Magnetic Disturbance on September 25tk and 26th, IS^l. 155 

 Deelinatio7i Magnetometer, Trevandrum, September 1841. 



Mr. Caldecott having arrived at Trevandrum in May 184-1, and established 

 his magnetic instruments in a temporary building until they were removed 

 into the permanent observatory early in October, had not at that time deter- 

 mined the value of the readings of the Vertical-Force Magnetometer. For 

 this reason a complete account of the disturbance of that component of the 

 force cannot be given ; and considering the very small amount of the whole 

 vertical force at Trevandrum, it may perhaps be sufficient to state, that the 

 readings of the instrument indicate an unusual difference in the force at the 

 Gdttingen hours of noon, 2, 4, and 6 p.m., from its value at the other magnetic 

 hours of the same daj'. 



Second Postscript, Dec. 20th — Whilst the Plate (I.) accompanying this 

 notice was still in the engraver's hands, the arrival of the abstracts for the 

 month of September from the magnetic observatory at St. Helena has fur- 

 nished the means of adding to the present account the observations of this re- 

 markable disturbance made at that station. 



An unusual movement of the magnetometers appears to have been noticed 

 by Lieut. Lefroy at an earlier period than at any observatory from which ac- 

 counts have yet been received ; extra observations having been commenced at 

 St. Helena between 11 and 12 a.m., Gott. mean time, on Friday the 24th. 

 At 2 P.M., the disturbance appearing to have subsided, they were discontinued, 

 but were resumed at 8 p.m., and continued thenceforward without intermis- 

 sion for twenty-six hours, until midnight of Saturday the 25i\\. During these 

 twenty-si.\ hours of consecutive observation, the declination-magnetometer 

 was observed at intervals of 5 minutes, and the iiorizontal and vertical-force 

 instruments each at intervals of 10 minutes. The observations are given in 

 the sul)join('d tables. 



The element principally affected at St. Helena was the horizontal force, 

 which underwent fre(juent fluctuations of unusual amount, and sustained, on 

 the whole, u considerabh; diminution of intensity. Between 2 p.m. and 8 p.m. 

 of the 2tth, the losb of force amounted to about -0048 of its whole value : 



