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one affecting the safety of navigation, as well as the question 

 that has been raised, whether the water areas of the globe 

 are as a whole more, or less magnetised than the land areas. 



To possess charts of iso-magnetic lines for even a few 

 countries is an evidence of considerable advance in the 

 knowledge of terrestrial magnetism, for if reference be 

 made to Sabine's lines of intensity in his contribution on 

 the magnetic survey of North-West America it will be 

 found that he rejected certain observations he considered 

 abnormal and defective, which Lefroy the observer con- 

 sidered to be his best and naturally retained in his map ; 

 the result being a considerable difference in the form of the 

 curves adopted by the two magneticians, Sabine giving 

 normal curves, Lefroy iso-magnetics. 



Respecting the local disturbances of the needle which 

 have been so clearly proved, the question naturally arises, 

 whence the cause of these disturbances ? It is now believed 

 by many, if not finally accepted, that Rlicker and Thorpe 

 have answered the question by the results of their laborious 

 survey of the British Isles, coupled with Riicker's elegant 

 investigations as to the permeability of specimens of the 

 rocks taken from the localities in which magnetic dis- 

 turbances were found. Their answer is to the effect that 

 these disturbances, which have been found to extend over 

 a region 230 miles long by about 110 miles broad, are 

 due to induction by the earth's magnetism in rocks of dif- 

 ferent permeability, either present as in the basalts on the 

 surface or concealed by superficial deposits. 



These results are distinct from the extraordinary dis- 

 turbances of the needle when in the immediate vicinity of 

 permanently magnetised rocks, and when the radius of dis- 

 turbance may be only as many feet as the extent of the 

 regional disturbance is in miles. 



The points of interest in the question of regional 

 magnetic disturbance are not confined to the magnetician, 

 for the geologist cannot afford to neglect the valuable in- 

 formation the magnetic needle affords. Thus although 

 Rlicker and Thorpe have since made a second and more 

 elaborate survey of the British Isles, their remark of 1890 



