548 A Magnetic Survey of the British Isles. [Apr. 11, 



tude and direction of the disturbing force at each station was deter- 

 mined. 



It was found that the Malvern Hills attract the north pole of the 

 needle strongly. 



The well-known fact that the difference of the declination at Kew 

 and Greenwich is much greater than the difference of longitude will 

 explain, is found to be connected with a widespread regional mag- 

 netic disturbance within the area of which these observatories lie. 



Several methods of argument all point to the conclusion that the 

 centre of this disturbance lies between Windsor and Reading, and a 

 little to the north and east of the latter town. Towards this point 

 all the calculated disturbing forces in the neighbourhood converge. 



The authors adopt, as a working hypothesis, the view that this 

 attraction is due to the same cause as that observed at the Malverns, 

 viz., the presence of igneous rocks, and they prove that the range of 

 the disturbance extends from Kenilworth to the Channel, and from 

 Salisbury to the North Sea, a total area of about 10,000 square miles. 



As the centre is approached the excess of the observed downward 

 vertical force above that given by calculation increases, and it reaches 

 a maximum at Reading close to the point which a study of the hori- 

 zontal forces had indicated as the centre. 



Extending the same method to the rest of the country, though this 

 has not been studied by them in the same detail as south-eastern 

 England, the authors prove (1) that the results obtained on re- visiting 

 the same station indicate that even in disturbed districts, the direction 

 of the disturbing force can in general be determined by a single set 

 of observations to within 15, and in most cases to within a much 

 smaller limit ; (2) that the directions of the disturbing forces were the 

 same when Mr. Welsh surveyed Scotland in 1857 as they are now ; 

 (3) that the horizontal disturbing forces tend towards districts in 

 which the vertical disturbing force is a maximum ; (4) that certain 

 regions in which crystalline rocks occur display a marked attraction 

 on the needle ; (5) that in certain other regions, and notably in lines 

 running respectively from London to the South Wales coal-field and 

 from the Lincolnshire Wolds to the Lake District, though no crystal- 

 line rocks appear on the surface, magnetic attractions, similar to that 

 observed near Reading, are in play, which indicate the existence of 

 crystalline rocks at no great depth ; (6) that there are in Great Britain 

 five principal regions of the two kinds referred to in (4) and (5), 

 towards which the horizontal disturbing forces act. Their positions 

 may be defined approximately by means of the following lines, which 

 pass through their central parts, viz., (<*) the line of the Caledonian 

 Canal ; (/3) a line somewhat to the west of the basaltic masses in the 

 Western Isles ; (<y) a line passing through the Scotch coal-field, in 

 which crystalline basaltic rocks occur ; () a line certainly parallel to, 



