On the Magnetic Permeability of Rocks, fyc. 505 



" On the Relation between the Magnetic Permeability of 

 Rocks and Regional Magnetic Disturbances." By A. W. 

 ROCKER, M.A., F.R.S. Received May 30, Read June 19, 

 1890. 



The investigation, of which an account is given in this paper, was 

 undertaken with the object of throwing light on the causes of local 

 magnetic disturbances. The two main theories which have hitherto 

 been proposed attribute local perturbations of the needle to earth 

 currents and to magnetic rocks respectively. 



In the Bakerian Lecture for 1889 (' Phil. Trans.,' A. 1890, p. 53), 

 Dr. Thorpe and I compared the directions of the disturbing magnetic 

 forces found by us to exist near Melton Mowbray and Reading with 

 the results of a survey of the local earth currents made in the neigh- 

 bourhood of those places under the direction of Mr. Preece, F.R.S. 

 No connexion could be traced between either the intensities or the 

 directions of the currents and the magnetic forces, and the result of 

 the investigation was thus opposed to the view that they are cause 

 and effect. As far as I am aware, however, no attempt has hitherto 

 been made to determine whether the mere presence in the earth's 

 magnetic field of such iron-bearing rocks as actually exist, and which 

 must certainly produce magnetic distui-bances, suffices to account for 

 such disturbances as are actually observed. This enquiry is obviously 

 complementary to the comparison of the disturbing forces with the 

 earth currents in the same neighbourhood, but the necessary data 

 have only lately been accumulated. The recent magnetic survey has 

 for the first time placed at our disposal facts as to the magnitude of 

 the disturbing magnetic forces in the United Kingdom, and the 

 measurements described below give us some idea of the order of the 

 magnitude of the permeabilities of magnetic rocks. The present in- 

 vestigation is thus divided into two parts, viz. : 



(1.) A determination of the magnetic susceptibility of a number of 

 rock specimens. 



(2.) An enquiry as to the order of the magnitude of the magnetic 

 disturbances which the presence of such rocks in the earth's magnetic 

 field would produce. 



The first part has been carried out by Mr. Hightield, Assoc. 

 N.S.S., and Mr. Jarratt, Scholar Elect of Trin. Coll., Cambridge, 

 both of whom are students in the Physical Laboratory of the Normal 

 School of Science and Royal School of Mines. I am under a great 

 obligation to these gentlemen for their share in the work. They 

 constructed all the special apparatus required, and have made all the 

 measurements recorded in this paper, and I am indebted to them not 

 only for care and skill, but also for several very useful suggestions. 



