PRODUCT OF MAGNETIC ENERGIES. 19 



from the magnet's pole. The experiment of vibration 

 may be extended to a position of the needle much 

 nearer to the center of the magnet than is the magnet's 

 pole. And thus we arrive at the Third Law of 

 Magnetism, that the magnetism collected in or near 

 the pole of a magnet acts equally, as to sense, in all 

 directions. In this respect, magnetism resembles gravi- 

 tation. (The law of force, as depending on the distance, 

 will be a subject of future inquiry.) 

 -. 



13. Experimental proof of the Fourth Law of 

 Magnetism, that the attraction or repulsion (as the case 

 may be) between two masses of magnetism, estimated as 

 a statical force, is proportional to the product of their 

 magnetic energies. Definition of the units of the ele- 

 ments used in succeeding investigations ; novel unit of 

 statical pressure. 



Without at present giving an algebraical or nu- 

 merical definition of magnetic energy, it may be under- 

 stood as being, in needles of similar form, proportional 

 to the force by which, under the action of terrestrial 

 magnetism, the red end is drawn to the north and the 

 blue end to the south. The successive steps of experi- 

 ment bearing upon the law now under consideration 

 will be the following : 



(a) Provide a light axis capable of receiving, at 

 pleasure, one, two, three, or more needles, made as 

 similar as possible, and charged as nearly as possible 

 with the same amount of magnetism. (This is easily 



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