Page:Scientific Memoirs, Vol. 2 (1841).djvu/51

the mirror, 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 little as possible. The thread supporting the stirrup is not fastened immediately 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 vertical pivot, the upper end of which supports the staples $A\;A$, and is surrounded by the rotating stirrup. The stirrup itself rests upon the periphery of the torsion circle, but is prevented from turning by its friction against it. At the end of the magnet bar $D\;D$ is observed the mirror-holder, which at $E$ forms a sheath incasing the magnet bar, to which it can be tightly fastened by screws. To this sheath is attached a frame $F\;F'$ turning round a vertical axis. Small pressing and tightening screws, which serve for placing and fixing this frame, are behind it in this view, and therefore are not seen. With this first frame $E\;F'$, turning round a vertical axis, is connected a second frame $H\;H$, turning round a horizontal axis at $F'$, which can be adjusted to the first 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 firmly 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 $F\;F\;F'\;F'$. 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 figure vertical) axis is attached to this first frame at $F'\;F'$, around which the second frame $H\;H\;H\;H$ can revolve. Opposite to this axis both frames have small projections, whose relative distance can be adjusted by pressing and tightening screws. Three small incisions are shown, $H\;H$, $H\;H$, $H\;H$, into which three small sliders can be inserted and fastened. This