156 Magnetic Disturbance on September 25th and 26th, 1841. 



this weakened state remained, witli little change, until 2 a.m. on the 25th, 

 when the force again augmented, and had regained about half the original 

 loss at 10 A.M., the period when the disturbances at Greenwich and Toronto 

 occasioned extra observations to be commenced at those observatories. At 

 this epocli anotlier great diminution of the horizontal force took place at St. 

 Helena, and the intensitjr continued to weaken until 7'' 42'" 30^ p.m., when 

 the loss since 10 a.m. amounted to "OllS, and since noon of the preceding 

 day, to '015 of the whole force. The magnetic inclination at St. Helena being 

 — 21°, the horizontal force forms by much the larger component of the total 

 magnetic force : the approximate absolute horizontal value in English units 

 may be taken at 5'9. 



In the account of the Toronto observations it has been noticed that the 

 greatest disturbance of the instruments took place after the observations at 

 Greenwich had been discontinued ; namely, after the midnight of Saturday at 

 Greenwich. St. Helena being nearly in the same meridian as Greenwich, 

 the observations ceased at nearly the same time and i'or the same reason : St. 

 Helena does not therefore afford any observations corresponding to those of 

 the second disturbance at Toronto. 



On Sunday 26th, the instruments were observed at 9 and 11 a.m., 3 and 8 

 P.M.; and on Monday 27th, extra observations Mere commenced at 1'' 30™ a.m., 

 at intervals of 30*", and continued until ] 1 a.m., when the intervals were 

 changed to 5"' for the declination-magnetometer, and to 10"^ for each of the 

 force-magnetometers, the extra observations being finally discontinued at 

 midnight of that day. 



A table is subjoined of the mean positions of the magnetometers at the 

 several magnetic hours during the month of September, showing the mean 

 diurnal curve for the month of each element ; also a table of the mean po- 

 sitions of the instruments on each day of the month, showing the monthly 

 curve. It is seen by the latter table that the horizontal force was considera- 

 bly below its average intensity on the 25th, and that it did not wholly recover 

 the loss before the end of the month. 



The fluctuations of the declination at St. Helena, as at Trevandrum, were 

 far less striking or remarkable than those of the horizontal intensity, or than 

 those of the declination at Toronto and Greenwich. The magnetic disturb- 

 ances at the tropical stations have not however always this character. During 

 a disturbance observed at St. Helena on the 26th of June, 1840, the extreme 

 range of the declination-magnetometer amounted to 1° 20'; and during se- 

 veral of the most extensive movements of the declination bar, Lieut. Lefroy 

 states that the horizontal-force bar remained perfectly at rest. The follow- 

 ing passages are extracted from Lieut. Lefroy 's report of the disturbance 

 here referred to (26th June, 1840), on account of their striking resemblance 

 to some of the remarks in Mr. Airy's account of the recent disturbance at 

 Greenwich : — 



" On taking the reading for noon, my attention was called to the disturbed 

 " state of the magnet ; I found it making rapid and irregular movements, with 



" sudden jerks and momentary pauses, too rapid to allow readings Two suc- 



" cessive movements frequently occurred in the same direction ; and another 

 " unusual circumstance was, that the magnet, after a violent movement, came 

 " instantly to rest ; it is for this reason that but two readings instead of three 

 " are sometimes given." 



With a view to a more complete examination of these singular phenomena, 

 it is deserving of consideration, whether some slight modification might not 

 be introduced in the mode of observing on such extraordinary occasions. For 

 example, peculiarities of movement might be more advantageously studied 

 und described by an observer, whose attention was not distracted by the ne- 



