1891.] on Magnetic BocJcs. 419 



regional magnetic disturbances are due to the inductive action exerted 

 by the earth's magnetic field on rocks. It is, however, necessary to 

 discuss the arguments which can be brought forward on the other 

 side of the question. 



The neighbourhood of Melton Mowbray is the source of a consider- 

 able magnetic disturbance. Mr. Preece, F.R.S., was good enough to 

 cause an earth-current survey to be made between the post offices in 

 that district. The earth currents appeared in all cases to run out 

 from Melton. This might raise a doubt as to whether the currents 

 were not largely due to small differences between the earth plates 

 causing them when connected to act as a battery. If this is so the 

 differences of potential to which the earth currents are due must be 

 less than those due to the plates. 



If, however, we assume that real earth currents were measured the 

 directions were not in all cases such as would produce the observed 

 deviations of the magnet. The potential differences were also much 

 less than those which at Greenwich produce or are at least connected 

 with similar deflections of the needle. The difference is not small. 

 If in both cases the earth currents are the cause, equal potential dif- 

 ferences must produce at Greenwich magnetic effects a hundred times 

 less powerful than those produced at Melton Mowbray. 



Perhaps, however, the strongest argument against the earth- 

 current theory is based on Captain Creak's generalisation as to the 

 magnetic properties of islands. If opposite poles are attracted in the 

 two hemispheres, disturbance currents must circulate round the 

 island in opposite directions. No adequate physical cause has been 

 suggested why current eddies of contrary directions of circulation 

 should be produced in the two hemispheres. 



If then we accept the view that the balance of evidence at present 

 inclines towards the rock theory, it is evident that in a survey of 

 magnetic disturbances the lines towards which the magnet is at- 

 tracted are in general loci of nearest approach of the magnetic rocks 

 to the surface, or of centres of highest magnetic susceptibility, or of 

 both of these combined. 



It is thus possible that from such observations we may learn 

 something as to the distribution of basic rocks at depths far below 

 those which ordinary geological methods can reach. It is therefore 

 interesting to note that the results obtained in the United Kingdom 

 have received a remarkable confirmation from France. Correspond- 

 ing to a ridge (or locus of attraction) which runs south from 

 Reading and enters the channel near Chichester, is another which 

 emerges from the channel almost exactly oj)posite to it and passes to 

 the south of Paris. The southern termination is not yet known, but 

 the magnetic disturbance increases as the latitude diminishes. There 

 can be little doubt that a well-marked locus of attraction for the north 

 pole of the needle runs from Reading to the south of Paris. 



[A. W. R.] 



