TEURESTRIAL MAGNETISM. 115 



Petersburg. From observations with a dipping-needle 

 at 8 A.M. and 10 P.M., the mean north dip appeared to be 

 greatest at 8 A.M. 



Toronto, in Canada. Prom hourly observations during 

 5J years with horizontal and vertical force magnetometers, 

 a principal maximum is found in all months of the year 

 about the hour of 4 P.M., occurring, however, somewhat 

 earlier from April to September than from October to 

 March ; and a principal minimum about 1 A.M. or 11 A.M., 

 occurring also earlier from April to September than from 

 October to March. The progression from the maximum at 

 10 or 11 A.M. to the minimum at 4 P.M. is continuous and 

 rapid. From April to September the inclination increases, 

 with occasional very slight interruptions, from the minimum 

 at 4} P.M. to the maximum at 10 A.M. At this season, 

 therefore, the horary variation scarcely differs from a single 

 progression, the decrease taking place in the six hours from 

 10 A.M. to 4 P.M., and the increase, more slowly, in the 

 remaining eighteen hours. In the opposite season, i. e. from 

 October to March, a secondary maximum shows itself at 

 from midnight to 2 A.M., and a secondary minimum at 

 about 6 A.M. (Sabine, Toronto, Yol. ii. p. Ixx.) The north 

 dip is greater in the six months when the Sun is in the 

 southern signs (75 17''S4) than in the six months when 

 the Sun is in the northern signs (7 5 1 6'*5 7) . The intensity 

 of the total force is also greater by about two-thousandth 

 parts of its whole amount in December and January, when 

 the Earth is nearest to the Sun, than in June and July, 

 when the Earth is most distant from the Sun. (Toronto 

 Obs. Vol. ii. pp. Ixxxvii. and xcii. and xciii.) 



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