Royal Institution. 315 



one instance as strikingly showing how these phsenomena appeared 

 to indicate a distinct localization of their cause. Thus there were 

 two disturbances of horizontal force at five stations (Catharinenburg, 

 St. Petersburg, Greenwich, Gottingen, Milan), occurring at an in- 

 terval of about a quarter of an hour; one of them showed increase 

 of force at all the stations, the other showed decrease at the two first- 

 named stations and increase at the others ; it appeared evident here, 

 that the cause of the first was exterior to Europe, and the cause of 

 the second was within Europe. 



The division of the subject to which the lecturer then came was 

 the cause of these phsenomena. He illustrated by a model Hansteen's 

 conception of two large magnets within the earth, stating that he 

 understood it to be put forward only as an imaginary construction, 

 generally (but not very accurately) representing the facts, butnot to 

 be taken for a representation of a real state of things. He then 

 adverted to Gauss's beautiful and general investigation of the effects 

 of a magnetic earth, supposing that every part of it was magnetic 

 in every conceivable variety of manner and degree ; and stated that, 

 by proper adaptation of certain constants in this general theory (a 

 theory which it is totally impossible to express in ordinary language), 

 all the recorded observations of the mean positions of the magnets 

 might be well represented. But M. Gauss had stated the following 

 as one consequence of the theory : — Supposing that every part of 

 the earth has equal magnetism in the most favourable direction for 

 producing the known effects with the smallest expense of power, 

 then the quantity of magnetism in one cubic metre of the earth is 

 equal to the magnetism of eight of the best steel magnets weighing 

 1 lb. each. This, in the lecturer's opinion, made the whole theory 

 difficult to be received. 



Connected with the theory of general magnetism of the earth, is 

 Canton's explanation of the diurnal inequality. He supposed that 

 if there were, near the equator, two magnets in north and south 

 positions, one more east and the other more west than England, the 

 rising sun would heat the eastern magnet, and thus (by a law which 

 applies to steel magnets) would diminish its magnetic power, and 

 the effect of the western magnet would then turn the English needle 

 to a position verging more to the north-west and south-east, until 

 the two magnets were equally heated. 



The lecturer then exhibited experimentally CErsted's discovery, 

 that a simple hehx of wire, through which a galvanic current passes, 

 possesses all the properties of a bar-magiiet, its opposite ends ex- 

 erting opposite effects upon one pole of a magnet, and these effects 

 being reversed upon testing it on the other pole of the magnet. 

 From this it followed naturally, that a model of a sphere surrounded 

 by a spherical helix carrying a galvanic current would nearly re- 

 present the condition of magnetism upon the earth, — and Barlow's 

 experiment to that effect was exhibited : and it was shown that its 

 action on a free dipping-needle is generally similar to the earth's 

 action. He then adverted to Lubeck's discovery — that the applica- 



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