35(j MR BALFOUR STEWART OM EARTH-CURRENTS DURING MAGNETIC CALMS, 



We are thus provided with a mass of apparently unexceptionable observa- 

 tions of earth- currents made during a period when the magnet was comparatively 

 tranquil, and the question arises, What connection have these with the daily 

 changes which take place in the magnetism of the earth ? 



In attempting to answer this question, it is, I think, necessary to divide the 

 observations of earth-currents into two categories, for the following reason. 



It has been shown by General Sabine, that magnetic disturbances obey laws 

 of time and place very different from those followed by the ordinary daily mag- 

 netic changes. Now, if we consider that such disturbances, more or less violent, 

 are of very frequent occurrence, we shall see at once the necessity, in magnetical 

 discussions, of separating the disturbed observations from the undisturbed, in 

 order to obtain accurately the laws of both. 



Let us bear this in mind, and even without speculating on the connection 

 between earth-currents and magnetic changes, we cannot, I think, expect that a 

 magnetic disturbance of a certain definite type shall be connected in precisely the 

 same way with an earth-current, as an ordinary daily magnetic change of the 

 same type as the disturbance. 



This will appear evident, if we reflect that although both changes may be of 

 the same kind here, yet, if we alter our position and go into another portion of 

 the globe, they may be quite different one from another, since a change of posi- 

 tion will affect the disturbance and the daily variation according to very different 

 laws. 



On any hypothesis, therefore, the disposition over the earth's surface of that 

 current which is connected with the disturbance, will be different from that 

 which is connected with the ordinary daily change. For these reasons it would 

 appear to be highly necessary to ascertain which of the earth-currents correspond 

 in time of occurrence with disturbances of greater or less amount, and which with 

 undisturbed observations. 



I have been enabled to ascertain this, at least approximately, by means of 

 the photographic records of the earth's magnetism at Kew. The portions of the 

 Kew declination and horizontal force curves, corresponding in time with the 

 observations of earth-currents, have been carefully inspected, and whenever any 

 appearance of disturbance presented itself, this has been noted by the side of the 

 corresponding earth-current observations, and these have by this means been 

 divided into the three following classes, viz. : — 



Class I. Obsei-vations during Magnetic Calms. 



II. Observations during smaller Disturbances. 

 III. Observations during greater and more abrupt Disturbances. 



These are recorded in the three following tables, Mr Walker's notation being 

 adopted : — 



