Magnetic Disturbance on September 9,5th and 96th, 1841 . 151 



Remarks relative to the Magnetic Disturbance at Toronto on the 

 25th and 26th of September, 1841. 



The disturbances appear to have commenced at Toronto nearly at the same 

 absolute time as at Greenwich, and to have been generally simultaneous at 

 both stations. Additional observations were taken at Greenwich early in the 

 morning of the 25th, the needles being in an agitated state, and an aurora be- 

 ing visible. Though additional observations were not commenced at Toronto 

 at this time, the difference between the regular observations at 2 and 4 a.m., 

 Gott. mean time, show a considerable disturbance ; the change of declina- 

 tion between the hours of 2 and 4 amounting to 15'*3; the change of horizontal 

 force to '00444 of its whole value; and of vertical force to -00202 of its whole 

 value. The additional observations at Greenwich were soon after discon- 

 tinued, the disturbance appearing to be over. 



The readings at 10 a.m., Gott. mean time, exhibited so great a change at 

 both stations, that extra observations were resumed at Greenwich, and com- 

 menced at Toronto. 



The additional observations at Toronto were continued without intermission 

 for 20 hours, from 10 a.m., Gott. mean time on the 25th, until 6 a.m., Gott. 

 mean time on the 26th, each instrument being observed at intervals of 5 mi- 

 nutes, in the following order, viz. : Declin. 00" 0^— H. F. 2" 00^— V. F. S"* 

 30^— Declin. 5" 00% &c. 



The disturbance in the first hour (10 to 11) was not very great, compared 

 with those that followed; but in the next 5 minutes, viz. from 11'' 00™ to 

 jjh Q-m^ (.}jg change of declination amounted to 19''6 ; and between ll'' 27'" 

 and ll*' 52*" the horizontal force had decreased by 'OlOSO of its whole value. 

 These changes are specially noted, in consequence of there apparently having 

 been no corresponding disturbance at Greenwich. 



The disturbance at Toronto was at its height from 1 1 a.m. to 4 p.m., Gott. 

 mean time : its general effect appears to have been that of causing a decrease 

 of easterly variation and of total intensitj'. 



The agreement in direction of the changes of horizontal and vertical 

 force deserves remark : the minimum and maximum of each occurred simul- 

 taneously, or as nearly so as can be learned from the observations, the mini- 

 mum of both being observed at the second reading after noon, and the maxi- 

 mum of both at precisely the same interval after the following midnight. 



The observations at Toronto, being continued until midnight of Saturday 

 at that station, which was 6 a.m., Gott. mean time of the 26th, lasted about 

 five hours longer than those at Greenwich ; and during these five hours a 

 second disturbance was observed even greater than the preceding. 



Between !• and 10 p.m., Gott. mean time, the disturbance had been much 

 lessened ; it then increased rapidly, and the changes between 12 p.m. and 6 

 a.m. were very remarkable: in 15 minutes the declination magnetometer 

 moved through an angle of 52''3, and in 25 minutes more had returned 

 tlirough an angle of62'*8 in the opposite direction. Tlie horizontal and ver- 

 tifal-force magnetometers were also greatly disturbed, tiie change in the hori- 

 zontal force amounting in 20 miimtes to -0122 of the whole force. 



Tiie changes of horizontal and vertical force in the last six hours have been 

 reduced to the cfpiivalent changes of inclination and total intensity, and 

 are printed, for the convenience of comparison at these observatories, where 

 the variations of the inclination and total intensity are directly observed. 

 As the tw(f magnetometers were not observed precisely simultaneously, the 

 changes of inclination and total intensity can only be regarded as approximate, 



