a7ul of the AUiiiide and Azimuth Instnment. 17 



at the same station,) the busmess of observing by repetition 

 may be commenced as follows. Set the fore telescope to zero, 

 as is nsual; or what is better, as was said before, read oiFwhat 

 the indices (being clamped) happen to show ; and, by turning 

 round by the general motion, place the intersection of the mid- 

 dle wires exactly on the object to the left. Then, by its own 

 motion in the same manner set the back telescope to the object 

 on the right ; and examine if the angle between the objects be 

 accurately comprehended between the two telescopes. Now 

 by the general motion, without touching any thing else, move 

 the back telelescope until its wires coincide with the object on 

 the left. To complete the first operation imclamp the fore 

 telescope, and carry it round to the object to the right ; when 

 its indices will have advanced upon the graduated Umb through 

 an arc equal to double the angular distance of the objects. To 

 read oft' this double result would be rather detrimental than 

 useful ; instead of which, with the fore telescope fixed at this 

 position, the three steps of another operation, as described 

 above, should be taken in order to obtain a second double re- 

 sult A third, fourdi, &c. course of operations must succeed, 

 until it is judged that sufficient has been done to produce the 

 accuracy required. At last the indices must be read, and the 

 total number of degrees, minutes and seconds, that have been 

 passed over by the iiidices, taken and divided by double the 

 number of operations ; when the simple angle between the ob- 

 jects will appear. If all the results had been read, the inter- 

 mediate errors of division would have come into the account, 

 and produced an effect that has been avoided by the process 

 described : for, except at the beginning and end, the observa- 

 tions were carried on as if there had been no divisions. It is 

 in this solely, that this instrument claims an advantage over 

 others, and justly; lor they have a beoinning and end to every 

 double result ; but this, as fer as graduation is concerned, has 

 only a begiiniing and an end to a whole course of observations.^ 

 In geodesy, the levels are of no use, except in the operation of 

 bringing the plane of the circle into that of the two objects: and 

 it may be observed here, that in astronomy the back telescope 

 is altogether unnecessary. 



To observe zenith distances of the licavenly bodies by repe- 

 tition, is a process so similar to what has been described, that 

 a shorter course may be taken to ey-jjlain it. The instrument 

 being adjusted, and the indices set or read, by the general mo- 

 tion' (the level being horizontal) bisect the star, and examine 

 that both are correct at the same time : now turn the instru- 

 ment lialf round in azimuth, cf)rrecting with the foot-screw the 

 position of the level if recjuired, and move the telescope by its 



Vol, fJO. No. 291. J///y 1822. C own 



