November 24, 1922] 



SCIENCE 



593 



measurements are a)t present in progress at tlie 

 Department's laboratory in Washington. 



In order to take advanitage of the previous 

 expeiiienee gained in earth-current work, and 

 to ascertain the direction in which further study 

 is desiratole, tlie writer undertook a discussion 

 of the available daita, especially of the 11-year 

 series, 1910-1920, obtained at the Observatorio 

 del Ebro, Tortosa, Spain. For the first time 

 comparisons could toe made .between the phen- 

 omena of terrestrial magnetism, earth currents, 

 and atmospheric electricity, as dependent upon 

 extensive observations at the same station. Ac- 

 cordingly, it has been possible not only to con- 

 firm and extend certain results previously 

 reached toy others, but also to draw important 

 new conclusions. 



It is hoped that the present investigation, 

 which had to be confined to a discussion of the 

 observational data on magnetieally-calm, or on 

 eleotrically-calm days, may be supplemented 

 later by a discussion of eanth-current data on 

 disturbed days. 



The chief conclusions may be stated as fol- 

 lows : 



(a) The resultant horizontal earth-cui'rents, 

 as observed at the E,bro Observatory, flow, on 

 the average for the year, in the direction from 

 about 29° west of North to 29° east of South, 

 or, approximately, in the direction from the 

 Magnetic North Pole towards south-southeast. 

 The average value, for the magnetically-calm 

 days during 1914-1918, of the potential gra- 

 dient of the component of the current flowing 

 from true North to South was 0.20 volt pea- 

 kilometer, and that of the component towards 

 geographic East was 0.11 volt per kilometer, 

 or about one half of the north-south component. 

 The resultant horizontal potential-gradient was 

 0.23 volt per kilometer, which during electric 

 or magnetic storms may reach a value 0.8 to 

 1.0 volt per kilometer. 



(b) The annual variations of the earth-cur- 

 rent potential-gradients and of the components 

 of the Earth's magnetism, as observed at the 

 E'bro Observatory, may be related to one an- 

 other as cause and effect only to a very minor 

 extent; tooth sets of variations may have to be 



referred, more or less, to common causes. The 

 range of the annual variation of the north-south 

 electric component is about 2.5 times that of 

 the west-east component. 



(c) The diurnal variation of earth currents 

 as observed at the Ebro Observatory along lines 

 somewhat over one kilometer long is remark- 

 atoly similar to that observed at Berlin along 

 telegraph lines, 120 and 262 kilometere in 

 length, from 1884-1887. In both eases the di- 

 urnal variations for itJie component of the cur- 

 rent along the meridian is considerably more 

 pronounced (2-3 times) than that along the 

 parallel of latitude. The diurnal variation in 

 the north component of the earth's magnetism 

 is not such as to correspond to the direct mag- 

 netic effect of the diurnal variation of the west- 

 east component of the earth currents. A simi- 

 lar conclusion had to be reached with regard 

 to the east component of the earth's magnetism 

 and the north-south component of , the earth 

 cuiTents. The general conclusion was that the 

 north-south earth-current might be the result 

 of electro-magnetic induction, caused by the 

 fluctuation during the day of the west-east com- 

 ponent of the earth's magnetism. If it be re- 

 called that all analyses of the diurnal varia- 

 tion field of the earth's magnetism have shown 

 ■ that the magnetic diurnal variation is in part 

 to be ascribed to electric currents circulating 

 in the regions overhead and in part to cur- 

 rents circulating within the earth's crust, exact 

 agreements between magnetic variations and 

 earth-current variations are not to be expected. 

 It further remains to point out that until we 

 have some knowledge of the actual course or 

 distribution of the earth currents in the earth's 

 crust and as to how the conductivity of the 

 crust may vary with temperature and other 

 meteorological causes during the day and at the 

 actual place of observation, attempts to find a 

 quantitative relationship between terrestrial- 

 magnetic and earth-electric effects may be fu- 

 tile. 



(d) The horizontal vector-diagrams both 

 for the magnetic and earth-eleotric components 

 vary during the sun-spot cycle in about the 

 same proportion. The earth-current vector- 



