USE OF THE BIFILAR MAGNETOMETER. 273 



is desired to increase or diminish the directive force, they 

 should be moved through an equal quantity at both extremities, 

 it is by no means necessary. The change in the interval of the 

 wires may be effected below only, but in such case to a greater 

 degree. The apparatus figured is, in fact, so arranged, that, 

 with a mean distance of the upper ends, every necessary in- 

 crease or diminution of the directive force can be produced by 

 a displacement of the suspension screws at the stirrup ; how- 

 ever, for the sake of completeness, the apparatus is provided with 

 an arrangement at top for an equal displacement of the two 

 cylinders, over which the wire is conducted, and by which its 

 two vertical suspended ends are kept separate from each other ; 

 so that, if it is desired, the upper distance may always be ren- 

 dered equal to the lower. In case it is not desired to retain the 

 power of making this upper displacement, these hvo cylinders 

 may be united into a roller of a suitable diameter, and the axis of 

 this roller, like that of a friction wheel, may be allowed to run on 

 wheels, so as to diminish the friction, and cause the two wires to 

 have an equal tension ; — a point which is of great importance in 

 absolute determinations. 



2. On the separate parts of the Bifilar. 



The description of these is reduced almost wholly to a descrip- 

 tion of the stirrup, because it unites nearly all the parts which 

 in the unifilar are distributed among the stirrup, the ceihng, and 

 the extremity of the bar. It is also unnecessary to speak of the 

 theodohte and its stand, the clock, the scale, or the mark, as all 

 these have been treated of in the account of the former insti-u- 

 ment. But as so many arrangements are united in the stirrup, its 

 construction requires to be particularly explained. Plate XIV. 

 gives three different views of the instrument, of the natural size, 

 and as arranged for the 25lb. bar ; the small and compound parts 

 have been represented in a separate section, so as to exhibit their 

 interior mechanism. It requires an attentive consideration on 

 account of the many impor:ant parts compressed into so small a 

 space at the stirrup : a cleai comprehension of its mechanism will 

 be obtained when we know the various concentric rotations which 

 are performed at the stirrup,— the mode of checking and mea- 

 suring these, — and their objects. The rotations are the following. 



1. Of the mirror on its pivot;— the whole of the other por- 

 tions of the instrument remaining unchanged. 



