258 



REPORT — 1861. 



Table III. 



The direction of (he isoclinal lines in England thus found for January 1, 

 1860, is from N. 71° 22' E. to S. 71° 22' W. The direction found in the 

 previous survey (by observations at 132 stations by five observers) was from 

 N. 65° 05' E. to S. 65° 05' W. (Brit. Assoc. Report, 1838, pp. 85 and 86). 

 The central geographical positions are only a few miles distant from each 

 other, being respectively 52° 38' N., and 2° 07' W. in 1837, and 52° 20' N., 

 and 1°41'W. in 1860. From the large amount of the difference in the 

 direction of the lines at the two epochs (6° 17'), it is scarcely possible to 

 doubt that in the interval between 1837 and I860 a real change has taken 

 place in this respect, and that the isoclinal lines passing across England have 

 increased the angle which they make with the geographical meridians ; a 

 change implying that in the interval the secular diminution of the dip has 

 been greater on the West than on the East side of the island. 



In the survey of 1837, r was found =0'-575, and in that of 1860=0'-624'; 

 the geographical distance between the lines has therefore increased in the 

 interval in the proportion of 0'-624 to 0'-575 ; a change implying that the 

 secular diminution of the dip has been greater in the Southern than in the 

 Northern parts of England. 



The difference in the rate of secular change on the east and on the west 

 sides of England may be also shown directly by the comparison of the obser- 

 vations at two stations, Margate and Lew Trenchard, one on the east and 

 the other on the west side ; the stations were common to both surveys, the 

 observer being the same at the two periods and the localities identical : the 



