Which occurred on the 2nd and Uh of July, 1842, 139 



vibratory movement, notwithstanding the magnitude of the 

 changes. 



There seemed to be a faint auroral light in the N.W. hori- 

 zon, but without streamers. 



When the regular observations were recommenced, on 

 Monday the 4-th instant, the disturbing forces were found to 

 be still in activity. At 2 and 4 a.m. the instruments showed 

 a very considerable decrease of declination, accompanied by a 

 great decrease of horizontal intensity. At 6 a.m. the declina- 

 tion exceeded the mean of the hour by a still greater amount j 

 and the horizontal intensity had also increased, though still be- 

 low its mean value. All the magnets were then vibrating 

 through very large arcs. The series of observations at short 

 intervals was then begun, and continued (almost without in- 

 terruption) for ten hours. 



At 6 h 24 ra the declination reached its maximum, the devia- 

 tion then amounting to 43'2 minutes. The horizontal inten- 

 sity also attained its maximum very nearly at the same mo- 

 ment. The two elements then began to diminish rapidly and 

 simultaneously; and between 7 and 8 a.m. there was a double 

 minimum of both, separated by an intervening maximum, 

 that of the horizontal intensity taking place a few minutes 

 earlier than the other element. 



At 9 a.m. the disturbance was extremely rapid. The mag- 

 nets were hurried to and fro with a violent movement; and 

 these changes of mean position were accompanied by a large 

 vibration, amounting in some instances (notwithstanding the 

 copper rings) to 20 divisions of the scale. 



This combination of movements rendered it difficult to seize 

 the moment of greatest deviation, or to determine its precise 

 amount. The declination attained a minimum at 9 a.m., 

 which was followed by a marked maximum at 9 1 * 22 m , the 

 range of the oscillation being 29*4 minutes. There was a 

 corresponding change of the intensity, but somewhat later in 

 time, — the minimum occurring at 9 h 14 m , and the maximum 

 at 9* 1 50 m ; and the range amounting to *0147» 



The changes of declination which occurred afterwards did 

 not present any remarkable features ; but the horizontal in- 

 tensity, which was previously less than in its mean state, after 

 reaching a minimum at l h 44 m , suddenly increased to an 

 amount exceeding its mean value, and reached a maximum 

 at 2 h 5 m p.m. The period of this maximum was characterized 

 by a sudden increase of the arc of vibration, as if by impulse. 

 The intensity continued above its mean value (though with 

 some considerable oscillations) during the remainder of the 

 time of observation. The disturbance ceased about 5 p.m. 



The induction inclinometer was observed, in conjunction 



