284 
MR. A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
Vertical Force should occur at the same point, and fig. 18 shows that we find it to be 
at Reading, which is our nearest station to the two points found as above described. 
All three elements then combine to indicate that the centre of the Thames Valley- 
disturbance lies between Reading and Windsor, and is a few miles to the north and 
east of Reading. 
Pig. 17. 
Horizontal Force disturbances in terms of O'OOl metric unit. 
+ Indicates that the obsei’ved is greater than the calculated value of H. 
- Locus of no declination disturbance. 
-Locus of no horizontal force disturbance. 
The Map of Vertical Force disturbances, however, teaches us a good deal more. If 
we draw contour lines to enclose all stations at which the Vertical disturbing Force is 
greater than O’OlO and 0’015 metric unit respectively, the first comprises two 
independent curves, the one embracing a large area around the focus, and the other 
surrounding Chichester. 
If the cause of the phenomenon were an underground mass of igneous rock we might 
picture it as a sub-terrestrial mountain of which the peak is near the focus. The 
slope would be most rapid towards the south-east. Two ridges would run north-east 
and north-west towards Oxford and Cambridge, a third, less lofty, to Chichester, and 
a fourth, nearly due east, terminating very abruptly at Kew. If all this were so, the 
