GAUSS AND WEBER OX TERRESTUIAL MAGNETISM. 31 



stirrup with its torsion-circle, the suspender with screw and 

 suspension thread, the mirror and mirror-holder, the torsion-bar, 

 the scale, and the quieting bar; to which must be added, for 

 measurements of intensity, the measuring scales, the deflecting 

 bar, the weights, and the weight-holder. The magnet bar, in its 

 connexion with the stirrup and the torsion-circle, (which again 

 is connected by the suspension thread with the suspender,) and 

 with the mirror and mirror-holder, is re})resented in Plate X. 

 fig. 3 and 5. 



5. The scale. — Fig. 10 gives a specimen of the scales hitherto 

 employed, which must be at least one metre in length. M. Ritt- 

 miiller of Gottingen has lithographed such a scale, and has had 

 it printed on white card-paper. 



6. The plumb-line at the object glass of the telescope. — A fine 

 wire of dark colour with a weight at its lower extremity, is 

 fastened in such manner to the upper rim of the object glass, 

 that it hangs correctly over its centre. In order to fix this 

 wire, the small notches of the grooved frame of the object 

 glass may be used ; or a ring, constructed specially for this pur- 

 pose, may be slid over the frame, having two shts diametrically 

 opposite each other. The upper slit serves for the fastening of 

 the wire, and the ring is so arranged that the wire passes freely 

 through the lower. If we now view the image of the scale in 

 the mirror through the telescope, we see at the same time the 

 image of this wire projected on the white surface of the scale, 

 and can thus find that point of the scale which lies in the verti- 

 cal plane of the optical axis of the telescope. The spot where 

 the prolonged plumb-line touches the ground is carefully marked, 

 and serves as a means of testing the immobility of the theodolite 

 stand. 



7. The mirror and mirror-holder. — The mirror of the mag- 

 netometer must be perfectly plane, because othei'wise, with 

 a magnifying power of 30, the image of the scale wo\dd be in- 

 distinct. The plane mirrors from Utzschneider's optical manu- 

 factory in Munich have hitherto proved the best. The min'or 

 should be somewhat broader than it is high, as, by the vibration 

 of the magnet bar, the right and left side of the mirror alternately 

 enters the field of the telescope. The best dimensions of the 

 mirror are from 50 to 70 millimetres in height, and from 70 

 to 100 in breadth. In measui'ing the distance of the mirror 

 from the scale and from the mark, the refraction of the rays 



