186 TERRESTRIAL MAGNETISM. 



as it is possible to form an opinion, tliese conditions seem to be satisfied. 

 There lias certainly been no noticeable change in the chief loci of attrac- 

 tion in the five years which have elapsed bet^yeen the epochs of our two 

 surveys. Mr. Welsh's observations made in Scotland in 1857-58 fit in 

 well with our own. Such evidence is not, however, inconsistent with 

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

 the inducing forces alter, the disturbing induced orces must alter also. 

 But this change would be slow, and as the horizontal force is in these 

 latitudes comparatively 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 i:»robably 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' i^eriod 

 are more likely to be connected with earth currents. It becomes there- 

 fore a matter of interest to disentangle the two constituents of local 

 disturbances; and there is one (juestion to which I think an answer 

 might be obtained without a greater expenditure than the importance 

 of the investigation warrants. Are the local variations in secular 

 change waves which move from place to place, or are they stationary 

 fluctuations, each of which is confined to a limited area beyond which 

 it never travels? Thus if the annual decrease in the declination is at 

 one time more rapid at Greenwich than at Kew, and five years after- 

 wards 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 dis- 

 tricts which have i^erhaps 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 declination could be determined several 

 times annually at each of a limited number of stations in the neighbor- 

 hood 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 magnetization by the earth's field of mag- 

 netic matter concealed below its surface, the question as to the nature 

 of this material still remains to be solved. Is it virgin iron or pure 

 magnetite, or is it merely a magnetic rock of the same nature and prop- 

 erties as the basalts which are found in Skye and Mull? There is of 

 course no a priori reason why all these difterent materials should not 

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



