MAGNETIC DISTUKBANCES. 135 



case for long and uninterrupted periods during the night 

 of the 24th September, 1806, lasting on the first occasion 

 from 2h. Om. to 3h. 32m. A.M., and next from 3h. 57m. 

 to 5h. 4m. A.M. 



In general, during unusual or larger magnetic disturbances 

 (magnetic storms), the mean of the arc of the oscillations 

 exhibited an increase either westward or eastward, although 

 with irregular rapidity, but in a few cases, extraordinary 

 fluctuations were also observed, even when the variation 

 was not irregularly increased or decreased, and when the 

 mean of the oscilli 'ions 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 

 violent perturbation without any progression to either side 

 was very striking. 70 One special modification of the motion, 



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

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

 out a change in the variation. He wrote to me as follows, in the course 

 of that year : " I have communicated to the Academy the results of 

 our simultaneons observations. I am surprised to notice the oscilla- 

 tions which the dipping needle occasionally exhibited at Berlin during 

 the observations of 1806, 1807, and of 18281829, even when the mean 

 declination was not changed. Here (at Paris) we never experience 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 frequently unaccompanied by any 

 oscillatory movement." The condition here described is, however, en- 

 tirely opposite to the phenomena which were observed at Toronto 

 (43 91' N.lat.) during the years 1840 and 1841; and which correspond 

 accurately with those manifested at Berlin. The observers at Toronto 

 have paid so much attention to the nature of the motion that they 

 indicate whether the vibrations and shocks are " strong" or "slight," 

 and characterise the disturbances in accordance with definite and 

 uniform subdivisions of the scale, following a fixed and uniform nomen- 

 clature. Sabine, Days of Unusual Magn. Disturbances, vol. i, pt. i, 

 p. 46. Six groups of successive days (146 in all) are given from the 

 two above-named years in Canada, which were marked by very strong 

 shocks, without any perceptible change in the horary declination. 

 Such groups (see op. cit. pp. 47, 54, 74, 88, 95, 101), are designated as 

 " Times qf Observations at Toronto, at which the magnetometers were dii' 



