GAUSS AND WEBER OX TERRESTRIAL MAGNETISM. 33 



meter in the meridian of the telescope. After such a shding of 

 the suspender on the ceiUng, which need be performed but very 

 seldom, it isne. Jssary to place on the opposite wall a new mark, 

 to which the telescope may be directed without departure from 

 the meridian. The thread to which the magnet bar is suspended 

 consists of 200 parallel fibres of raw silk, each of which would sup- 

 port thirty grammes without breaking. The weight which this 

 thread has usually to sustain amounts to nearly 2000 grammes, to 

 which, in the measurements of Intensity, two weights of 500 

 grammes are added when determining the moment of inertia of 

 the magnetic bar. The thread, therefore, never carries more 

 than half the weight with which it would break. It is about two 

 metres long, and has a torsion force, the moment of which amounts, 

 for small deviations, to about the 1000th part of the magnetic 

 force. The thread may be prepared by winding a single fibre 

 twenty-five times round two glass tubes, distant fi-om each other 

 about four times the intended length of the thread ; the two 

 ends of the fibre are then tied firmly together, and the twenty- 

 five-fold skein, thus formed, is stretched by drawing the two glass 

 tubes further from each other. A small hook, carrying a weight, 

 is then attached to the skein, midway between the two tubes, 

 which are then raised and brought together, and the two loops 

 are united in one. Thus a hundred-fold thread is prepared, 

 M-hich forms a loop at top and bottom, and which, being again 

 brought together in a similar manner, forms the thread to which 

 the magnet bar is suspended. 



9. Tlie stirrup and torsion-circle. — The force of torsion of the 

 thread to which the magnet bar is suspended must not be entirely 

 neglected in absolute measurements of declination and intensity, 

 even though this thread be very long and fine. In order to mea- 

 sure the magnitude of this force, and to diminish its influence, 

 so that the thread in the mean position of the magnet bar may 

 be brought to its natm'al position when its moment of torsion is 

 zero, it is necessary to be able to turn the thread, at one of its 

 two extremities, round itself, in such manner that the angle of 

 torsion may thereby be measured. In order to have the means 

 of effecting this at hand, the apparatus for this purpose must be 

 at the lower extremity of the thread ; but, that the magnet bar 

 may not be turned with it, the stiri-up is composed of two paits, 

 an alidade and a circle, which revolve only round a common 

 vertical axis. The alidade supports the magnet bar, and is itself 



VOL. II. PART v. c 



