30 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



I 



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,/'; a line drawn from the theodolite telescope 

 to the mark designated by the ari'ow on the opposite wall, would 

 represent the magnetic meridian. Towards the centre the sus- 

 pender of the magnetometer is fixed to the ceiling; from this is 

 suspended the thread carrying the stirrup, in which is placedthe 

 magnet bar, to the anterior extremity of Mhich the mirror is fast- 

 ened vertically. The distance of the mirror from the telescope 

 and its distance from the centre of the scale, (before Avhich 

 passes a plumb-hne 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 

 fi-om 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 measure- 

 ments 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 pui-pose a clock w hich 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 aU establishments where magnetic 

 obsei-vations are to be executed in the most perfect manner, 

 the magnetometer consists of the follow ing parts, which are ne- 

 cessary for measurements of declination : — the magnet bar, the 



