VARIATIONS OF THE MAGNETIC NEEDLE. 283 



electric currents may be supposed to operate in producing the 

 magnetic effects. 



An electric current, traversing the earth's crust in a horizontal 

 direction, may affect a horizontal magnetic needle above its 

 surface in two ways. For the current may either act directly 

 upon the needle, according to the known laws of electro-magnetic 

 action ; or it may induce temporary magnetism in the earth itself, 

 which will thus affect the needle differently from before. I believe, 

 with Dr. Lament, that the former hypothesis is inadmissible, at 

 least as regards the principal part of the observed effect. In 

 addition to the reason assigned for this by Dr. Lamont, I may 

 adduce the known similarity in the course of the magnetic changes 

 over considerable portions of the earth's surface, a similarity incom- 

 patible with the supposition that the magnet is directly acted on, 

 to any great extent, by the subjacent current. We must suppose, 

 therefore, that the earth is acted on, to a considerable depth, by 

 the wave of currents which sweep over its surface, and which alter 

 by induction the magnetism of the subjacent mass, and that the 

 effect produced upon the freely suspended magnet at its surface is 

 the result of this induced change. On this supposition, the mag- 

 netic phenomena, whose laws we are considering, are the indirect 

 effects (not of the subjacent current merely, but) of the entire 

 wave traversing an extended portion of the earth's surface. We 

 can thus understand the cause of the similarity in the more rapid 

 magnetic changes to which we have adverted ; and we may even 

 frame some idea of the depth acted on by the superficial current, 

 from the geographical limits of the phenomena. 



Upon this supposition, also, we are prepared to expect differences 

 in the laws of the observed and computed currents, such as have 

 been above noticed. For the galvanometric measures belong only 

 to currents at the place of observation ; while the magnetic changes 

 are, by hypothesis, the mean results of currents occupying a con- 

 siderable portion of the earth's surface. Hence, also, it follows 

 that the complete identification of the two classes of phenomena 

 can only be made by the help of simultaneous observations of 

 earth-currents at numerous points in an extended district. 



14. Before concluding this part of the subject, I must refer 

 briefly to the previous investigations of Dr. Lamont connected 

 vvith it, so far as they have been yet made public. 



In a letter, dated July 29, 1SG1, which was read by the 



