632 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
line turns north through Stroud, then bends west to Ross. A centre of attraction is 
very clearly marked on the borders of Hereford and Brecon. 
The directions of the arrows indicate its position very exactly, as in the Black 
Mountains due west of Pontrilas. After this the line is deflected south. There is 
another centre of attraction about 10 miles north of Swansea, and finally the ridge 
line enters the Irish Channel a little to the north of Haverfordwest. 
A third centre of attraction is situated in the Welsh Coal Field, but, as it is 
separated fiom the magnetic ridge we have just discussed by a valley line, we include 
it in the next district, the exact constitution of which is, however, by no means 
clear. 
District 15. — The Bristol Channel. 
This district extends on each side of the Bristol Channel, from Monmouth to the 
south of Dorsetshire, and from Exmoor to the eastern end of the Mendips. 
Its most remarkable characteristic is the smallness of the Horizontal Disturbing 
Forces. Counting stations on the valley line, it includes 28 stations, and at 16 of 
these the Horizontal Disturbing Force is less than the limit of the accurate deter¬ 
mination of direction. 
There is certainly a centre of attraction to the north of Cardiff and Newport, and 
the magnetic cols at Dolygaer (Merthyr Tydfil) and Blakeney suggest that this is 
closely connected with the main ridge line to the north. 
We made a number of observations near the Mendips. These hills appear to be 
on a magnetic ridge line, but the Horizontal Disturbing Forces are so small that the 
evidence is not very conclusive, and no clear connection is established with the 
surrounding districts. 
District 16. — Devonshire. 
In our previous memoir we came to no conclusions as to the magnetic state of 
Devonshire and Cornwall. The number of stations at which observations had been 
made was too small. We have now traced a ridge line through Devonshire which 
may, perhaps, run into the Bristol Channel District. 
The Vertical Disturbing Force is a maximum at Drewstelgnton, to the north of 
Dartmoor. 
District 1 7. — CornivaU. 
At all the stations on the South Coast, from Weymouth to the Land’s End, the 
Disturbing Force is directed southwards, as though there were a ridge line in the 
Channel. This general tendency is as clear close to the serpentine of the Lizard as 
on less disturbed ground. 
