Rocks and Regional Magnetic Disturbances. 



529 



west between Doncaster and Brigg, which arc near the centre of the 

 southern loop of the figure of eight is about 27 miles, or 45 kilom. 



It may, therefore, fairly be assumed that the order of the forces 

 within the district will be the same as that of those over a rectangle 

 60 kilom. in length and 30 in breadth. The linear dimensions of such 

 a figure are somewhat less than the extreme length and breadth of 

 the district, but its area is 1*6 times greater. Let us then superpose 

 upon the plateau a rectangular mass, 60 kilom. long by 30 broad, and 

 3'5 deep, the upper surface of which is, therefore, at a depth of 

 0-5 kilom. or 1638 feet. 



The vertical disturbing forces due to this mass along a line passing 

 over its centre and parallel to its longer edge, are as follows : 



Distances are expressed in kilometres. 



Taking the mean of the numbers at the edge and at a distance of 

 5 kilom. from it as applying to the district between them, the mean of 

 the positive vertical disturbances is 83. 



If this minor mass were placed on the larger one, so that their 

 longer edges were parallel, and that the median line of the smaller 

 mass was 20 kilom. from the edge of the larger one, thus correspond- 

 ing with the position 8 in fig. 3, p. 527, and if, lastly, the distribution 

 of magnetic matter were as is shown in that figure, then the mean 

 vertical disturbance along the median line of the small mass would be 

 101 + 83 = 184. 



In comparing the observed and calculated values it is convenient 

 to take the mean force in the negative districts as a datum ; so that 

 the disturbance as calculated is 95 + 184 = 279, and as observed 

 80 + 236 = 316. 



This latter number is obtained by excluding the high values at 

 Market Weighton, Butterwick-on-Trent, and Market Rasen, but 

 these can easily be imitated by placing small masses on the upper 

 surface. 



A plate, 5 kilom. square and 0'25 kilom. thick, approaching the 

 surface to within 0'25 kilom., or 820 feet, would increase the force by 



VOL. XLVIII. 2 



