274 W. WEBER ON THE ARRANGEMENT AND 



2. Of the mirror, with its pivot and alidade, on the circle to 

 which the suspension-screws of the wires are fixed, and on which 

 the stirrup and its alidade rest. 



3. Of the stin-up with its alidade, on the circle on which it 

 rests. 



In order to complete the view of all the rotations, we may here 

 add, 4. That of the two upper extremities of the wire around one 

 another, i. e. around the same axis as that on which the other ro- 

 tations take place. 



The first rotation ^vill be sufficiently intelligible from figs. 1 

 and 3, PI. XIV. The arrangement is simple, because its amount 

 does not require to be measured. Its object is merely to allow of 

 perfect freedom iu fixing the theodolite ; the axis of the mirror 

 can always be made to revolve, to suit the position of the telescope 

 and the scale, wherever they may be placed. The image of the 

 scale which appears in the mirror serves itself to regulate the ro- 

 tation, and no further arrangement for measuring it is required. 

 A screw, as exhibited in the figs. 1 and 3, fixes the mirror in its 

 position. 



For the second rotation, the three pieces, the mirror, the pivot, 

 and its alidade, are firmly connected as one piece, and revolve 

 together in the cavity of the circle ; they are represented, together 

 with the latter, in a cross section, at fig. 4. The mirror is placed 

 on the upper end of the pivot JB at ^ ; C is the ahdade of the 

 pivot ; D is the circle. The only essential difference between the 

 second rotation and the first is that in the second the angular 

 amount can be measured. As the revolving alidade of the pivot, 

 situated beneath the circle, embraces at its two extremities the 

 edge of the circle, it forms on its upper and graduated surface 

 two verniers, the inner mai'gins of which lie close to the outer 

 margin of the divided circle. A clamp, by which the alidade of 

 the pivot can be pressed firmly against the circle, is seen in the 

 section at E, fig. 4. 



The second rotation alone would be sufficient if there were at no 

 time an impediment to its use. The verniers on the aUdade of 

 the pivot come in certain cases beneath, and are hidden by the 

 alidade of the stirrup. In the instrument represented in PL XIV. 

 much care has been employed to restrict this within very narrow- 

 limits, as will be plainly perceived in fig. 2 ; but, in order to 

 meet the rare cases in which it does still occur, without having 

 to alter the position of the theodolite, both rotations may be em- 



