THE SIX GATEWA YS OF KNOWLEDGE. 259 



We all know a little about the mariner's com- 

 pass, the needle pointing to the North, and so on ; 

 but not many of us have gone far into the subject, 

 and not many of us understand all the recent 

 discoveries in electro-magnetism. I could wish, 

 had I the apparatus here, and if you would allow 

 me, to show you an experiment in magnetism. 

 If we had before us a powerful magnet, or say 

 the machine that is giving us this beautiful electric 

 light by which the hall is illuminated, it, serving 

 to excite an electro-magnet, would be one part 

 of our apparatus ; the other part would be a piece 

 of copper. Suppose then we had this apparatus, 

 I would show you a very wonderful discovery 

 made by Faraday, and worked out admirably by 

 Foucault, an excellent French experimenter. I 

 have said that one part of this apparatus would 

 be a piece of copper, but silver would answer as 

 well. Probably no other metal than copper or 

 silver certainly no other one, of all the metals 

 that are well known, and obtainable for ordinary 

 experiments possesses, and no other metal or 

 substance whether metallic or not, is known to 



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