36 Mr. C. Brooke on a remarkable Magnetic Disturbance. 



as a favourable specimen of the present state of magnetic 

 registration. 



The scale of time is marked on the photographic base-line, 

 and is about 12 inch to one hour; the hours denote Got- 

 tingen mean astronomical time. The scale of angular space 

 is 5 inches to 1° in azimuth, and is measured in a direction 

 perpendicular to the base-line, the position of which corre- 

 sponds nearly to 23° il' of W. declination. 



During 22'^ and 2'6^ the W. declination ranged between 

 22° 17' and 23° 37', but the greatest change is that which is 

 represented in Plate I. This comprises the variations of de- 

 clination from 24"^ 11*^ to 22^. The declination is observed to 

 decrease rapidly from 23° 14' at about ll'^ 40™, to 21° 53' at 

 about 12^ 5™; and after several considerable fluctuations, to 

 increase to 23° 35' at about 13'^ 13™, indicating a variation of 

 1° 42' in little more than one hour. The breaches of conti- 

 nuity in the line during these large excursions, indicate that 

 the magnet has not rested at any one point long enough to 

 mark the paper. The least time during which the tracing 

 pencil of light will produce a visible impression is probably 

 about 4^ 



Between IG** and 19'' the excursions of the magnet are so 

 incessant and considerable, that periodic observations, even at 

 the least periods at which an observer could continuously 

 record them, would scarcely give an adequate ideaof the pro- 

 gress of the disturbance. This vibratory movement of the 

 magnet probably corresponds with the "peculiar mechanical 

 agitation " mentioned by Captain Lefroy in an account of the 

 disturbance of the 24th of September at Toronto, published 

 in the November Number of this Journal, and is quite distinct 

 from mere oscillation of the magnet about its mean place. The 

 secondary vibrations accompanying the larger excursions of 

 the magnet are very well shown between 18'^ and 21^^. These 

 have very rarely been observed in the photographs thus ob- 

 tained during the last two years. They would appear to 

 indicate the simultaneous existence of two distinct disturbing 

 causes; of which that producing the larger and more gradual 

 excursions is far more frequently in operation. 



The register has been disconnected at 18^ for the conve- 

 nience of the Plate; and the register of the bifilar maoneto- 

 meter, obtained on the same paper,in order to avoid confusionj 

 has been altogether omitted. It may however be remarked, 

 that at the time when the variation of horizontal force was 

 most considerable, amounting to more than one-tenth of the 

 whole force, the northern hemisphere was brilliantly illuminated 

 with coruscations, as described by Mr. Glaisher, and at this 

 period the disturbance of the declination was not proportion- 



