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attain, respectively, the maximum and the minimum amount which 

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

 mediate epoch might he 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 1 780 of the Magnetismus der Erde with 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 1780 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- 

 ticular year in which this feature attained its limit, and an opposite 

 change commenced, cannot now perhaps be precisely determined ; it 

 was probably somewhat earlier than 1840. But from the comparison 

 of the magnetic surveys of the British Islands in 1836-37 and 

 1857-58, it is certain that the change in the direction of the isoclinal 

 lines in this part of the globe has entered upon the contrary phase 

 to that which had previously existed. The observations of the late 

 Mr. Welsh in Scotland in 1857-58 (Brit. Assoc. Reports, 1859), 

 when compared with those of the Scotch Survey made in 1836-37, 

 published in the British Association Reports for 1836, show, according 

 to Mr. Balfour Stewart's calculation, that an increase of several degrees 

 in the angle at which the lines cut the meridians in passing across 

 Scotland has taken place between the epochs of the earlier and the 

 later surveys. The same general conclusion follows from a comparison 

 of the magnetic surveys of England at nearly the same epochs ; every- 



