The Astronomer Royal on a recent Magnetic Disturbance. 507 



in 2 s or 3 s the needle frequently moved through many mi- 

 nutes of arc, and then suddenly became stationary again. 



At 3 h 36 m 20 s p.m., a bold sweep carried the north end 

 of the needle towards the west 56' of arc, in one minute 

 of time; at 3 h 37 m 20 s the theodolite read 245° 22'—; and 

 for two minutes after this time the cross carried by the needle 

 was out of range of the telescope *, being in a position, in 

 which, if the telescope had been exactly directed to it, the 

 theodolite would have read less than 24-5° 22'. At 3 b 40 m 20 s 

 p.m., Gottingen mean time, the needle had moved back 25'; 

 at 3 h 43 m 50 s the theodolite read 246° 19'; at 3 h 44 m 20 8 the 

 needle had moved so far (north end towards the east), that the 

 theodolite reading was 247° 38' + , and the cross carried by the 

 needle was out of range of the telescope on the opposite side. 



This extraordinary magnetic stride of more than 2^°, was 

 traversed in about 8 m , the extremes being only separated by 

 that interval of time. 



At both extremities the needle was without swing, all mo- 

 mentum from such a wide sweep being quite destroyed. 



After remaining at this place for more than two minutes, 

 the needle slowly returned, and at 3 h 48 m 54 s the theodolite 

 read 246° 27' ; at 3 h 53 m 21 s it read 246° 37'. Within 5 s after 

 this, another bold sweep carried the cross out of the range of 

 the telescope ; when found, at 3 h 54 m 40 s , the theodolite read 

 245° 31'+ ; instantaneously it rushed back, with a violently 

 agitated motion, across and out of the field on the other side; 

 the circle-reading, at 3 h 55 m s , was 246° 43' + , and there- 

 fore this arc of 1° 12'+ was passed over in 20 s of time. 



This was the last very great excursion ; the needle was in- 

 cessantly watched for 9 h longer, during which time it was 

 much agitated. The most remarkable changes are the fol- 

 lowing : — 



is o ' 



Gottingen mean time, theodolite read 246 8 



34 



8 



21 



10 



* This arose from the great inclination of the axis of the collimator (carried 

 by the magnet) to the magnetic meridian ,• in consequence of which, the 

 pencil of rays passing through the object-glass of tlje collimator was thrown 

 entirely on one side of the object-glass of the theodolite. In ordinary 

 cases, the change of angle has been sufficiently slow to allow the observer 

 to shift the suspension of the magnet, but the rapid changes on Sept. 25 

 did not allow the observer to leave his place. Observations with the 

 mirror in Gauss's manner are free from this inconvenience. 



2 L 2 



