MAGNETIC DISTURBANCES. 



131 



stance, was the case for long and uninterrupted periods dur- 

 ing the night of the 24th September, 1806, lasting on the 

 first occasion from 2h. Om. to oh, o2m., and next from 3h. 

 57m. to 5h. 4m. A.M. 



In general, during unusual or larger magnetic disturb- 

 ances (magnetic storms), the mean of the arc of the oscilla- 

 tions exhibited an increase either westward or eastward, al- 

 though with irregular rapiditj, but in a few cases extraor- 

 dinary fluctuations were also observed, even when the vari- 

 ation was not irregularly increased or decreased, and when 

 the mean of the oscillations did not exceed the limits apper- 

 taining to the normal position of the needle at the given 

 time. We saw, after a relatively long rest, sudden motions 

 of very unequal intensity, describing arcs of from 6^ to 15^, 

 either alternating with one another or abnormally inter- 

 mixed, after which the needle would become suddenly sta- 

 tionary. At night this mixture of total quiescence and vio- 

 lent perturbation, without any progression to either side, was 

 very striking.* One special modification of the motion, which 



This was a small disturbance, which was succeeded by great calm 

 until 5h. 6m. A.M. ^ ^^p*-, 1806, about 2Ii. 46ra. A.M. a great but 

 short magnetic storm, followed by perfect calm. Another equally 

 great magnetic disturbance about 4h. 30m. A.M. 



The great storm of |^| September had been preceded by a still 

 greater disturbance from 7h. 8m. till 9h. 11m. P.M. In the following 

 winter months there was only a very small number of storms, and 

 these could not be compared with the disturbances during the au- 

 tumnal equinox. I apply the term great storm to a condition in which 

 the needle makes oscillations of from 20 to 38 minutes, or passes be- 

 yond all the scale parts of the segment, or when it is impossible to 

 make any observation. In small storms the needle makes irregular 

 oscillations of from five to eight minutes. 



* Arago, during the ten years in which he continued to make care- 

 ful observations at Paris (till 1829), never noticed any oscillations 

 without a change in the variation. He wrote to me as follows, in the 

 course of that year : " I have communicated to the Academy the re- 

 sults of our simultaneous observations. I am surprised to notice the 

 oscillations which the dipping-needle occasionally exhibited at Berlin 

 during the observations of 1806, 1807, and of 1828-1829, even when 

 the mean declination was not changed. Here (at Paris) we never ex- 

 perience any thing of the kind. The only time at which the needle 

 exhibits violent oscillations is on the occurrence of an Aurora Borealis, 

 and when its absolute direction has been considerably disturbed, and 

 even then the disturbances of direction are most frequmihj unaccom- 

 panied by any oscillatory movement." The condition here described 

 is, however, entirely opposite to the phenomena which were observed 

 a,t Toronto (43° 91' N. lat.) during the years 1840 and 1841, and which 

 nrrespond accurately with those manifested at Berlin. The observ- 

 ftrs at Toronto have paid so much attention to the nature of the mo- 



