32 GAUSS AND WEBER ON TERRESTRIAL, MAC5NETISM. 



of light at the anterior surface of the glass must be consi- 

 dered : that plane is the reflecting plane which is equidistant 

 from the anterior and posterior surfaces of "^e mirror. The 

 mirror is fixed to that end of the magnet bar which is turned 

 towards the telescope, and must form with it so solid a system 

 that no reciprocal disarrangement of either may be feared 

 during the experiment, although the magnet bar be taken 

 out of the stirrup and replaced in it in a reversed position. 

 Moreover, the mirror must have such a position relatively to 

 the bar, that the normal of the mirror shall be quite, or very 

 nearly, parallel to the magnetic axis of the bar. The mirror- ^ 

 holder rejoresented at fig. 4. may serve both these purposes ; its } 

 frame is attached by screws to the bar. The frame-work sup- ' ' 

 porting the mirror may be turned by screw motion round two 

 rectangular axes, by which it may be brought into the required i 

 position. T 



8. The suspender, elevating screio, and suspension thread. — It is 

 very advantageous to fix to the ceiling the thread, which is to 

 carry the magnet bar, as by this it is sufficiently insulated from 

 the floor and protected from all shaking, and because a proper 

 length may in this manner be given to the thread. If, for the sup- 

 port of the magnetometer, we employ, not a metal wire, (the ela- 

 sticity of which for an equal tenacity is almost ten times greater 

 than that of one formed of silk fibres,) but a thread composed of 

 parallel fibres of raw silk, it lengthens greatly, especially at first ; 

 and it is therefore requisite from time to time to raise it, so that 

 the magnet bar and the mirror fixed to it may regain theiroriginal 

 height. In raising the thread it is necessary that it should not be 

 displaced in a lateral direction. A screw may be employed for 

 this purpose, in the grooves of which the thread lies, and 

 upon which it can be wound up still further, while the end of 

 the screw works into a fixed nut. The groove in which the thread 

 places itself, by the turning forwards of the screw, takes then, 

 of itself, (from the advance of the whole screw) the place which 

 the vertically suspended line had before occupied. Ths; fixed 

 nut, with solid rest, through which the screw pin passes freely, 

 is let into a wooden slider which is mortised into a large plank 

 fixed to the ceiling, and can slide therein in a direction parallel 

 with the north or south wall of the room. If the position of the 

 magnetic meridian should change in the lapse of time in any 

 considerable degree, this slider will serve to retain the magneto- 



