October 27, 1922] 



SCIENCE 



465 



time it has fceen known that the two phenomena 

 are physically related, hut there is, as yet, no 

 agreement as to the exact nature of this rela- 

 tion. 



In this connection it should he mentioned 

 that two classes of magnetic and electric varia- 

 tions have been observed and are frequently, if 

 not universally, assumed to have the same ex- 

 planation. It is well known that at times of 

 great sun-spot and auroral disturbances there 

 are also very great and irregular magnetic and 

 earth-current disturbances. It was these ab- 

 normal earth-currents which first attracted the 

 attention of physicists, and it is these which 

 have received the most attention from writers 

 on the subject. But in addition to these there 

 is a regular diurnal variation in both the mag- 

 netic declination and inclination and in the 

 direction and intensity of the earth-currents. 

 In the literature of terrestrial magnetism, 

 variations of the irregular class are called mag- 

 netic disttirbances while the regular daily and 

 seasonal changes are known as the magnetic 

 variations. The relation between the magnetic 

 changes and the earth-current changes seems 

 to be different in the two cases. Thus in the 

 case of the magnetic disturbances the corre- 

 sponding earth-current changes seem to be 

 simultaneous with the magnetic changes, while 

 in the case of the ' regular diurnal variations 

 the magnetic changes seem to lag ^behind the 

 corresponding earth-current changes b}' a 

 period which may be as great as two or three 

 hours. 



This would seem to suggest that if the reg- 

 ular variations in terrestrial magnetism and 

 earth-euiTents are related in the sense of cause 

 and effect the irregular magnetic and earth- 

 current disturbances are not so related, but are 

 both apparently due to some external phe- 

 nomenon which acts simultaneously upon both 

 classes of phenomena. This fact seems not to 

 have been heretofore recognized by writers 

 upon the subject. 



The first extensive study of the relation of 

 earth-currents to terrestrial magnetism seems 

 to have been started at the Greenwich Observa- 

 tory in 1863. Two earth-current lines, one 

 north and south and the other east and west, 

 were established in that year, and continuous 



records of the earth-currents in these two direc- 

 tions were made. In 1868, Airy, the astronomer 

 royal, pulblished the results of his comparisons 

 of these variations with accompanying mag- 

 netic changes. As a result of this comparison, 

 he says : 



I think that on repeatedly examining the agree- 

 ment of the two systems of curves, it is impossi- 

 ble to avoid the conclusions that the magnetic 

 disturbances are produced by terrestrial galvanic 

 currents below the magnets. ... At the present 

 time we are unable to say whether the records of 

 the galvanic currents throw any light upon the 

 origin of the diurnal variations in the magnetic 

 elements. 



In 1870 Airy poblished another paper in 

 which he undertook to account for the diurnal 

 magnetic variation, as well as the magnetic dis- 

 turbances, by earth-currents; but the theoret- 

 ical curve of magnetic variation which he con- 

 structed from earth-current data, while agree- 

 ing in general shape with the observed curve 

 of variation, is not coincident with it in time. 



Since the work of Airy, the most extensive 

 comparison of the variations in earth-currents 

 and in the magnetic elements of the earth has 

 been made in Germany. In 1883, two under- 

 ground telegraph lines, one from Berlin to 

 Thorn and the other from Berlin to Dresden, 

 were set apart for the measurement of earth 

 currents, and continuous records of these cur- 

 rents were kept from that time until 1891. The 

 material thus collected was turned over to Pro- 

 fessor B. Weinstein, who, with a corps of com- 

 petent assistants, went over all the records and 

 estaiblished for' the five years 1884-1888 what 

 he regarded as the constant resultant earth- 

 current in the region between these cities. The 

 diurnal and seasonal variations of this current 

 were then compared with the corresponding 

 variations in the magnetic elements at Vienna 

 and Wilhelmshaven. As a result of this com- 

 parison, Weinstein says : 



Personally, I have arrived at the conviction 

 that almost the whole of the changes observed by 

 means of a magnetometer and classed as terres- 

 trial magnetic variations are due to earth-currents 

 which act upon the magnetometer as a galvano- 

 meter. 



Notwithstanding the very close relation 

 which the curves published by Weinstein show 



