188 TERRESTRIAL MAGNETISM. 



that rejiulsive forces are in play, or that there is another magnetic 

 ridge line still farther to the south. Dr. Fritsche thinks that these 

 observations explain the gravitational anomalies without recourse to 

 the somewhat forced hypothesis of a vast subterranean cave. He 

 assumes that there is a concealed mass of iron which approaches near to 

 the surface at Moscow and also along two loci to the south and north of 

 the city. He attributes the magnetic irregularities to the attraction 

 of the central iron hill, the detlections of the plumb line to the flanking 

 masses. It is perhaps not inconceivable that such results might follow 

 in a special case, but without the support of calculation it certainly 

 appears that the magnetic experiments point to the existence of the 

 principal attracting mass under the town. This is in fact the arrange- 

 ment shown in the figure with which Dr. Fritsche illustrates his 

 hypothesis. If this is so, the theory would prima facie seem to require 

 that the bob of a plumb line should be attracted toward and not, as 

 is actually the case, away from the center of the magnetic disturbance. 

 On the whole, then, though the coexistence of large magnetic and 

 gravitational disturbances in the same place is suggestive, I do not 

 think that they have as yet been proved to be different eifects of the 

 same hidden mass of magnetic matter. 



In a few weeks an international geodetic conference will meet at 

 lunspruck, at which the Royal Society will be represented. It is, I 

 believe, intended to extend the detailed investigation of the relations 

 between the nature of the earth's crust and the gravitational and 

 magnetic forces to which it gives rise. We may therefore hope that 

 special attention will before long be given to localities where both 

 may combine to give information as to facts outside the range of the 

 ordinary methods of geology. 



The second phenomenon on which more light is desirable is the per- 

 manent magnetization of magnetic rocks. It is known that fragments 

 of these are strongly but irregularly magnetized, but that the effect of 

 very large masses at a distance appears to be due to induced rather 

 than to permanent magnetism. There are three questions to which I 

 should like an answer: Are underground masses of magnetite ever 

 permanently magnetized ? Are large areas of surface masses — say a few 

 hundred square yards in extent — ever permanently and approximately 

 uniformly magnetized in the same sense? Is there any relation between 

 the geological age and the direction of the permanent magnetism of 

 magnetic rocks'? 



Inquiries such as these can only be taken up by individual workers, 

 but I venture to think that the comparison of the observatory instru- 

 ments and the fluctuations of secular change outside the observato- 

 ries could best be investigated under the auspices of a great scientific 

 society. The cooperation of the authorities of the observatories will 

 no doubt be secured, but it is most important that the comparison 



