528 Prof. Riicker. On the Magnetic Permeability of 



Although, therefore, the method of testing the theory is necessarily 

 very rough, it is nevertheless evident that the range of the vertical 

 forces over a mass of magnetic matter of almost the same area as the 

 region of positive vertical force disturbance in England, and within 

 60 kilom. of its edge, might agree fairly with the facts if the edges of 

 the mass were steep, and if its magnetic properties were the same as 

 those of the Mull basaltic rocks. The observed range is 140, and 

 widely differing hypotheses give calculated values between 107 and 

 159. 



It is not, however, sufficient to account for the differences of 

 vertical force over large areas. Within there are smaller but still 

 large districts in which the vertical force disturbance considerably 

 exceeds the mean, and the explanation of the phenomena by rock 

 magnetism would be imperfect if the calculated forces were insuffi- 

 cient to explain the characters of these. It might easily be that, to 

 produce an average vertical force disturbance equal to that which 

 obtains over the whole plateau, it was necessary to bring the upper 

 surface so near to the surface of the earth that the remaining depth 

 was insufficient to account for the additional forces in play on the 

 area of greater disturbance within it. 



The largest area of very high vertical force which has at present 

 been mapped in detail lies in Lincolnshire and South Yorkshire. 



The following table exhibits the names of stations within it, and the 

 vertical force disturbance at each : 



If these places are connected by straight lines, an irregular figure of 

 eight is produced, the area of which is about 425 square miles, or 

 1100 square kilom. 



It is, of course, unlikely that the stations are all close to the edge 

 of the attracting mass, but, on the other hand, it may be deeply cut 

 into by valleys or regions of less disturbance. The latter hypothesis, 

 if correct, would make it easier to explain the high forces, whereas 

 an extension of the district beyond the limits actually observed 

 makes any such explanation more difficult. The distance between 

 the most northerly and most southerly stations (Sutton-on-Derwent 

 aud Market Rasen) is 45 miles, or 75 kilom. The distance east and 



