90 



system in North America, about the meridian of the middle of Cali- 

 fornia, and that of the weaker and moving system, about the meri- 

 dian of the British Islands, having a progressive motion towards the 

 east. Now as the resultant phenomena in the north of America, 

 though influenced principally by the nearer and stronger system, 

 would still exhibit in a slighter degree the influence of the weaker 

 and moving system, the isogonic lines in that part of the globe 

 should have, according to the hypothesis, a movement of translation 

 from west to east conformably to the motion of the weaker system, 

 until the difference in longitude between the poles of the respective 

 systems should amount to 180, an event which would constitute an 

 epoch in the secular magnetic variations, characterized (amongst 

 other circumstances) by the reversal of the motion of the isogonic 

 lines in America, which would thenceforward take place from east to 

 west, as the distance between the poles should diminish on the Sibe- 

 rian side of what Halley termed the American Pole. Now it is well 

 known that the expedition of MM. Hansteen, Erman, and Due, across 

 the continents of Europe and Asia in 1828 and 1829, had, for its 

 principal object, the determination of the magnetic phenomena around 

 the point of maximum attractive force of the weaker or moving sy- 

 stem ; and that the position those gentlemen assigned to it in longi- 

 tude at the time of this expedition was about 115 East of Greenwich, 

 to which meridian it had progressively moved in the interval which 

 had elapsed since Halley assigned its position near the meridian of 

 our Islands. Fully recognizing that in the present, as in the earlier 

 state of magnetical science we can only regard such assignments as 

 approximate, we have still full reason to believe that about the time 

 of the memorable expedition of MM. Hansteen, Erman, and Due, 

 i. e. a few years earlier or a few years later than 1828-29, the epoch 

 must have occurred when the points of greatest attraction of the two 

 systems in the northern hemisphere must have passed through their 

 greatest longitudinal distance from each other, and when, according 

 to Halley' s hypothesis, the direction of .the movement of translation 

 of the isogonic lines in the northern parts of America should be 

 reversed, which we find to have now taken place. 



I have ventured to think that these few remarks, recalling to 

 recollection an hypothesis which was not framed without a most 

 laborious coordination and sagacious grouping of the phenomena 



