Page:Scientific Memoirs, Vol. 2 (1841).djvu/42

30 Remarks on the separate parts of the Magnetic Observatory, and of the Magnetic Instruments.

1. The room.—Plates II. and III. are a perspective view and a ground-plan of the room. In the first the southern wall is supposed to be removed; in front, on the right, is seen, a, the foundation for the theodolite; b, the stand of the theodolite; c, the theodolite; d, the scale attached to the stand; e, the plumb-line suspended from the centre of the object glass. Near to it is stationed the clock, f; a line drawn from the theodolite telescope to the mark designated by the arrow on the opposite wall, would represent the magnetic meridian. Towards the centre the suspender of the magnetometer is fixed to the ceiling; from this is suspended the thread carrying the stirrup, in which is placed the magnet bar, to the anterior extremity of which the mirror is fastened vertically. The distance of the mirror from the telescope and its distance from the centre of the scale, (before which passes a plumb-line let fall from the theodolite telescope,) are, together, equal to the distance of the telescope from the mark.

2. The theodolite.—For observing the changes of declination, a telescope, having motion in a vertical plane, so that it may from time to time be directed either towards the mirror or towards the mark, is quite sufficient. This movement serves to ascertain and verify the stability of the telescope. For absolute measurements of declination a theodolite is employed instead of such a telescope. As the divisions of a scale divided into millimetres must not only be seen but even their subdivisions estimated, it is necessary that, at a distance of five metres of the scale and of the telescope from the mirror, the telescope should possess a magnifying power of at least thirty.

3. The clock.—All observations must be made accurately to time, for which purpose a clock which beats seconds must stand near the observer, with its face towards him. A chronometer may serve the purpose.

4. The magnetometer.—Besides a clock and a theodolite, which must be supposed present in all establishments where magnetic observations are to be executed in the most perfect manner, the magnetometer consists of the following parts, which are necessary for measurements of declination:—the magnet bar, the