PART III. KARTH CURRENTS AND EARTH MAGNETISM. CHAP. I. 



As Lament and Bachmetjevv employed only short lines, and Palmier! made his observations on the 

 ope of Vesuvius, their determinations in this respect must be treated with great reservation. If we 

 ut them on one side, we may draw from the above the conclusion that the earth-currents as a whole 

 ill be inclined to flow in a direction N S in Europe; but local circumstances at the various places 

 ill often cause the currents to deviate considerably from this main direction. 



In the United States of America it has been found that during magnetic storms it is the lines 

 inning E W, or NE SW which are most strongly affected. 



In India the directions of the earth-currents, from a number of observations on telegraph-lines, 

 as found to be N S. 



DIURNAL VARIATION OF EARTH-CURRENTS. 



142. As first shown by Barlow and later by Airy, the earth-currents recorded on calm days show a 

 ;ry marked diurnal period. On this point all authorities who have entered into the question seem to 

 jree. The result is confirmed by Wild. Tromholdt, observing on telegraph-lines in Norway, found a 

 incipal maximum at about 7 9 p.m. 



The most extensive and complete treatment of the diurnal variation is that of B. Weinstein. He 

 und the average diurnal variation for the five years from 1884 1888, and also the variation for the 

 air seasons of the year, and finally the diurnal variation for each month. The type of variation is 

 -ry similar all through the year, but the amplitude is greatest in the summer and smallest in the winter 

 ason The diurnal period, whatever may be its cause, appears to be a very definite thing, showing 

 jite definite properties. In accordance with Walker and Airy, Weinstein says: 



"After this I think that we already by looking at these curves can draw no other conclusion than 

 lat the phenomenon with which we here have to deal is a real one, and that its origin is due to a 

 rocess of a more general character(')". 



From the comparison made with the diurnal period of terrestrial magnetism, it appears, as the 

 isult of all the efforts made to find a connection, that no simple relation is found between the two 

 nenomena. 



Weinstein finds a similarity as regards variation of earth-currents and that of the total intensity, 

 lit such a similarity seems very difficult to interpret physically, for the effect of a surface-current 

 ^tending over a large area should distinctly be felt in a similar manner in the horizontal elements, i. e. 

 edination and horizontal intensity. Weinstein, however, is of the opinion "that nearly all the total move- 

 lent observed on the magnetometers generally named terrestrial magnetic variations, are only caused 

 1; variations of the earth-current, which affect the magnetometers in the same way as galvano- 

 ieters"( 2 ). But this result of Weinstein's does not seem very convincing while he takes for granted 

 lat "when the current-sheet has a horizontal position, there should not exist any horizontal magnetic 

 irces worth mentioning( 3 )". Lately van Bemmelen ( 4 ), from records observed in Java determined 

 te diurnal variation and found by comparison with magnetometer records "that the direction 

 ( the earth-current is such that it can be regarded as causing the variations of the magnetic com- 

 pnent and that the vibrations for them correspond"; but he finds "that the magnetic component is 

 itarded with respect to the earth-current", and finally "that the ratio of the amplitudes of corresponding 

 'brations decreases with the duration of that vibration, so that those of the earth-current are relatively 

 Irger with a shorter duration". 



(') loc. cit. p. 18. 



<?\ loc. cit. p. 78. 



( 3 ) loc. cit. p. 69. 



| J ) v\.\ BKMMKI.KN loc. cit. p. 513. 



