34 GAUSS AND WEBER ON TERRESTRIAL MAGNETISSI. 



supported by the circle ; the circle is provided ^ith a pivot 

 which passes through the alidade, and has, at its upper extre- 

 mity, two hooks to receive the pin fixed to the thread. With 

 this arrangement of the stirrup, it is important that the ali- 

 dade in which the magnet bar lies should rest on the rim of 

 the circle ; otherwise, the friction, taking place near the axis of 

 rotation, would produce a displacement of the parts relatively 

 to each other, in consequence of the impulse arising from the 

 vibrating bar. Moreover, the stirrup is so constructed that 

 the magnet bar fits in either on its broad or narrow side. This 

 is done for the purpose of determining accurately, by observa- 

 tions of declination in any of the various positions of the magnet 

 bar in the stirrup, the position of the mirror relatively to the 

 magnetic axis of the bar. 



10. The box and the measuring scales. — The box which pro- 

 tects the magnetometer from the influence of currents of air is 

 constructed so as to afford ready access to the instrument within. 

 It forms a cylinder of 800 millimetres in diameter, and 300 in 

 height. The cyhndrical form is given to it for this reason ; in 

 the measurement of intensity, in order to ascertain the moment 

 of inertia, a wooden rod 700 millimetres in length is placed at 

 right angles on the magnet bar of 600 millimetres in length, 

 and this rod, to which weights are suspended, must find a place 

 in the box along with the magnet bar, and must vibrate freely. In 

 order to perform these experiments with convenience, it is also re- 

 quisite that the box should admit of being entirely opened at the 

 top, and of being tightly closed again, so that there should only 

 remain an apertui-e at the top for the suspension thread, and one 

 for the mirror at the side. The latter may be closed with a small 

 wooden slider, to exclude air when not observing. The box is 

 closed above by two semicircular lids, which must fit exactly, one 

 of which is provided with a small aperture for the thread. This 

 aperture is not situated in the centre of the circle formed by the 

 two semicircular hds, but is so placed that the thread passing 

 fi'eely through it, the mirror of the magnet bar may hang close 

 before the aperture in the side of the box. This arrangement is 

 necessary, in o.-der that a small aperture may suffice to allow the 

 light to pass from the scale to the mirror, and from that to the 

 telescope. iVi'ound the case are fixed the measui'ing scales on 

 which may be placed a second magnet bar to the south or north, 

 to the east or west of the magnetometer, at prescribed distances 



