80; 



ON MAGNETISM. 



points of the needle are observed by means of micro- 

 scopes which are attached to a revolving frame that 

 carries verniers by which the graduations of an external 



Fig. 30. 



vertical circle are read. The true position of the needle 

 (including all effects of friction, uncertainty of reading, 

 &c.) is rarely doubtful to the extent of 2'. A modified 

 form of the instrument, adapted to the use of needles 

 of different lengths, and with other fittings, is mounted 

 as a permanent instrument at the Royal Observatory, 

 Greenwich. 



In. all cases, the instrument is so mounted that it 



