472 Royal Society : — 



necessary to state the arrangement adopted at Greenwich. There 

 are two wires proceeding from the Observatory, one ending near 

 Croydon and the other near Dartford (nearly at right angles) ; and 

 these are well insulated throughout their entire length, and have 

 good earth -connexions at their extremities. 



From these, by means of galvanometers, the intensity of earth- 

 currents is recorded for two very favourable directions ; and from 

 these again the intensity of those currents which flow in the magnetic 

 meridian, and in a direction perpendicular to it, may be very easily 

 deduced. The standard for direction is the current which flows 

 through the wire of a battery from the zinc pole, and which is called 

 the zinc-current. With this explanation the earth-current diagrams 

 appended to this paper will be quite intelligible ; and with regard to 

 the magnetic curves, it is only needful to remark that increasing or- 

 dinates denote decreasing declination and decreasing horizontal force, 

 and that the normal lines, which have been furnished through the 

 kindness of General Sabine, denote the position which the curves 

 would have occupied had no disturbance supervened. But before 

 proceeding to compare together the two sets of curves, it will be 

 necessary to advert to a peculiarity of disturbances which enters as 

 an essential element into all such discussions. It has been found by 

 General Sabine that if the disturbances of declination be divided into 

 two categories, easterly and westerly, these obey very different laws 

 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 effect 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 



