizontal movement. Besides the usual 

 mode of illuminating the field through the axis, there are 

 also facilities for illuminating the circus in a dark field. 

 The circles are four feet in diameter, and cast in one piece, 

 and both circles are graduated on silver from to 300, 

 divided into live minute spaces. There are eight micro- 

 meter-reading microscopes attached to the granite piers, 

 being four for each circle. Four of these are seen at 

 A B C D, the other four being on the opposite side of the 

 pier. These micrometers bisect diametrically both piers. 

 The arm E, attached to the pier, supports an additional 

 microscope, which serves as a pointer to indicate approxi- 

 mately the degrees and minutes. For levelling the axis 

 a striding level is employed ; and this, combined with a 

 method of reflecting from quicksilver at the nadir point, 

 affords means of ascertaining the amount of colliination 

 of the middle wire without reversal of the pivots. There 

 is, however, an apparatus for reversing the instrument 

 when required. The object glass of this circle is by 

 Mcrz of Munich ; the fitting by Simms of London. 



The mode of observation with the mural circle is as 

 follows : The telescope having been set approximately 

 to the culminating star, the observer bisects the object 

 by the movable horizontal wire as it passes the meridional 

 vi-rtic.il fibre. lie then "reads off" the points and the 

 nix microscopes, as well as the telescope micrometer, by 

 a combination of which a concluded circle reading may 

 be obtained. It may be necessary to mention that the 

 instrument is furnished with a clamp and a slow motion 

 crew, by which the horizontal wire may be brought on 

 the star, after the telescope has been approximately 

 directed to it If, by inadvertence, or other causes, the 



VOL. r. 



object is not observed at the meridian wire, it will 

 involve a "correction of curvature," which is thus 

 investigated. 



For the purpose of recording the position of every 

 star within range by means of electro-magnetism, tlio 

 telescope is firmly clamped to remain in its position, 

 while the observer, sitting with his eye at the telescope, 

 has but to press his finger upon a key at the instant a 

 Fig. 213. s ^ ar i s seen to pass the 



wires, which, for this 

 purpose, are divided 

 into two systems (Fig. 

 213). Those for right 

 ascension are thirty-five 

 in number, divided into 

 groups of five each, the 

 intervals between the 

 ! wires being from, two 

 to three seconds. The 

 wires for difference of 

 declination are also 

 thirty-five, and ar- 

 ranged in similar 

 groups. In order to 

 prevent confusion be- 

 tween observations for right ascension and declination, 

 the rule is, to observe for right ascension on one set of 

 wires first, and denote the magnitude of the star by 

 telegraphic symbol ; and afterwards to observe for 

 declination on the inclined wires. 



When an object is observed by the mural circle off the 



Fig.,214. 



meridian, the north polar 

 distance found from the 

 circle reading is the N. P. 

 distance of a point of the 

 meridian, which is inter- 

 sected by a great circle 

 passing through the place of 

 the object at the time of ob- 

 servation. If the object 

 moved in a great circle, this 

 would be its north polar distance when on the meridian ; 

 but as it moves in a parallel to the equator, a correction 

 is required, which, as it is evident, will be positive for 

 stars north of the equator, and negative for those south 

 of it. It is thus computed : 



Let P be the pole (Fig. 214) ; S the place of the object ; 

 = its north polar distance ; a = its distance from tho 

 meridian ; S x = the observed N. P. D. ; then wo 

 have 



cos. S = cos. a cos. (S z) = (l 2 sin. 2 J a) (cos. icos. x 



+ sin S sin. x) 



cos. S = (1 2 sin 2 a) (cos. S X sin. X x sin I") 

 = cos. 8 + sin. S X sin. 1* 2 sin H a cos. S 



2 sin. sin. 2 J a X x sin. 1 



If a be small, the last term will vanish, and by making 

 sin. 4 a = ici sin. 1", we have 2 sin. 2 ^ a = Ja 2 siu. 2 V 

 .-. sin. & X x sin. 1* = cos. S x 5 a 2 sin.' 1". 



. sin. 1* , 

 x = cos. S, n"" 



The distance between the wires is supposed to ba 

 20s. = one interval ; and if n be tho number of intervals, 

 a = n X 20 ; 



.". a* = n 2 x 400, consequently x = cot. S ., " 



x = tan. declination X (sin. \" X 200) n 3 . 



In tho case <>f a star near the pole, the following is tho 

 investigation for curvature of path (Fig. 215): 



Let Z S' = Z = the observed znnith dist.ince ; Z S x 

 zenith distance on meridian ; Z P S = ;/ the difference 

 between the star's K. A. and the sidereal time of obs> i - 

 vatiou ; d = star's declination ; L = the latitude of tl e 

 place of observation. Then in the triangle Z S' P, wo 

 havo 



6p 



