40 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



arrangement serves the purpose of adjusting the space necessary 

 for the reception of the miiTor. These three small sliders ter- 

 minate at their southern extremity, in tkree smaU vertical circular 

 surfaces, on which the edges of the mirror are placed ; while the 

 head of a screw, Avhose grooves fit into the sliders beneath the 

 edge of the muTor, press on its front surface. In this figure 

 the sliders themselves are not seen, but merely the heads of the 

 three screws, which fit into and conceal them. 



After these explanations of the first figures, a few short re- 

 marks respecting tiie others will suffice. 



Fig. 5. In this view of the stirrup, torsion-circle, magnet bar, 

 and mirror-holder, seen fi'om above, the torsion circle is more 

 distinctly presented to view, as also the form of the stirrup. In 

 the centre of the cu'cle is also visible the end of the pivot passing 

 through the alidade, and the double staple attached to it, with 

 its two pivot holes. The brass pin, whose points fit into these 

 holes, is removed, for the sake of perspicuity. In this figure, 

 moreover, is seen how the mirror is fastened to the min'or- 

 holder. 



Fig. 6. In this figure, which has often been referred to previ- 

 ously, is chiefly seen in Avhat manner the points of the pin, to 

 which the suspension thread is fastened, fit into the holes of the 

 staples, which latter are connected by a centre-piece proAided with 

 a square apertui'e in its own centre, into which the 4-sided pin of 

 the torsion circle is inserted, and held fast by a screw. Since the 

 stirrup, together with the magnet bar, must be raised when the 

 latter has to be inverted for the purpose of finding its magnetic 

 axis, the pin to which the thread is fastened Avould then fall out, 

 but for a small spring beneath, Avhich is visible in this figure, 

 and which then retains the pin in its position. Tlie wooden 

 rod, above 700 millimetres in length, which in tliis figui-e is laid 

 across the centre of the magnet bar, and serves for the support of 

 two half-kilogrammes which are to increase the moment of inertia 

 of the magnet bar, is furnished with 6 points, on which the two 

 weights can be placed at different distances. The two central 

 points are at a distance of 100, the next two at a distance of 

 400, and the extreme points at a distance of 700 millimetres from 

 each other. The first and last are fixed ; the two intermediate 

 ones can be taken out and placed in other notches, situated at 

 distances of from 50 to 50 millimetres asunder. The distances of 

 aU these points must be measvu-ed with microscopical accuracy. 



