612 



of the larger magnets is to be suspended within the wooden 

 box, which should be placed on the same stand with the di- 

 vided circle, at a distance not less than one foot from its 

 centre. The optical axis of the telescope, and that of the 

 magnet-collimator, are then to be brought nearly into the 

 same right line, by an azimuth movement of the top of the 

 stand, and by a small parallel movement of the box. The 

 torsion of the suspension thread is then to be determined by 

 the help of the brass cylinder, and to be removed by means 

 of the torsion cap. The magnet being then replaced, the 

 coinciding division of the scale is noted, with the magnet di- 

 rect and inverted, and the mean of the two readings is the 

 division corresponding to the magnetic axis. The verniers 

 of the circle being then read, the telescope is to be turned 

 until the division so found coincides with a fixed mark, whose 

 azimuth is to be determined at leisure. The latter deter- 

 mination is made by the help of the same Theodolite, used 

 in combination with the Chronometer or Sextant. 



2. Observation of Vibration. 



The upper plate of the circle is to be moved to its original 

 position, and clamped there. 



The coefficient of torsion of the suspension thread being 

 determined, by the help of the torsion cap and glass scale, 

 the magnet is to be set in vibration, and the lime of i300 vi- 

 brations determined in the ordinary manner. The arc of 

 vibration should be noted, by the help of the glass scale, at 

 the commencement and end of the observation, and the tem- 

 perature recorded at the same times. 



3. Observation of Deflection. 



The wooden box being removed, the metal box and the 

 measuring rod are to be attached to the upper plate of the 

 instrument. One of the smaller magnets is then to be sus- 

 pended ; and the larger magnet being transferred to its sup- 

 port upon the measuring rod, at a fixed distance, the upper 

 plate and telescope are to be turned until the collimator line 



