TERRESTRIAL MAGNETISM. 187 



As regards the United Kingdom 1 liave, both in a i>ai)er on the 

 "Permeability of magnetic rocks" and in the description of the recent 

 survey, made cak'ulations which tend to prove that if we suppose that 

 the temperature of the interior of the earth is, at a depth of 12 miles, 

 such as to deprive matter of its magnetic properties, and if we further 

 make the unfavorable assumption that down to that limit the suscepti- 

 bility is constant, the forces which are observed on the surface are of 

 the same order of magnitude as those which could be produced by 

 large masses of ordinary basalt or gabbro. It would not, however, be 

 wise to generalize this result and to assume that in all places regional 

 disturbances are due to basic rocks alone. 



We know that local effects are produced by iron ore, for the Swedish 

 miners seek for iron with the aid of the magnet, and in some other 

 cases magnetic disturbances of considerable range are so intense as to 

 suggest that material of very high 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 i)lumb 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 pub- 

 lished by Dr. Fritsche.^ 



Observations made about thirty years ago by a former director of 

 the Astronomical 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 neighborhood of Moscow itself, 

 and the suggestion has been made that there is either a huge cavity — 

 a bubble in the earth crust — under the town, or that the matter beneath 

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

 side at a distance of 10 or 12 miles. 



As long ago as 1853 Captain Meyen made magnetic observations 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 places within 50 miles of 

 Moscow. The exi>eriments were all made witliin 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 



'"Die magnetischea Localabweichungen bei Moskau uiid ihre Beziehungeu zur 

 dortigen Local-Attractiou," Bulletin de la Society Iinp^r. dea Naturalistes de Moscou, 

 1894, No. 4. 



