444 Magnetic Surveys in the British Isles. [Apr. 17, 



ing forces was due to some source of error, stations in the extreme 

 south of the Hebrides would in all probability be similarly affected. 

 If the directions of the forces were due to a physical cause, such as a 

 centre of attraction out at sea to the west of Tiree, then the disturb- 

 ing forces in the Southern Hebrides would almost certainly be directed 

 southwards towards it. 



The observations made last summer prove (1) that the direction of 

 the disturbing horizontal force at Bernera, which is the southernmost 

 island of the Hebridean group, is due south ; and (2) that, as this 

 point is approached from the north, the downward vertical disturbing 

 attraction on the north pole of the needle regularly increases, which 

 exactly agrees with the supposition that a centre of attraction is 

 being approached. 



There is, therefore, now no doubt that there is a centre of attraction 

 on the north pole of the needle to the south of the Hebrides and to the 

 west of Tiree. 



(3) In one of the maps communicated to the Society last year we 

 drew two lines, bounding a district about 150 miles long and 40 miles 

 broad, in Yorkshire and Lincolnshire, and gave reasons for the belief 

 that a ridge line or locus of attraction lay between them. 



This conclusion has now been tested by means of thirty- five addi- 

 tional stations, with the following results : — (1) At all stations (with 

 one exception) on or near the two lines, the horizontal disturbing 

 forces tend towards the centre of the district they bound. 



(2) The downward vertical disturbing forces are greater in the 

 centre of the district than at its boundaries. In particular, there 

 are two well-marked regions of very high vertical force. 



(3) The greatest vertical force disturbances occur at Market 

 Weighton, where the older sedimentary rocks are known to approach 

 the surface, and at Harrogate, which is on the apex of an anticlinal. 



(4) The central ridge line runs from the Wash parallel to the line 

 of the Wolds to Brigg. Thence it appears to turn west, and reaches 

 Market Weighton via Butterwick and Howden. One or two addi- 

 tional stations are, however, required to determine whether this bend 

 is real, or whether the line runs direct from Brigg to Market 

 Weighton. From the latter town it passes to the limestone district 

 of Yorkshire and traverses its centre. It has not yet been traced 

 west of the Jine of the Midland Railway between Settle and Hawes, 

 but there is ground for believing that it continues to the Lake 

 District. 



Although, therefore, one or two points of detail remain for furthe?" 

 investigation, the existence of a line of attraction 150 miles long w 

 proved beyond the possibility of doubt, and for about 90 miles its 

 position is known "bo within 5 miles. 



There are, then, even in those parts of England where the super- 



