EARLIEST ISOCLINICS AND FORCE OBSERVATIONS. 403 
Whiston’s directions are fairly trustworthy, they will furnish 
a valuable contribution to our knowledge of the secular vari¬ 
ation of the dip over England. In the accompanying sketch 
the mean isoclinics are shown by full lines for the epochs 
1720, 1837, 1860, and 1886, as drawn by Felgentraeger. It 
will be seen that between 1720 and 1837 the isoclinics moved 
anti-clockwise and are now moving in the contrary direction. 
It would be interesting to determine when this motion was 
reversed. The first one to call attention to this retrograde 
motion w T as, I believe, Sabine,* who says: “ The angle of in¬ 
tersection of the meridian and isoclinics has been diminish¬ 
ing up to about 1840, when a reversal took place, and the 
angle is now increasing.” So the position for 1837 is prob¬ 
ably not far from the lower western extreme position. The 
angle of shift between 1720 and 1837 appears large. Doubt¬ 
less enough observations made between 1720 and 1837 may 
be found to enable us to draw intermediate positions of the 
* Sabine (Sir Edward). Proc. Royal Soc., 8°, London, 1862, vol. xi, p. 144. 
