284 ON EARTH-CURRENTS, AND THE DIURNAL 



Astronomer Boyal at the last meeting of the British Association, 

 Dr. Lament stated that he had found " that electric currents, or 

 (as they may be more properly termed) electric wares, varying in 

 direction and intensity, are constantly passing at the surface of 

 the earth, and that these waves correspond perfectly with the 

 variations of terrestrial magnetism." The correspondence here 

 referred to seems to relate to the epochs of the two classes of 

 changes, and not to their amount. For in the latest account of 

 Dr. Lament's researches, of which I am aware, and which is con- 

 tained in a letter to Professor De la Eive, dated October 30, 1861, the 

 writer states that the galvanometer indicates, not the earth-current 

 itself, but its momentary changes. In fact the needle of the galva- 

 nometer, in his observations, appears to have been affected only 

 during a rapid increase or decrease of the earth-current, and to 

 return to zero when the current became equable. It is obvious 

 that, if such were the case generally, there could be no corre- 

 spondence, such as has been pointed out in the preceding pages, 

 between the magnitude of the magnetic changes and the deflections 

 of the galvanometer. It is stated, in fact, by Dr. Lamont, that in 

 such circumstances the galvanometer does not vary much from its 

 mean position, even when the magnetometers have changed con- 

 siderably; and he expresses his doubt whether the constant part 

 of the action of the earth-current can be observed at all with our 

 present means. 



Dr. Lamont ascribes the singular effect above described to the 

 double conductor, the current moving (as he believes) at first in 

 the wires, and afterwards diffusing itself in the earth below. I 

 venture to suggest that it may be due to some disturbing cause, 

 which operates more powerfully (in relation to the principal effect) 

 in short than in long wires. No effect of a similar kind appears 

 to have occurred in the observations of Mr. Barlow, which were 

 made upon telegraphic wires of great length. And, on the other 

 hand, it is a fact well known to telegraphists, that the currents, 

 produced by chemical action upon the terminal plates, interfere 

 with the primary current much more in short, than in long wires. 

 In confirmation of the same view, I may mention the fact observed 

 at many points of the earth during the remarkable aurora of 

 August and September, 1859, namely, that the proper earth- 

 current was strongest, caten's paribus, in the longest lines of 

 telegraph. 



