36 GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 



when measuring the duration of vibrations, to make the com- 

 mencing arc no greater than 2 or 3 degrees, and in observiii;^- 

 changes of direction, to make the arc as small as possible, ne\(r 

 allowing it to exceed 2 or 3 minutes. This end is attained with 

 the quieting bar, in the use of which every observer must prac- 

 tise himself. It is a magnetic bar half the length and breadth, 

 and four times hghter than the principal bar. When this bai' is 

 held by the observer behind the theodolite in a horizontal posi- 

 tion, and at a right angle with the magnetic meridian, it will 

 cause at this distance (about 5t meters), if it is strongly 

 mao-netized, a deviation of about one minute, westerly if its 

 north pole is held easterly, and vice versa. This deviation 

 becomes smaller in proportion as the bar is removed from 

 the horizontal position, and disappears entirely with its ap- 

 proach to the vertical position. No inconvenience is therefore 

 occasioned by such a bar standing by the wall or near the clock- 

 case (as in Plate II. and III.), till wanted. The use of the quiet- 

 ing bar in magnetic measurements is manifold ; and it is im- 

 portant, in order to attain perfect and skilful facility in the 

 performance of these experiments, to become accurately ac- 

 quainted with its mode of operation. A separate article will 

 therefore be allotted subsequently to the explanation of the rules 

 and laws for its various uses and modes of action. 



Finally, the building may be situated in the neighbourhood 

 of other buildings without any injmy to the observations. The 

 magnetic observatory in Gottmgen, for instance, could not, w ith- 

 out causing many difficulties, be situated far from the astrono- 

 mical observatoiy. The magnetometer is stationed about 60 

 metres westward of the astronomical observatory. At this 

 distance moderate magnetic forces exercise so small an influ- 

 ence on the magnetometer, that it has been found unobjection- 

 able to erect in a room of the astronomical observatoiy an auxi- 

 liary magnetic apparatus, which is of very essential sendee in 

 absolute measurements. 



More accurate directions will be gi^■en subsequently for deter- 

 mining the influence of a distant magnet, according to its force 

 and position relatively to the magnetometer; and will especial- 

 ly serve this purpose, that when several magnetical apparatus 

 (for instance, a principal magnetometer, an auxiliaiy magneto- 

 meter, and an inchnatorium) are to be fixed in neighbouring 



