652 REPORT— 1894. 



and always opposed on opposite sides of tlie equator. For this supposition no 

 satisfactory explanation is forthcoming, and therefore, with all reserves and a full 

 consciousness that in sucli matters hypothesis ditiers but little from speculation, it 

 appears to me that the theory that induced magnetism is the main cause of the 

 disturbance has the greater weight of evidence in its favour. 



If this be granted, it is evident that the positions of the main lines and centres 

 of attraction would be approximately constant, and, so far as it is possible to form 

 an opinion, these conditions seem to be satistied. There has certainly been no 

 noticeable change in the chief loci of attraction in the five years which have 

 elapsed between the epochs of our two surveys. Mr. Welsh's observations made in 

 Scotland in 1857-8 fit in well with our own. Such evidence is not, however, in- 

 consistent with minor changes, and it is certain that, as the directions and magnitude 

 of the inducing forces alter, the disturbing induced forces must alter also. But 

 this change would be slow, and as the horizontal force is in these latitudes com- 

 paratively weak, the change in the disturbing forces would also be small, unless 

 the vertical force altered greatly. It is, at all events, impossible to attribute to 

 this cause oscillations which occupy at most eight or ten years. It is possible to 

 suggest other changes in the state of the concealed magnetic matter — alterations 

 of pressure, temperature, and the like — to which the oscillations of secular change 

 might be due, but probably there will be a general consensus of opinion that if the 

 slowly changing terms in the disturbance function are due to magnetic matter, 

 the more rapid fluctuations of a few years' period are more likely to be connected 

 with earth currents. It becomes, therelore, a matter of interest to disentangle 

 the two constituents of local disturbances; and there is one question to which I 

 think an answer might be obtained without a greater expenditure than the impor- 

 tance of the investigation warrants. Are the local variations in secular change 

 waves which move from place to place, or are the}' stationary fluctuations, each 

 of which is confined to a limited area beyond which it never travels F Thus, if 

 the annual decrease in the declination is at one time more rapid at Greenwich than 

 at Kew, and five years afterwards more rapid at Kew than at Greenwich, has the 

 maximum of rapidity passed in the interval through all intervening places, or has 

 there been a dividing line of no change which has .separated two districts which 

 have perhaps been the scenes of independent variations." The answer to this 

 question is, I take it, outside the range of our knowledge now; but if the declina- 

 tion could be determined several times annually at each of a limited number of 

 stations in the neighbourhood of London, to this inquiry, at all events, a definite 

 answer would soon be furnished. 



There are two other lines of investigation which, I hope, will be taken up sooner 

 or later, for one of which it is doubtful whether the United Kingdom is the best 

 site, while the other is of uncertain issue. 



If, however, it be granted that the principal cause of local and regional magnetic 

 disturbances is the magnetisation by the earth's field of magnetic matter concealed 

 below its surface, the question as to the nature of this mateiial still remains to be 

 solved. Is it virgin iron or pure magnetite, or is it merely a magnetic rock of 

 the same nature and properties as the basalts which are found in Skye and Mull ? 

 There is, of course, no d, priori reason why all these different materials should not 

 be active, some in one place and some in another. 



As regards the United Kingdom, I have, both in a paper on the Permeability 

 of Magnetic Rocks and in the description of the recent survey, made calculations 

 which tend to prove that, if we suppose that the temperature of the interior of the 

 eartii is, at a depth of twelve mUes, such as to deprive matter of its magnetic proper- 

 ties, and if we further make the unfavourable assumption that down to that limit 

 the susceptibilit}' 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 generalise this 

 result, and to assume that in all places regional disturbances are due to basic rocks 

 alone. 



We know that local efl'ects are produced by iron ore, for the Swedish miners 

 seek for iron with the aid of the magnet, and in some other cases magnetic disturbs 



