28 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



tical axis of the telescope ; and when the magnetometer is sus- 

 pended by the pkimb-Hne, the distances of the reflecting plane of 

 the magnetometer (see mirror and mirror-holder) from the scale 

 and from the telescope, may be together equal to the distance 

 of the telescope from a point on the opposite wall, m hich is to 

 serve as a mark, to Avhich the telescope may be directed. At 

 the point of the ceihng whence the plumb-line is let fall, the 

 suspender of the magnetometer, together with the elevating screw 

 and suspension thread, must be fixed. Let a Meight be provi- 

 sionally attached to the thread suspended from the elevating 

 screw, in the manner of a plumb-hne ; adjust the suspender 

 on the ceiling until the thread coincides with the plumb-line, 

 making the length of the suspender parallel to the north or south 

 wall of the room. After this, measure the height of the sus- 

 pender, of the telescope, and of the scale from the ground. 

 From the first height, subtract half the sum of the two latter, 

 and form a thread of parallel fibres of raw silk {coconfdden), 

 whose length is equal to this difference, and which is sufficiently 

 strong to carry the magnetometer, and one kilogramme of addi- 

 tional weight. The upper extremity of this thread is to be 

 fastened to the screw, and the lower to the stirrup, {shiffchen), 

 in which the magnet bar is placed. A wide box is placed under 

 the magnet bar, at the bottom of which are two cushions, upon 

 which the magnet bar would fall, in case the raw silk fibres 

 should break, without endangering the mirror attached to the 

 front extremity of the magnet bar. After these preparations the 

 more accurate measurements may be commenced. These are : 



1. To place the magnetic axis of the magnet in a horizontal 

 direction, and the miiTor perpendicular to it ; or to measure the 

 small angle which the axis of the muTor forms with the magnetic 

 axis. 



2. When the magnet is in its mean direction, to bring the force 

 of torsion of the thread to zero, or to measure the small remain- 

 ing torsion. (Vide seq. torsion bar.) 



3. To determine the ratio of the moment of torsion of the 

 thread, and the magnetic moment of the bai", in a deflection. 

 (Vide seq. stirrup and torsion circle.) 



4. To ascertain by measurement the place for the mark on the 

 wall opposite to che telescope. 



The apparatus is then ready for measurements of the Declina- 

 tion. These consist : 



1. In the measurement of the azimuth of the mark. 



