300 ON THE PKOBABLE CAUSES 



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 h 30 m A. M. ; southward, at 2 h 30 m 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 l h 30 m 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 maxi- 

 mum occurs, on the average, at 8 h 30 ra 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.M., and is 

 observed at almost all the stations. 



The foregoing facts leave no doubt that the sun is the primary 

 cause of the currents ; and the only question is as to the mode of 

 its agency. Upon this point I concur with Dr. Lament in believ- 

 ing the electrical currents (or waves) on the earth's surface to be 

 due to disturbances of equilibrium of statical electricity; but I 

 regard these derangements of equilibrium to be simply the effects 

 of solar heat, and not (as Dr. Lament believes) the results of an 

 electrical force emanating directly from the sun. 



It is well known that the earth and the atmosphere are, in 

 ordinary circumstances, in opposite electrical states the electricity 

 of the earth being negative, and that of the atmosphere positive. 

 It is also known that the electricity of the air increases rapidly 

 with the height, a few feet and in some cases even a few inches 

 being sufficient to manifest a difference of electrical tension. The 

 rate of this increase is very different at different periods of the 

 day, the difference appearing to be due to the greater or less 

 conductibility of the lower strata of the atmosphere, causing a 

 greater or or less interchange of the opposite electricities. 



