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. Rittmüller of Göttingen 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 purpose, may be slid over the frame, having two slits 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 vertical 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 magnetometer must be perfectly plane, because otherwise, with a magnifying power of 30, the image of the scale would be indistinct. The plane mirrors from Utzschneider's optical manufactory in Munich have hitherto proved the best. The mirror 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 measuring the distance of the mirror from the scale and from the mark, the refraction of the rays
