154 Magnetic Disticrbance on September 25 fh and 26th, 1841. 



than the average of the preceding twenty-four days of the month, by the 

 quantities shown in the above table ; and at the Gottingen hours of noon, 

 2, 4, and 6 p.m., when great disturbances were taking place at Greenwich 

 and Toronto, the observations at Trevandrum show a decrease of intensity 

 much exceeding the usual fluctuations. Mr. Caldecott has annexed the fol- 

 lowing remark to the readings at these hours : — " These irregular readings 

 " examined into at the hours they were made, and found not to arise from 

 " any instrumental irregularity. — J. C." 



On the 26th, being Sunday, no observations were made ; but the following 

 table, exhibiting the mean position of the magnetometer in each day in Sep- 

 tember, shows that during the remaining days of the month the horizontal 

 intensity did not return to its previous average value ; corresponding with the 

 remark deduced by Professor Kreil from ten perturbations observed at Prague, 

 namely, that " the horizontal intensity remains weaker for some time after the 

 " great oscillations have ceased, and only gradually resumes its ordinary 

 " force*." 



Declination Magnetometer. — The effect of the disturbance on the declina- 

 tion magnetometer at Trevandrum appears to have been comparatively small. 

 The north end of the magnet was, however, during the whole day to the east- 

 ward of its average position at the same hours in the preceding part of the 

 month, as is shown in the subjoined table. The second part of the table ex- 

 emplifies the small amount of the fluctuations which take place from day to 

 day in the mean position of this magnetometer at Trevandrum. The mean 

 position for the month, corrected for torsion of the thread, was 253*48 scale- 

 divisions, or the mean declination for the month = 0° 43' 45*7" East. A 

 scale-division in this instrument = 39*85"nearly. 



* Letter to Lieut.-Col. Sabine, translated in Phil. Mag., Third Series, vol. xvii. p. 429. 



