GENERAL BEARINGS OF MAGNETIC OBSERVATIONS. 113 



are there large ''regional" magnetic disturbances extending over large 

 areas of land, but that in moderate depths of water where the largest 

 ship can navigate freely, the land below is also found to have consider- 

 able areas of local magnetic disturbance which, if not allowed for, 

 may in thick or foggy weather lead ships into danger by seriously 

 disturbing their compasses. 



The United States has done excellent work in producing charts of 

 isomagnetic lines, or charts in which the chief local magnetic disturb- 

 ances are recognized, and the full results of observation recorded. 

 The magnetic surveys of Eiicker and Thorpe in the British Isles, of 

 Moureau in France, of Bijckevorsel in Holland and elsewhere, have 

 thrown considerable light on the magnetic conditions of those countries, 

 but there remain whole continents to be covered by the observer. 



The direction of the isomagnetics, too, from the deep sea to the dry 

 land of the coasts is an extension of the subject which the observer 

 has hardly touched as yet, but 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 magnetized than the land 

 areas. 



To possess charts of isomagnetic lines for even a few countries is an 

 evidence of considerable advance in the knowledge of terrestrial mag- 

 netism, for if reference be made to Sabine's lines of intensity in his 

 contribution on the magnetic survey of Northwest America it will be 

 found that he rejected certain observations he considered abnormal and 

 defective, which Lefroy, the observer, considered 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 giv- 

 ing normal curves, Lefroy isomagnetics. 



Eespecting 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 

 Eiicker and Thorpe have answered the question by the results of their 

 laborious survey of the British Isles, coupled with Bucker's elegant 

 investigations as to the permeability of specimens of the rocks taken 

 from the localities in which magnetic disturbances 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 different per- 

 meability, either present as in the basalts on the surface or concealed 

 by superficial deposits. 



These results are distinct from the extraordinary disturbances of the 

 needle when in the immediate vicinity of permanently magnetized 

 rocks, and when the radius of disturbance 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 disturb- 

 ance are not confined to the magnetician, for the geologist can not 

 SM 95 8 



