TERRESTRIAL MAGNETISM. 139 



where all the attendant circumstances are so diligently noted, 

 I find this purely vertical trembling ("constant vertical 

 motion, the needle oscillating vertically") mentioned three 

 times in the case of the declination magnet at Hobarton;( 171 ) 

 and frequent notices of the vibration of the vertical and 

 horizontal force magnets at Toronto when no change took 

 place in their mean readings. 



It appeared to me, that the average hour of the occurrence 

 . of the larger magnetic disturbances at Berlin was the third 

 hour after midnight, and that they ceased most usually at 

 5 A.M. We observed small disturbances in the afternoon 

 between 5 and 7 P.M., often on the same days in the 

 month of September when there ensued after midnight such 

 magnetic " storms/' that the magnitude and rapidity of the 

 oscillations made any reading, or any appreciation of the 

 middle point of the oscillation, impossible. I was very early 

 so convinced that the magnetic storms occur in groups, re- 

 turning on successive nights, that I announced this pecu- 

 liarity to the Berlin Academy, and invited friends to visit 

 me at predetermined hours, to enjoy the sight of the 

 phenomenon : and our. expectations were more often 

 realised than disappointed. ( 172 ) Kupffer, during his journey 

 to the Caucasus in 1829, and afterwards Kreil, in his valu- 

 able Prague observations, corroborated this disposition of 

 the magnetic storms to return at the same hours.^ 173 ) 



Since the establishment of the British Colonial magnetic 

 observatories, the rich mass of materials which they 

 have afforded, and the able treatment of these materials 

 by Colonel Sabine, have caused the facts relating to the ex- 

 traordinary disturbances of the declination, which I had 

 recognised only in a general manner in my equinoctial and 

 solstitial observations of 1806, to become one of the most 



