ON THE MAGNETIC DISTURBANCE AT TORONTO, &C. 

 Declination Magnetometer, Trevandrum, September 1841. 



349 



Mr. Caldecott having arrived at Trevandrum in May 1841, and established 

 his magnetic instruments in a temporary building imtil 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 

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

 hours of the same day. 



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 movemsnt of tiie 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 25th. During these 

 twenty-six hours of consecutive observation, the declination-magnetometer 

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

 instruments each at intervals of 10 minutes 

 the subjoined tables. 



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

 which underwent frequent fluctuations of unusual amount, and sustained, on 

 the whole, a considerable diminution of intensity. Between 2 p.m. and 8 p.m. 

 of the 24th, the loss of for(;e amounted to about -0048 of its whole value ; 



The observations are given in 



