ELECTRICAL CUllRENTS OF THE EARTH. 215 



otlier, and determined with a rheostat the conductibility of the two cir- 

 cuits. The ditterence between the two was, from the tirst, very small, 

 aud it was ouly necessary to excavate by a centimetre, (^„ of an incli.) 

 or thereabouts, the two hohis of tlie line which had the greater resist- 

 ance, iu order to render both of equal and constant resistance. Proof 

 of homogeneity was also made by tilling two large holes, which had 

 been excavated in close contiguity, with the same earth procured in 

 forming the holes at the extremities of the lines. In the holes thus tilled 

 I formed the two capsules already described, and introduced therein the 

 usual electroiles; when these were in operation, having been hrst well 

 prepared, no current was found on the introduction of the electrodes 

 into the holes. 



After these preliminary arrangements I commenced a series of observ- 

 ations which were continned about a month, from the 12th of ]\I;irch to 

 the 15th of April, 18G4, being the season generally of clear skies and of 

 cool and dry air; there were two or three days of a strong easterly wind 

 and one of storm with rain. For the spac3 of ten days the observations 

 were never interrupted night or day, aiul two soldiers Vv'ho relieved one 

 another were stationed as a guard at each hole. Tlie galvanometer used 

 was one of 1,500 coils and gave atixed deviation of 00^, with ;i Danielfs 

 battery introduced into one of the mixed circuits. 



The following- were the results obtained from the long series of ob- 

 servations made in the manner and at the time indicated : 



1. In mixed circuits, formed of a copper wire, aud a stratum of earth 

 very nearly horizontal and about kilometres (1 miles) long-, there is 

 always an electric current which circulates with intensity and in deter- 

 minate directions according to the direction of those circuits in regard 

 to the magnetic meridian; this current cannot be absolutely attributed 

 to the heterogeneity of the electrodes, or of the terrestrial strata in con- 

 tact with those electrodes. 



2. These currents have an intensity which increases in ])ro])ortion to 

 the depth at which the electrodes are sunk beneath the surface of the 

 earth, from 50 centimetres (20 inches) to 2 metres (7 feet.) This greater 

 conductibility possessed by the mixed lines iu proportion to the depth to 

 which the electrodes are sunk, explains the variation discovered in the 

 iutensityof the electric currents of the earth under these circumstances. 

 This result is in conformity with that which is observed after rain, and 

 which is due to the greater humidity of the ground in contact vrith the 

 electrodes. 



3. When the cavities iu which the electrodes are sunk have a depth 

 of 2 metres (7 feet) or more, or when the electrodes are immersed in the 

 water of wells, the extension of the lamina} of zinc and the diameter 

 of the porous vases have little influence on the intensity of the terres- 

 trial currents. 



4. In the circuit extended along the magnetic meridian or south-north, 

 the electric current had always a constant direction aud an intensity 

 which varied very ieebly vrhile manifesting a certain period. For a 

 month, several hundreds of varied observations showed that the terres- 

 trial current was always directed in the metallic part of the circuit from 

 south to north, and that the needle of the galvanouu'ter never became 

 fixed at zero nor in the opposite quadrant, and that its oscillations were 

 always small and very slow. 



5. On comparing with one another the slightly unequal deviations ob- 

 tained in nine entire days of constant observation, it results that the 

 current iu the south-north circuit i)resents in twenty-lour hours two 

 viaxlma and two minima of intensity. The two minima occur, one iu 



