TRANSACTIONS OF SECTION A. 553 



ances of considerable range are so intense as to suggest that material of very higli 

 magnetic permeability must be present. 



If the concealed magnetic matter were iron, and if it were present in large 

 quantity, it is evident that the results of experiments with the magnetometer and 

 dip circle might be supplemented by observations made with the plumb-line or 

 pendulum. In such a case the region of magnetic disturbance would also be a 

 region of abnormal gravitational attraction. An account of a suggested connection 

 between anomalies of these two kinds occurring in the same district has lately been 

 published by Dr. Fritsche.' 



Observations made about thirty years ago by a former director of the Astro- 

 nomical Observatory in Moscow led to the conclusion that throughout two large 

 districts to the north and south of that city the plumb-line is deviated in opposite 

 directions. The deflections from the vertical are very considerable, and indicate 

 a relative defect in the attraction exerted by the rocks in the neighbourhood of 

 Mo.-cow itself, and the suggestion has been made that there is either a huge cavity 

 — a bubble in the earth-crust — a little to the south of the town, or that the matter 

 at that point is less dense than that which underlies the surface strata on either 

 side at a distance of ten or twelve miles. 



As long ago as 1853, Captain Meyen made magnetic obeervations in order to 

 determine whether the same district is also the seat of any magnetic irregularity. 

 His stations were hardly sufficiently numerous to lead to decisive results, but the 

 magnetic elements have recently been measured by Dr. Fritsche at thirty-one place's 

 within fifty miles of Moscow. The experiments were all made within eleven 

 days, so that no correction for secular change is required. They indicate a locus 

 of magnetic attraction running through Moscow itself. South of the town the 

 disturbance again changes in direction so as to show either that repulsive forces 

 are in play, or that there is another magnetic ridge line still further 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, 

 lie 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 

 deflections of the plumb-line to the flanking masses. It is perhaps not inconceiv- 

 able 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 

 arrangement shown in the figure with which Dr. Fritsche illustrates his hypothesis. 

 If this is so, the theory would 'primd facie seem to require that the bob of a plumb- 

 line should be .attracted towards and not — as is in some places actually the case- 

 away from the centre 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 

 effects of the same hidden mass of magnetic matter. 



In a few weeks an International Geodetic Conference will meet at Innsbruck, 

 at Aviiich 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 permanent 

 magnetisation of magnetic rocks. It is known that fragments of these are 

 strongly but irregularly magnetised, 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 



' ' Die magnetischen Localabweichungen bei Moskau und ihre Beziehungen zur 

 dortigen Local-Attraction,' Bulletin, de la SocUte Ivipcr. des Natiiralhtes de iMoncou,. 

 1894, No. IV. 



