248 M. Kreil's Magnetic Observations. 



to be taken from those which are ascribed to the first hour 

 in the morning when the apparatus was observed. It follows 

 that much fewer disturbances begin at that hour than in the 

 evenmg. It is remarkable that a disturbance hardly ever 

 begins in die latter hours of the forenoon. 



.19. The perturbations appear to commence for the most 

 part suddenly, as by shocks. At least it was so in those cases 

 in which the phsenomenon began under our eyes, that is to 

 say, at the time of an observation. It was so on the 18th of 

 February 1837, on which day an aurora showed itself with 

 a magnificence unusual in this country. Two series of transits 

 of a division of the scale across the wire had been observed 

 as usual. In the first series at 4'' 38™ these passnges agreed 

 to a tenth of a second of time; — a proof that the needle was 

 still performing its minute vibrations with perfect regularity ; 

 nor in the preceding observations of the same day was any 

 trace of disturbance to be discovered. In the second series, 

 which was made 12 minutes later, there were differences of 

 2 seconds of time, and the needle was visibly drawn back- 

 wards and forwards by the disturbing forces. I may be per- 

 mitted to mention one more among many similar cases. It 

 is that of the very great disturbance of the 14th of November 

 1837, at which time we were making magnetic observations 

 for three days uninterruptedly from 5 to 5 minutes, and when 

 necessary at still shorter intervals, for the purpose of exami- 

 ning whether the periodical phaenomenon of the falling stars 

 was or was not connected with magnetism. Between 10 and 

 1 1 in the evening the needle appeared pretty tranquil, al- 

 though earlier in the day it had been much disturbed. The 

 observer, Sig. Delia Vedova, was engaged in observing the 

 passage of a division of the scale across the wires, and had 

 chosen for that purpose a division nearly in the middle of the 

 arc of vibration, which was then about 7', when all at once, 

 at 11'^ 7', he saw that this division did not come to the wire, 

 although it had taken its direction towards it ; but before 

 reaching it, the needle had turned the opposite way, and im- 

 mediately after the scale disappeared from the field of view, 

 which was left quite dark. The observer, thinking that the 

 lamp which illuminates the scale had gone out, was about to 

 rise in order to light it again, when he saw the scale suddenly 

 reappear, move rapidly across the field, and disappear on the 

 opposite side. The rapidity of the movement indicated a 

 much larger arc of vibration. The arc had in fact increased 

 40 minutes without any apparent cause. As the observations 

 could not be exact whilst the vibrations were so great, Sig. D. 

 Vedova was about to employ a magnetic bar which is always 



