SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 631 
with the view that the Stratford-on-Avon ridge line is continued parallel to this 
valley and joins the Thames Valley ridge line near Reading. The magnetic peak, 
which we formerly discovered near Reading, appears to lie between Henley-on- 
Thames and High Wycombe, ^.e., a little further north than we anticipated. This 
point is not specially marked by very high Vertical Forces, though Reading and 
Henley lie on a curious line of maximum Vertical Disturbance which is almost a 
closed curve. Its form suggests the question v^hether it is an example of the effect 
ol a flat slab of magnetic matter with steep sides near the edges of which the Vertical 
Disturbance would be greater than over the central parts. 
We are not inclined to insist on any such interpretation, partly because v/e have 
but few stations in the central region, and partly because the corrections to the 
Vertical Forces, on transference from the isomagnetics of either survey to the mean are 
very large in this part of England. There can, however, be no possible doubt that 
the Ilsley Downs and the Chiltern Hills, from Marlborough to Tring, are a region of 
high Vertical Force, to which a number of magnetic ridge lines converge. 
One of these we have already traced. Another was described in our previous 
Memoir as running due south from Reading to the English Channel. A number 
of additional stations have confirmed our conclusions. This result is important, as 
M. Moureaux, who has for some years been engaged on a magnetic survey of France, 
has discovered a corresponding line which enters that country near Dieppe, and passes 
southwards beyond Paris. 
Another ridge can be followed eastwards from Reading to Deal parallel to, but 
about 15 miles north of, the Nortli Downs. Two stations of negative Vertical 
Disturbance occur close to the ridge at Dartford and Gravesend. 
At these the arrows point away from the ridge, but the occurrence of a positive 
Vertical Disturbance on the north bank of the Thames at Purfleet may prove that 
the ridge line is in this neighbourhood double; traces of a valley line connecting 
Stanmore and Southend can also be detected. 
In Mid-Kent and Sussex the Horizontal Disturbing Forces are extraordinarily 
small, being less than the limit of accurate determination of direction at a group 
of nine neighbouring stations. A very small change in the positions of the Terrestrial 
Isomagnetics would completely alter the apparent direction of the Horizontal 
Disturbing Forces at all these places. 
It is, however, interesting to observe that the Disturbing Forces at St. Leonards 
(a repeat station), which appear to be very different when the earlier and later 
observations are compared with the isomagnetics of the earlier and later survey 
respectively, are in good accord when referred to the mean lines. 
I he absolute values of the Vertical Disturbing Forces in this part of England must 
still be considered doubtful, though this does not in any way weaken the proof that 
they are relatively large near the North Kent ridge line. 
Returning now to the west of the Reading region of high Vertical Force, the ridge 
