185 



In that communication I refrained from offering any conjecture as to 

 the origin of the currents themselves. Every speculation of this kind 

 must remain a pure hypothesis, until it can he confronted and compared 

 with facts; and the magnetic phenomena presented at different points 

 of the earth's surface are so diversified, that a wide collection of the facts 

 is necessary in order to form the basis of any sound physical theory. For 

 these reasons, I have deemed it the more proper course to ascertain the 

 laws of the diurnal changes of the Earth-currents at many places, so far 

 as they may he inferred from the magnetic phenomena which they pro- 

 duce, before proceeding to the consideration of their causes. This pro- 

 cedure is in accordance with the acknowledged rules of the inductive 

 philosophy ; and the departure from it has given rise to speculations on 

 this subject, which, however well they might accord with the phenomena 

 with which they were compared, could not have been admitted for an 

 instant in the presence of a wider generalization. 



It has been shown, in the paper referred to, that the Earth-currents, 

 as inferred from the changes in the two horizontal components of the 

 magnetic force, observe certain general laws, which are common to all 

 the stations at which these changes have been observed ; while, on the 

 other hand, their departures from a common type are various and consi- 

 derable. "We thus learn that the phenomena are produced by a common 

 cause, the effects of which are greatly modified by the physical peculia- 

 rities of the parts of the earth where they are observed. The following 

 are the principal features of the phenomena common to all, or to most of 

 the places of observation. 



I. The point to which the resultant Earth-current is directed follows 

 the sun, although not at a uniform rate, throughout the day. In the 

 northern hemisphere its direction is eastward, on the average, at 10'' SO"" 

 A. M. ; southward, at 2^ 30" p. m. ; and westward, at 7 p. m. 



II. The intensity of the current is greatest between noon and 2 p.m., 

 the mean time of the maximum in the northern hemisphere being about 

 P 30"" p. M. The intensity of the current is least at an interval of about 

 twelve hours from the epoch of the maximum ; and the direction of the 

 current of least intensity is, in nearly all cases, opposite to that of the 

 greatest. 



III. There are two subordinate maxima, separated from the principal 

 maximum by intervening minima. The morning maximum occurs, on 

 the average, at 8*^ 30™ a. m. It may be traced in the diurnal curves of 

 the American and Siberian stations, and in those of the Cape of Good 

 Hope and Hobarton. The current is then northerly in the northern 

 hemisphere, and southerly in the southern. The evening maximum 

 occurs at about 10 p. ivr., and is observed at almost all the stations. 



The foregoing facts leave no doubt that the sun is the primar}^ cause 

 of the currents ; and the only question is as to the mode of its agency. 

 Upon this point I concur with Dr. Lamont in believing the electrical 

 currents (or waves) on the earth's surface to be due to disturbances of 



