Page:Scientific Memoirs, Vol. 2 (1841).djvu/52

40 arrangement serves the purpose of adjusting the space necessary for the reception of the mirror. These three small sliders terminate at their southern extremity, in three small vertical circular surfaces, on which the edges of the mirror are placed; while the head of a screw, whose grooves fit into the sliders beneath the edge of the mirror, 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 remarks respecting the others will suffice.

Fig. 5. In this view of the stirrup, torsion-circle, magnet bar, and mirror-holder, seen from above, the torsion circle is more distinctly presented to view, as also the form of the stirrup. In the centre of the circle 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 mirror-holder.

Fig. 6. In this figure, which has often been referred to previously, is chiefly seen in what 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 provided with a square aperture 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 would then fall out, but for a small spring beneath, which is visible in this figure, and which then retains the pin in its position. The wooden rod, above 700 millimetres in length, which in this figure 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 all these points must be measured with microscopical accuracy.