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diminution of the Dip in London might Le expected to continue until 

 the epoch should arrive when, by the easterly movement of translation 

 of the minor magnetic system in the northern hemisphere, the dis- 

 parity of the magnetic force prevailing in the European and American 

 portions of the hemisphere should have attained its maximum : which 

 is certainly not yet the case. 



Is there then, in the secular change of the Dip, no feature in 

 which, in conformity with the Halleian hypothesis, an alteration 

 might be expected to synchronize with the reversal in the direction 

 of the secular change of the declination ? Assuredly there is ; and 

 the facts which recent investigations have brought to our knowledge 

 manifest that such an alteration has taken place. I proceed to de- 

 scribe it. 



If we have recourse to those extensive generalizations which, under 

 the name of "Isoclinal Lines corresponding to particular Epochs,'* 

 present a connected view of the changes which have taken place from 

 time to time in the magnetic lines of the Dip over large portions of 

 the earth's surface, and enable us to anticipate with some degree of 

 confidence the changes which may be expected to take place in years 

 to come, we notice generally that the lines undergo two species of 

 modification, or peculiarities of change, which it is necessary to keep 

 separately and distinctly in view. In the British Islands, for example, 

 the Isoclinal Lines for little less than two centuries past have been 

 steadily advancing towards the north by a gradual movement of trans- 

 lation. This is one feature of the secular change ; but there is a 

 second feature, which, if not at first sight equally striking, is yet 

 equally regular and systematic in its operation ; viz. the direction of 

 the isoclinal lines as they pass across our country from the south-west 

 towards the north-east undergoes a small but sensible change from year 

 to year, by which, in the lapse of several years, the angle at which 

 they cut the geographical meridians is materially altered. By the 

 joint operation of these two processes, the general configuration of the 

 lines over large portions of the earth's surface, as well as their values 

 in particular localities, are both subject to systematic alteration; a 

 remark which is not limited to the isoclinal lines alone, but is the case 

 also in the isogonic and isodynamic lines. Those who are conversant 

 with Dr. Halley's writings, will be aware that, in correspondence 

 with his views, between the epochs when the Dip in London should 



