524 Prof. Riicker. On the Magnetic Permeability of 



On the other hand, the absence of vertical disturbing force does 

 not necessarily prove that no magnetic matter exists between the 

 magnetic floor and the point of observation. The upper and lower 

 surfaces of a plate, of which the horizontal dimensions are very large 

 compared with the distance of either from the surface, would produce 

 equal and opposite effects over the central parts. 



On approaching the plate from a distance the vertical disturbing 

 force would increase near the edge, and then die out as the centre was 

 approached, the phenomena so far corresponding with those which 

 occur when the observer crosses an underground ridge of magnetic 

 rocks. The two ca,ses, however, could be distinguished by the fact 

 that the direction of the horizontal forces would be the same on both 

 sides of the line of maximum vertical force if the disturbance were 

 due to the edge of a plate, but different if it were produced by a 

 magnetic ridge. 



I now pi'opose, therefore (1), to describe the distribution of vertical 

 force disturbance over England and Wales ; (2) to show that the 

 presence beneath the surface of rocks which possessed in situ the 

 same magnetic properties as basic rocks on the surface would produce 

 disturbing forces of the same order as those which are actually 

 observed. 



In discussing the first point, it will be convenient to measure dis- 

 turbances or departures of the magnetic force from its normal calcu- 

 lated value as terms of O00001 C.G.S. or O'OOOl metric unit, which 

 may be regarded as the unit of disturbing force. Vertical disturbing 

 forces are positive when they urge a north-seeking pole downwards. 



It is fully explained in the published account of the magnetic 

 survey that vertical force disturbances are measured from an arbi- 

 trary datum, and that there is no proof that this is uniform all over 

 the kingdom. 



The largest area of positive disturbance occurs in the east and 

 south-east of England. It extends from the English Channel to the 

 north of Yorkshire, i.e., about 230 miles north and south, and in 

 parts it is about 1 10 miles wide. 



It is, however, deeply cut into by a narrow gulf-like region of 

 negative disturbance in the Midlands, and by others in Kent, Sussex, 

 and South Lincolnshire. 



Taking this into account, and also the fact that in the north it 

 is very narrow, it is fair to assume that it may be approximately 

 represented by a rectangular figure 180 miles long and 108 miles 

 wide. The regions of negative vertical force which bound it are 

 relatively small, say from 15 to about 50 miles wide in the west, 

 while to the north they are wider. 



To the east and south the sea prevents our determining the exact 

 limits of the district. It may, however, be taken as a rough but 



