Page:Scientific Memoirs, Vol. 2 (1841).djvu/40

28 axis of the telescope; and when the magnetometer is suspended by the plumb-line, the distances of the reflecting plane of the magnetometer (see mirror and mirror-holder) from the scale and from the telescope, may be together equal to the distance of the telescope from a point on the opposite wall, which is to serve as a mark, to which the telescope may be directed. At the point of the ceiling whence the plumb-line is let fall, the suspender of the magnetometer, together with the elevating screw and suspension thread, must be fixed. Let a weight be provisionally attached to the thread suspended from the elevating screw, in the manner of a plumb-line; adjust the suspender on the ceiling until the thread coincides with the plumb-line, making the length of the suspender parallel to the north or south wall of the room. After this, measure the height of the suspender, of the telescope, and of the scale from the ground. From the first height, subtract half the sum of the two latter, and form a thread of parallel fibres of raw silk (coconfäden), whose length is equal to this difference, and which is sufficiently strong to carry the magnetometer, and one kilogramme of additional weight. The upper extremity of this thread is to be fastened to the screw, and the lower to the stirrup, (shiffchen), in which the magnet bar is placed. A wide box is placed under the magnet bar, at the bottom of which are two cushions, upon which the magnet bar would fall, in case the raw silk fibres should break, without endangering the mirror attached to the front extremity of the magnet bar. After these preparations the more accurate measurements may be commenced. These are:

1. To place the magnetic axis of the magnet in a horizontal direction, and the mirror perpendicular to it; or to measure the small angle which the axis of the mirror forms with the magnetic axis.

2. When the magnet is in its mean direction, to bring the force of torsion of the thread to zero, or to measure the small remaining torsion. (Vide seq. torsion bar.)

3. To determine the ratio of the moment of torsion of the thread, and the magnetic moment of the bar, in a deflection. (Vide seq. stirrup and torsion circle.)

4. To ascertain by measurement the place for the mark on the wall opposite to the telescope.

The apparatus is then ready for measurements of the Declination. These consist:

1. In the measurement of the azimuth of the mark.