152 Magnetic Disturbance on September 25th and 26tk, 1841. 



though the difference in the times of observation having been only 1^ minute, 

 the errors occasiontd flieicbj' are probablj' seldom verj' great. 



The observations of tiie second disturbance show a striking connexion be- 

 tween the changes of declination and intensity, an increase of force corre- 

 sponding to an increase of easterly declination, and vice versa : tiie same con- 

 nexion WcUs observed in the Toronto observations of the 29th of May, 1840. 



Auroras. — An aurora vas visible at Greenwicli both in the morning and 

 evening of the 25th. At Toronto the morning was heavily clouded, with rain, 

 consequently no aurora could be seen : in the evening the aurora Avas visible 

 at intervals from 7 to 10 p.m., Toronto time, or 1 to 5 a.m., Gdtt. time, the 

 period of the second great disturbance ; the remainder of the night was 

 heavily clouded. A gale of wind occurred on the following day (26th), and 

 in the evening another aurora was seen. 



The extreme changes of the declination, horizontal, and vertical force, du- 

 ring the two disturbances, were as follows: — of the declination, 1" 05' ; of the 

 horizontal force, "02438 of its whole value; and of tlie vertical force, •01288 

 of its whole value. The days of occurrence, and the extreme ranges of the 

 principal disturbances observed at Toronto in 1840 are subjoined, in order 

 that the relative extent of the present disturbance may be estimated. 



Dates. Extreme Ranges. 



1840. Declination. Horizontal Force. Vertical Force. 



o / 



May 29 1 59 Off scale, exceeding '044 Off scale, exceeding "024? 



August 28 1 44 0-03521 0-00846 



September 22. . 1 01 Not observed, being out of adjustment. 



25.. 30 0-00184 ' 0-00112 



December 21.. 122 0*01522 0-01074 



1841. 



Sept. 25 and 26 1 05 0-02438 0-01288 



An aurora was observed on each of the above days in 1840; that of the 

 29th of May was the most brilliant of any seen since the establishment of the 

 Observatory. Very few additional observations Avere taken either on the 22nd 

 or 25th of September, 1840; on the last day especially, the few that were 

 taken were not commenced until the greater part of the disturbance was ap- 

 parently over ; consequently the actual changes were, in all probability, much 

 greater than those observed ; the recurrence of so great a disturbance on the 

 same day in the following year is remarkable. Additional observations 

 have occasionally been taken in the course of every month in 1841, in con- 

 sequence of unusual disturbances, but the changes have never equalled those 

 above-mentioned. 



The months of September and October appear to be those of greatest dis- 

 turbance. 



Gauss's arrangement of the scale and mirror has been adopted for the 

 horizontal-force magnetometer at Toronto, in consequence of the disturb- 

 ance of the 29th of May, 1840, having driven the magnet beyond the range 

 of the collimator scale. The range of the declination scale (on Professor 

 Lloyd's plan) being about 6°, — or three times greater than the extreme range 

 hitherto observed, — there is little probability of its ever being exceeded. 





