GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 39 



the miiTor, which must be of such thickness that it may not 

 bend, all the other parts are constructed of thin brass, so as to 

 increase the moment of inertia of the magnetometer as httle as 

 possible. The thread supporting the stirrup is not fastened im- 

 mediately to it, but to a pin which fits below the staples A A, so 

 that without unfastening it may be disengaged from the stirrup. 

 The pin is provided with two small points, at a distance of about 

 40 millimetres from each other, which fit into two depressions on 

 the staples A A. The torsion circle B B is furnished with a ver- 

 tical pivot, the upper end of which supports the staples A A, and 

 is surrounded by the rotating stirrup. The stirrup itself rests 

 upon the periiihery of the torsion circle, but is prevented from 

 ^turning by its friction against it. At the end of the magnet bar 

 D D is obsen^ed the mirror-holder, which at E fonns a sheath 

 incasing the magnet bar, to which it can be tightly fastened by 

 screws. To this sheath is attached a frame FF' turning round 

 a vertical axis. Small pressing and tightening screws, which 

 ser^te for placing and fixing this frame, are behind it in this 

 view, and therefore are not seen. With this first fi-ame E F', 

 turning round a vei'tical axis, is connected a second frame H H, 

 turning round a horizontal axis at F', which can be adjusted to 

 the fiii'st by means of the screws shown above. The clamps 

 which are to receive the mirror are attached to this second frame. 

 Three such clamps exist ; but in this figure only two, K and K', 

 are visible, while the third is covered by the second at K'. 



Fig. 4. Serves to give a more distinct view of all the parts of 

 the mirror-holder, which, here seen from the south, are severally 

 better seen than in the foregoing view from the west. Each part 

 is designated by the same letter. The rectangle seen between 

 E and E" is the transverse section of the sheath inclosing the 

 magnet bar, to which it is fix'mly screwed. This case has on 

 one of its sides two projections, E E' , which form the vertical 

 (horizontal in the figure) axis of the frame F FF' F'. Opposite, 

 near E" , is a third projection, against which the screws act, 

 which serve for placing and holding fast this first frame. A 

 horizontal (in our figm-e vertical) axis is attached to this first 

 frame at F' F', around which the second frame HHHH can re- 

 volve. Opposite to this axis both frames have small projections, 

 whose relative distance can be adjusted by pressing and tighten- 

 ing screws. Three small incisions are shown, H H, HH, HH, 

 into wliich three small sliders can be inserted and fastened. This 



