BEITISH ISLES FOR THE EPOCH JANUARY 1, 1915. 
69 
would have to run a little west of Swindon, round by Salisbury, Southampton and 
Portsmouth. Then it would turn rather sharply, and passing to the north of 
Chichester, pass between Haslemere and Horsham and so towards Windsor and 
Reading. A gradual increase in the intensity as we move from tlie boundary 
towards Reading is required. 
The inferences in this case differ a little from those of Rucker and Thorpe, partly 
on account of the difference in the data, but mainly on account of the guiding 
principle used. This principle consists essentially in regarding the disturbing sources 
as doublets distributed over a considerable area, in place of single poles in a 
concentrated form. It is thus an extension of the method used by Rucker and 
Thorpe, the necessity for which was clearly adumbrated in their second memoir. 
We shall next consider the vicinity of the Malvern Hills. This case is of special 
interest because four points very close together were selected by Rucker and Thorpe. 
The data contained in the table'are shown graphically in fig. 6. The lines in red 
refer to Rucker and Thorpe’s survey and those in black to the re-survey. The 
horizontal disturbing forces are shown in magnitude and direction by full lines on a 
scale 1 mm. = lOy, and the numbers are the vertical disturbances. Rucker and 
Thorpe observed the inclination at station B only. 
The results of the two surveys ar»e in very good agreement and undoubtedly prove 
the attraction created by the Malvern Ridge. A quantitative explanation would be 
greatly facilitated by additional observation at the mean point. Another point of 
importance is suggested. The mean horizontal disturbance for the four stations is 
from the old survey N = —63, W = —21, and from the re-survey N = —79, 
W = —15. Now if this resulting disturbance is of any value in correlating Malvern 
with other stations over a wide area, the true effect of the Malvern Hills is better 
represented by compounding this mean force reversed with the original values. The 
result is shown by dotted lines. If, however, the original values are due solely to the 
Malvern Hills, our mean station is not representative and we have no right to use the 
mean value to connect Malvern with stations at a distance. The matter may be 
settled by direct observation at a new station to the south of the Hereford Beacon. 
It seems to me vital in the whole question of the explanation of disturbing forces to 
prove that the selected station is representative of at least a parochial area, and the 
Malvern Hills provide a particularly good test case deserving of further experimental 
investigation. 
We now pass to consideration of the Leicestershire region. This case is interesting 
on account of the presence of iron ore, known to exist in this part of the country. 
The value of the vertical disturbance at Loughborough ( — 39) appears to be some¬ 
what anomalous, but the enquiry slip from Greenwich states that the magnetographs 
there were very much disturbed during the time of my observations at Loughborough, 
and that the values supplied could not be relied on for my purpose. We ought, 
therefore, to attach little weight to the values for Loughborough, although it happens 
