Magnetic Disturbance on September 25th and 26tli, IS-il. 153 



Postscript, Dec. 14th. — Whilst these pages were in the press, I received, 

 through the kindness of Mr. Caldocott, the abstract of the observations made 

 in September at the Magnetic Observatory instituted by the Rajah of Tra- 

 vancore, at Trevanurum, in hit. 8° 30' 35-2" N., and long. 5^ 7" .59^ E. ; by 

 which I am enabled to state, that an unusual magnetic disturbance took place 

 in India simultaneously witli those observed in England and America: al- 

 though, in consequence of the easterly position of Trevandrum, the com- 

 mencement only of the disturbance Mas observed there. Each observatory 

 discontinued its observations when its own Saturday night arrived ; thus the 

 observations at Greenwich continued five hours later than at Trevandrum, 

 and at Toronto five hours later than at Greenwich ; the latest observations, in 

 each case, showing the continuance of the disturbance. 



The observations at Trevandrum consist of the regular two-hourly read- 

 ings of the three magnetometers, day and night. By comparing the positions 

 of the magnetometers at each of the magnetic hours of the 2.5th of Septem- 

 ber, with the mean position at the same hour in the previous twenty-four days, 

 we obtain what we may consider a measure of the magnetic disturbance of 

 that day. As the disturbance was indicated principally by the horizontal- 

 force magnetometer, we may commence with the comparison of that instru- 

 ment, premising that as the inclination at Trevandrum is only — 2" 50', the 

 horizontal intensity at that station comprises nearly the whole magnetic in- 

 tensity, the vertical component being extremely small. 



Horizontal- Force Magnetometer-, Trevandrum, September 25, 1841. 



During the 24th of September the horizontal intensity differed little from 

 its mean position, at the same hour, since the commencement of the month, 

 until 10'' 28'" I'.M., Trevandrum time, being the last observation of the day, 

 when it was weaker than the average at that hour by an amount c(iual to 

 about 7 scale -divisions. During the whole day of the 25th it was weaker 



