WEBER ON A TRANSPORTABLE MAGNETOMETER. 571 



drical, and is inclosed in a cylindrical brass box, on the under 

 surface of which are two small projections which fit into two 

 cavities in the copper case when the suspension thread of the 

 needle is let down. The brass box can be fixed in this position 

 by two screws brought through the upper part of the copper 

 case : the box being thus held fast, the needle va^Lj first be drawn 

 out through the opening in the back of the case, and a brass cy- 

 linder of the same form as the needle, inclosing a weak magnet, 

 may be placed in its stead, to try the torsion of the thread. 

 Secondly, for the purpose of measuring the error of collimation, 

 the needle may be turned in the box round its longitudinal 

 axis, by means of a key introduced through the aperture in the 

 back of the case. During the observations the apertures in the 

 front and the back of the case are closed with a plate of mica 

 to guard against currents of air. 



Fig. 2 represents a somewhat different and more simple con- 

 struction of the same instrument; the needle is not hollow, 

 is not enclosed in a brass case, and cannot be reversed. This 

 simplification is admissible when the use of the instrument is to 

 be restricted to the experiments which are to be made in the 

 open air, as detailed in the sequel. In this case the mirror is 

 included in the copper case, and its normal forms a right 

 angle with the magnetic axis of the needle. The glazed open- 

 ing in the side of the case does not impair its action as a damper, 

 and the opening may be made of any convenient size. 



Fig. 3 represents the outside box, in which the instrument is 

 packed for travelling, and which serves also for suspending the 

 deflecting bar when it is to be used for the experiments of 

 vibration. A mirror is fixed to the end of the bar, so that it 

 may be obsen'ed from a distance with a telescope and scale. 

 The box has a small opening which can be closed with a plate 

 of mica admitting the light. The figure shows the bar suspended 

 in the box, and loaded with two cylindrical weights, made of 

 brass, and connected by a silk thread passing over a bar parallel 

 to the needle, to keep the centres of gravity of the two weights 

 exactly the length of the bar from each other. The weights 

 serve for the deduction of the moment of inertia. 



The unifilar suspension of the bar can be changed for a bifilar, 

 if the variations of the intensity are to be observed. The box 

 must then be placed relatively to the theodolite and to the mag- 

 netometer in the manner represented in the ground plan, fig. 4, 

 namely, so that, according to the rule laid down in the Resultate 



