BRITISH ISLES FOR THE EPOCH JANUARY 1, 1915. 
f).3 
agreement in the horizontal components, while in the vertical component the 
residnals are distinctly greater than those obtained in the new survey. 
Moreover, the district vertical residuals of the older survey do not follow any very 
obvious law of dependence on the geographical co-ordinates, whereas in this element 
in the new survey the district residuals could be much reduced by supposing that 
there is a line of -f-vertical disturbance running from the south of Ireland to the 
north of England, while as we pass to N.W. or S.E. from this line, the downward 
vertical force diminishes. In this connexion it appears not unlikely that a reduction 
of Rucker and Thorpe’s observations by the method used in the re-survey, may be 
of considerable interest. 
The district table shows that both surveys agree in giving nearly the same 
decrease of W in passing from District V. to VI., and from VIII. to IX., and the 
result is important in Rucker and Thorpe’s theory of “ Ridge and Valley Lines.” 
The Disturbing Forces deduced for the new survey are shown on Chart 5. The 
arrows represent the magnitude and direction of the horizontal forces on a scale of 
1 mm. = 20y. The base of the arrow is the station, and the number adjoining is 
the vertical disturbance in units y, the + sign indicating a downward force. 
This chart may be compared with that prepared by Rucker and Thorpe. 
In the main qualitative features there is a general agreement between the 
disturbing forces in the two surveys. Thus, on the principles used by Rucker and 
Thorpe, the “ ridge” lines along the Caledonian Canal, from Ayr to Edinburgh, from 
Greenwich to Milford Haven, and from Portsmouth to Nottingham, are indicated by 
both surveys. Further, the peculiar features near the Wash and in the Pv,eading 
vicinity are exhibited in both. But, on the other hand, the evidence for the 
Yorkshireridge ” line in the new survey does not appear to me very conclusive, 
nor do I think that a simple “ ridge ” from Portsmouth to Reading adequately 
accounts for the data in the re-survey. 
Moreover, quantitative analysis has led me to such results that one must suppose 
that in some cases the disturbing forces are either of a very local origin, or else 
indicate the existence of maefiietic material on a somewhat colossal scale. I therefore 
introduce here a digression on the quantitative explanation of disturbing forces 
before resuming the main argument. 
If the “ disturbing forces ” assigned to the stations of a magnetic survey are 
regarded as independent and of very local origin (say within a range of 100 metres) 
their physical interpretation is easy, but of very local value. But if the disturbances 
are correlated even on a parochial scale (say 10 km.) a quantitative explanation 
leads to conclusions of a somewhat startling character. The quantity of magnetic 
material required for such explanation is specially great when the observed disturbing 
force is upwards. 
My attention was directed to this matter, in the first instance, when I considered 
the disturbing forces at Strachur and Lochgoilhead. I shall return to this case later, 
