1020 



tOHOMY. 



[THE MURAL CIUCLB. 



. Z - tin. L in. n + coa. L oo. d eoa, 



p') 

 i tin. L sin. d + oos. Leo*. tl(l 



But cos. Z x = sin. L sin. d + cos. L cos. d ; 



or since x is small, 

 oos. Z + sin. Z X z sin. L sin. <2 + cos. L cos. d ; 



cos. L oos. d X ^ 

 2 



therefore z sin. Z 



sin. Z X z sin . 1* cos. L cos. d X 



, ... Low / 



X number whose log. = 673673 



or log. z* 



log. cos. L + log. cos. d log. sin. Z 



4- 073073 + Zlog.p 



log. cos. L + log. cos. d + ar. co. log. sin. Z 



+ G 73073 + 2 log. p. 



For the purpose of obtaining a zero point of the mural 

 ike use of two different methods, of which 

 the first consists in the observations of stars by reflection 

 from a surface of mercury. The mode of operating is as 

 follows : Point the telescope to any known star when it 

 crosses the meridian, and record the reading of the circle ; 

 on tin- next night observe the same star as it crosses the 

 meridian, by pointing the telescope upon the imago of the 

 tar reflected from the surface of the mercury. As the 

 surface of a fluid at rest must bo horizontal, and as the 

 angle of reflection is equal to the angle of incidence, this 

 image will be just as much depressed below the horizon 

 at the star itself is above it. The arc intercepted on the 

 limb of the circle, between the star and its reflected 

 image, is the double altitude of the star, and its middle 

 the horizontal point of the circle, allowing for 

 the difference of refraction at the moment of observation. 

 .'..-HI that the image of a star seen by reflection 

 will be as much depressed below the horizon, as the object is 

 really abcve it ; and by combining the two readings of the 

 circle for the same star, observed by reflection and direct 

 MHJnn, tlio horizontal point of the instrument maybe 

 readily determined. This method of obtaining a reflec- 

 tion and direct observation of a star, at the game transit, 

 U daily practised at our principal observatories. 



The second method is by the use of a Bohnenbcrger's 

 eye-piece, previously explained. By causing the two 

 linage* of the wire of the declination micrometer to 

 . ami noting ita readings, the nadir, and conse- 

 qiifiitK |Miiitts of the circle may be determined 



ting the telescope vertically downward upon a 

 haiiu of mercury, the reflected image of the horizontal 

 wire being made to coincide with its direct image. The 

 telescope U directed towards the tui./ir, which is distant 

 W from the horizontal point, or 180 from the zenith 

 point. As this observation can be made at any tim. , 

 independently of the weather, it is a most valuable 

 method, and in many observatories U the one excln-ivelv 

 mplojred. The I 



..- i . ild dil i . ... tlj ;i.i fn m 



tlw smith |.int, mating eye- 



Thu eye-pioco, first suggested by Bohneuborger, 



consists of an ordinary eye-piece with an aperture A 

 ut in its side, and a plane perforated xpocu- 

 luni B B inserted between the two lenses, at an an-lu of 

 lfc.SU. 



45, with the optical axis, having a lamp held so as to 

 throw a strong light on the speculum. The 

 images of the wires may be seen with groat distinctness. 

 Instead of a perforated opaque speculum, a piece of pla:ie 

 glass with parallel faces, and without any perforation, is 

 sometimes used. The. observer looks through the plane 

 glass without difficulty, while sufficient light is reflected 

 from the lower surface to render the lines visible. If 

 the axis of the telescope be not horizontal, half the dis- 

 tance between the middle wire and its inia^e, <-on 

 for error of level, will give the error of collimatiou of the 

 middle wire. 



The tallies of refraction, made use of at the !< 

 Observatory, Greenwich, are those of Bessel, modified 

 and expanded from the Tabula Reyiomontarue. A nu- 

 merical example of their application is given further on. 



Example of a zenith point determination from the Green- 

 wich Observatory, 1849. (See, also, page 1008). 



Horizontal Point ^ M. , 



The adopted Zenith Point U 'J:i> 32' 49" -8D. 

 The printed form given as follows, will show fulh 

 method of re. hieing observations of zenith ciUt;uu-e. It is 

 arranged for the reduction of the zenith d: .-rva- 



tions with the transit circle, wliich differs from the ftl 



mcns, principally in the valuation of the nr 

 microscopes. The manner in which the revolutions and 

 parts of revolutions of tho microscojie-inicronieter are 

 converted into arc, are explained as follows in the 

 GrecnwiJi Obtervotioiu, 1852, page iv. : 



" I must premise that, as one revolution of < 

 microscope-micrometer does not diiter extravagantly/ 

 from 1' on the limb of the circle, we may con- 

 revolution as a nominal minute. Next, supposing the 

 number of integral revolutions, as shown by each m 

 scope, to be the Maine for all, we ought, in order to 

 obtain the mean of tho fractions of a revolution, to add 

 her the subdivisions as shown by the different 



'.to sum by (i. Third 1 



tho subdivisions are in the decimal scale, we should then 



multiply tlm mean by CO to reduce it to seconds. It is 



nt thus that tho number of nominal seconds to bo 



