133 TERRESTRIAL MAGNETISM. 



manifestation is the Aurora or polar light, the oscillations were 

 from 14 to 38 minutes of arc, each oscillation being performed 

 in from 1 to 3 seconds of time. On many occasions the 

 magnitude and inequality of the oscillations, which extended 

 far beyond the limits of the divided scale on one or other, or 

 on both sides, made any observation impossible. ( 169 ) For ex- 

 ample, on the night of the 24th of September, 1806, this was 

 uninterruptedly the case from 14h. 40m. to 15h. 32m., and 

 again for the still longer interval from 15h. 57m. to 17h. 4m. 

 Most often in magnetic storms (the " unusual" or "larger 

 magnetic disturbances") the middle point of the arcs of 

 oscillation was in course of constant though unequal progress 

 to one or the other side, *. e. east or west ; but in other 

 and more rare cases, extraordinary oscillations were remarked 

 without any increase or decrease of the declination, that is 

 to say, without displacement of the middle point of the 

 oscillations from its normal place at the particular hour. 

 After a long time of comparative repose, we sometimes saw 

 very unequal movements take place suddenly (describing 

 arcs of from 6 to 15 minutes, the different magnitudes being 

 either alternately or irregularly intermixed), and then the 

 needle would as suddenly become calm again. It was at 

 night that such alternations of total repose, and violent 

 fluctuation without progressive movement towards either 

 side, \\ere particularly striking. ( 17 ) Another peculiar 

 modification of these movements should be mentioned ; it 

 occurred very rarely, and consisted in a particular kind of 

 affection of the inclination of the north end of the needle, 

 continuing for ] 5 or 20 minutes of time, with only very 

 moderate horizontal fluctuations, or even with an entire ab- 

 sence of any. Jn the registers of the British observatories, 



