534 Prof. Riicker. On the Magnetic Permeability of 



investigation, I think, supplies for the first time a definite answer to 

 the enquiry whether the mere presence in the earth's magnetic field 

 of concealed magnetic rocks, such as those which exist on the surface, 

 would suffice to account for the observed local or regional magnetic 

 disturbances in districts where the superficial deposits are non-mag- 

 netic. 



The question is not answered by pointing to the large disturbances 

 produced close to basic rocks, for these may be, and probably often 

 are, due to permanent magnetism. But, as this is very irregularly 

 distributed in the surface rocks, we cannot regard it as a probable 

 cause of widespread disturbance, though locally it may produce very 

 intense effects. 



It is, I think, answered in the affirmative by the above discussion. 



In gauging the value of the answer, it must be remembered that 

 only one of the various constants involved has been at our disposal, 

 and that it would not have been possible to imitate the observed 

 results by assigning to it appropriate values, whatever arbitrary 

 assumption had been made as to the others. 



Thus, in the simple case of a rectangular magnetic slab, the average 

 vertical force produced over it in virtue of the earth's inductions, 

 depends on five variables, viz., its length and breadth, the distance of 

 its upper and lower horizontal surfaces from the surface of the earth, 

 and its permeability. Of these, the first two have been defined by the 

 observed magnitude of the areas of high vertical force. The depth 

 of the lower surface has not been fixed to suit the exigencies of the 

 argument, but deduced from the temperature at which iron ceases to be 

 magnetic. The permeability is at most that given by experiment on 

 the specimens of basic rocks from the west of Scotland. Only one dis- 

 posable constant remained, viz., the depth of the upper surface, and by 

 shifting this we can do no more than raise the average disturbing 

 force to a certain maximum, which might have been much less than 

 the observed disturbances. As a matter of fact, however, by choosing 

 suitable depths we are able to obtain forces of exactly the right order 

 of magnitude. With the constants chosen there appears to be some 

 difficulty in obtaining correct relative values of the vertical and hori- 

 zontal disturbing forces, though even here the order of their magni- 

 tude is unaffected. This is exactly the kind of difficulty which might 

 almost certainly be expected in calculations based only on more or less 

 probable assumptions. It will be diminished by any change which 

 increases the intensity of the calculated forces, and there are several 

 possible causes which might produce such an effect. 



Thus no account has been taken of the increase in the permeability 

 of magnetite with temperature. This is certainly a vera causa, and 

 will tend to bring observation and calculation into closer agreement. 



Again, the high specific gravity of the earth, as compared with that 



