On Rapid Periodic Variations of Terrestrial Magnetism. 7 



the latter effect seems to be due chiefly to the rigidity of the 

 suspension wire, it is advisable to use as fine a wire as possible. 

 In any future work, a thorough annealing of the wire after 

 hanging the suspended system is very desirable. 



The tripod support of the instrument rested on a thick glass 

 l^late, which was provided with a hole and a V-groove for 

 receiving the feet of the levelling screw, and was rigidly fixed to 

 the face of the stone pedestal. 



The lens in front of the instrument case was replaced with 

 another with a focal length of 5 m. 



The deflecting magnetic rod for determining the sensibility, 

 was fixed to a special holder'^ such as is usually attached to 

 Mascart's magnetograph for a similar purpose, and placed on the 

 support pasted on the pillar C3 in the same meridian as the 

 instrument. After starting the photographic apparatus, the 

 deflecting magnet was brought in position, i.e. horizontally in WE 

 direction, and reversed every 3 or 5 minutes. The magnetic 

 moment of the rod mostly used was M = 77 e.g. s. at the room 

 temperature. The distance of the deflecting rod from the needle of 

 the instrument was 123 cm., so that with the rod " side on," the 

 change of the WE-component corresponding to the reversal of the 

 deflector was 8'3 "/'. The sensibilit}'' was so adjusted as to make 

 the corresponding deflection nearly 50 mm.; i.e. 1 mm. on the 

 record corresponded to 0"17 ;-. The sensibility, though fairly 

 constant, showed occasionally a tendency to decrease slightly after 

 running for 24 hours, though not at all so serious as to affect the 

 general aspect of the results obtained. It was therefore considered 

 preferable to test the sensibility at least once every day and 

 redetermine the constant. This gradual decrease of sensibility is 

 probably due to the influence of a slight elastic time-effect of the 

 suspension wire, made apparent on account of the extreme position 

 of the bifilar sj^stem. The presence of a sensible time-effect in 

 the wire used, is also made evident on the photographic record, 

 wdien an abrupt deviation is effected by means of the deflecting 

 magnet. After an immediate deflection, a slow creeping up of the 



1) Mascart, Magnétisme Terrestre, p. 195, Fig. 46 c. 



