On the Secular Change in the Magnetic Dip in London. 237 



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 

 attain, respectively, the maximum and the minimum amount which 

 constitute its limits under the system of secular change, an inter- 

 mediate epoch might be anticipated, when the isoclinal lines passing 

 across the British Islands should attain their least angle of inclination 

 to the geographical meridian ; towards which they should have pro- 

 gressively advanced, and from which they would as progressively re- 

 cede. Now, if we compare the line of 70° of dip in the Isoclinal 

 Map of 1780 of the Magnetismus der Erdewith that of 1840 in Mr. 

 Keith Johnstone's Physical Atlas, plate 23, we may fix on a point 

 in about 42° North Latitude and 30° West Longitude, in which the 

 Dip has remained nearly stationary, and through which the line of 70° 

 of Dip passed, at both epochs ; and we may perceive that, in its easterly 

 course from that point or pivot, this line passed in 1 780 through the 

 middle of France considerably to the South of Paris (where the Dip 

 was then between 71° and 72°) ; whereas in 1840 it passed across 

 England considerably to the north of London (where the Dip had 

 diminished to little more than 69°). Therefore in the sixty years 

 which had elapsed between the two epochs, 1780 and 1840, the di- 

 rection of the lines as they impinged upon Western Europe had 

 become much less inclined to the geographical meridian (i. e. forming 

 a greater angle with the parallels of latitude) in 1840 than in 1780 : 

 and if we consult still earlier maps, we find that a change in the same 

 direction had been progressive from a still earlier period. The par- 



