USE OF THE BIFILAR MAGNETOMETER. 277 



which must be performed in fixing and adjusting the appa- 

 ratus. 



1. The clock, the theodolite, and the scale are fixed, and a 

 plumb-line dropped from the centre of the object-glass across 

 the scale. The theodolite is to be leveled. 



2. The telescope is directed to the opposite wall, on \vhich 

 there is a mark, serving to designate the terminal point of the 

 optical axis. The scale is placed perpendicularly to the vertical 

 plane of that axis. 



3. A place is sought in the vertical plane of the optical axis 

 for the mirror, the distances of which from the centre of the ob- 

 ject-glass, and fi'om the division of the scale across which the 

 plumb-Hne is suspended, are, together, equal to the distance of 

 the mark from the centre of the object-glass. The horizontal 

 plane of this point must bisect the plumb-line from the centre 

 of the object-glass. A plumb-line is let fall from the ceiling 

 through this point. 



4. The bearer is either fixed to the ceiling, or perpendicularly 

 above a hole made through the ceiling, from 80 to 100 milli- 

 metres wide ; so that the ends of a thread passed over it, and ex- 

 tended by small weights, pass freely through the aperture, and 

 are both situated in the vertical plane of the optical axis of the 

 telescope. 



5. One end of a steel wire, sufficiently strong to carry half the 

 weight of the instrument without danger of breaking, is fast- 

 ened to one end of the thread, and drawn up to the bearer 

 by drawing the other end of the thread doM'n (care being 

 taken that the wire and the thread should always be extended 

 in a straight line) ; it is passed over the two cylinders of the 

 bearer, and drawn down ; the thread is then removed, and the 

 two ends of the wire, weighted, are left to hang freely until they 

 have assumed their natural position. 



6. The two ends of the wire are cut off about 100 or 150 mil- 

 limetres below the place where the magnetometer is to be sus- 

 pended, and are fixed to the suspension screws. The stirrup 

 thus carried is then, with the help of the screws, w^ound up into 

 its proper position. 



7. A box, sufficiently large to contain the magnet-bar, is placed 

 underneath to protect the instrument in case the wires should 

 break, and to prevent currents of air. This box is closed on all 

 sides. Its lid consists of two halves, which fit close, and leave 

 only one round aperture, through the centre of which the pivot 



VOL, II. PART VI. T 



