C64 [June 18, 



of daily variation, and also that this difference is not of the same 

 description for all stations ; so that we are compelled to view a mag- 

 netic disturbance as the resultant eifect of two disturbances of differ- 

 ent character, superimposed upon one another. General Sabine has 

 likewise stated his opinion that this duality of action may perhaps 

 be due to the disturbing force entering the earth at two or more 

 points, one denoting magnetism of a more permanent kind, and the 

 other magnetism of an induced description. A study of the Kew 

 disturbance-curves tends to give confirmation to such an idea ; for 

 in these it is seldom found that the whole body of force which pro- 

 duces a disturbance is one which preserves the same type throughout 

 and only varies in intensity. Even if we suppose that this type will 

 vary with the hour of the day, we shall find, if we take disturbances 

 which last for several days, that the type of force at a given hour of 

 the first day is in very many cases different from that during the 

 Same hour of the second. "When, however, we confine our attention 

 to very abrupt changes of force, we find that the disturbance-type 

 which these display retains more of the same character throughout 

 a disturbance. In order to explain this, we may perhaps suppose 

 that there are two sets of magnetic particles in the earth one set 

 being of the nature of soft iron, and the other similar to hardened 

 steel. Now only the first of these would be acted on by any very 

 sudden change in the disturbing force, since it would require time in 

 order to influence the second set. We may thus perhaps account 

 for the fact that any very sudden change is of one type, since it only 

 influences one set of particles. Let us now consider what will take 

 place if a disturbance of the same primary nature continues for any 

 length of time. Here the hard-iron particles will also be influenced 

 to an extent compounded of the time and of the average value of the 

 disturbing force during that time. The magnetic needle, therefore, 

 will now be acted on by the joint influence of these two sets of par- 

 ticles, whereas at the beginning of the disturbance it was only acted 

 on by one of them, namely, the soft-iron ones. The type of force 

 will therefore have changed if the hard-iron particles are differently 

 distributed in our globe from the soft-iron ones ; and if, instead of 

 two, there are many sets of particles, we shall have a very compli- 

 cated effect. 



Now this duality of disturbing forces must be considered when we 



