

ASTRONOMY. 



[ZENITH DI.TAHCB. 



A, as in Fig. If -aohed and turned toward* 



|-.-:iit II, as in Fig. 183, when the instrument stand* 

 exactly in ita first position, as at 1 IT 1 .'; the 

 only difference tiring, that the index, which at the 

 beginning of the tint operation was at zero ot the gradu- 

 ated Male, hat, at the end ot the first operation, travelled 

 over it double the distance of the angle Bought ; and, at 

 the end ot the second operation, thin distance from zero 

 is quadrupled ; therefore, to get the exact value of the 

 angle, the portion of the circle thus travelled over has to 

 be divided oy four. In order to diminish the chances of 

 error in reading off the scale at the end of these opera- 

 tions, four different verniers, regularly subdivided, are 

 fixed upon the circle ; one of the indexes only which ac- 

 company these verniers, is employed to determine the 

 entire number of divisions of the limb which the tele- 

 scope has turned ; but the four verniers give each a value 

 of the fraction of division which is to be added to t!xj 

 entire number, and it is the mean of their indication.-. 

 which is given as the value of this fraction of division. 

 The microscopes M M (Fig. 175) are BO disposed that 

 the divisions of the vernier can be easily observed, as 

 well as the coincidence of one of them with the scale of 

 the limb itself. It is hardly necessary to add that the 

 re|H-tition of the operation s, rures immunity from error, 

 and that it alto gives its name to the -instrument. 



To MEASCKE THE ZENITH PI.-TAVK. In measuring 

 the angle, we have real objects to deal with ; it is other- 

 wise in the present operation. \\ , name that point of 

 the heavens the zenith to which the vertical line is 

 directed, and this direction is indicated by the plumb- 

 line (Fig. 184). The zenithal distance of a point 

 is the angular distance of this ]><>int and of the 

 zenith that is to say, the angle that the ray 

 directs towards the point made in the vertical, 

 at the place of observation. The zenith is not, 

 as wo have said, a point that we can observe 

 with an instrument ; but the zenith distance is, 

 nevertheless, to be found by the following ope- 

 ration : In order to render the repeating circle 

 available for measuring the zenith distance, the 

 axis of the column F (Fig. 18(5) is rendered 

 perfectly vertical. For this purpose three powerful 

 screws, G, are required, on which the instrument is to 

 fir. IKS. rest ; these screws are 



turned until it is per- 

 fectly level, which is as- 

 co -tuined by means of a 

 spirit-level (Fig. 185) fixed to the tube of the loner tele- 

 scope, in a horizontal position, while the circle itself is 

 nearly vertical, as in Fig. 187 ; the screws, M n g (Fig. 



Fig. 184. 





"d to regulate the position, and render the 

 whuU perfectly square. 



This position l> ..-.I, turn tho whole instru- 



ment round tho axis, F (Fig. isr.i, until tho spirit 

 is parallel to the lino p <i, and pwpendioolar to tho line 

 IH (Fig. 187). 



Tho axis of the column of tho circle and tho plane of 

 the circle being thus rendered perfectly vertical, the 



upper telescope is to bo turned upon the circle so that its 

 index stands at zero of the graduated scale, and the circle 

 is fixed in this position. The circle is then turned with 

 the telescope, first around tho axis of tho column so 

 as to load the vertical plane of tho circle by the point A 

 Fig. 188), and afterwards round tho axis of the circle so 

 as to train the optical axis of the 

 glass exactly towards the same point. 

 The circle being fixed in this position 

 by means of the tangent screw ^ 

 acting on the extremity of its axis, 2 

 a half turn is given to all the in- .j 

 strument, which carries it round "* 

 the axis of the column, placing it 

 in the position indicated (Fig. 189), 

 detaching the instrument, and train- 

 ing it along round the axis of the 

 circle in such a manner as to point 

 towards A (Fig. 190). It is clear 

 that in this movement the teles 

 has made an aniclc double the /.i-n; 

 distance A O Z (Fig. 103), which N 

 to be determined ; and that by 

 reading the number of degrees, 

 minutes, and seconds of the gradu- 

 ated scale to which the index, which 

 . I >anies tho telescope, corre- 

 sponds, wo have only to take the 

 half of this number to have the . 

 value of zenithal distance. 2 



The operation may hero termin 

 if the operator be satisfied with the - 

 measure obtained ; but should he 

 wish to verify the calculations by 

 increasing the multiple of the angle, 

 he may continue the operations by 

 making a half turn round tho axis 

 of tho column, crossing tho circle ^ 

 in its plane, so that tho t 

 which remains Ii\ 



point A (Fig. I'.Hi). 



The instrument is n\v in f\aeily 

 the same position as in Fig. 188. in 

 thw first operation ; but the index, 

 in plaeo of being at zero, is now at an 

 angulardi.stance from . ulilu 



the data we seek to value. A new 

 operation, exactly like tho i 

 s. <|iirntly gives, at its termination, ~ 

 a value of four times the value 

 of tho zenithal distance, ami tho 

 sum found, divided by four, will 

 give the precise value. 



THE TiiDonoi.rrr.. The- r 



great exactness tho angle A O I! ( l-'j.;. \\i:\), fon 1 by 



the right lines joining the two points A and K at the 



