EARTH'S MAGNETISM PRODUCED BY THE MOON AND SUN. 



degree). This is in accordance with the theory that the lunar atmospheric tide is the 

 main cause of the phenomenon, although, of course, it does not prove this to be 

 the case. 



6. So far reference has been made entirely to the lunar variation as determined 

 from a number of whole lunations, as has been generally done (the exceptions are 

 Trevandrum, Bombay, and Batavia). It will be remembered that SCHUSTER'S theory 

 of the solar diurnal variation involved the hypothesis of a variable conductivity 

 depending on the sun's hour angle. This should, of course, also affect the electric 

 currents which arise from the lunar atmospheric tide, and so make the lunar magnetic 

 variations depend on the sun as well as on the moon. In the course of a lunation, 

 however, the angle between the sun and moon, viewed from the earth, changes from 

 to 2-jr, and the mean lunar variation for such a period cannot be expected to show 

 any special dependence on solar time. At any particular lunar phase, however, the 

 solar day hours, during which (over a given part of the earth) the atmospheric 

 conductivity is greatest, occur at a definite part of the lunar day, this part changing 

 with the lunar phase ; and it has, in fact, been found* that the lunar variation 

 determined from the mean of a number of days all at the same lunar phase is not of 

 the semi-diurnal form. The variation curve goes through a regular cycle of change 

 with lunar phase, in such a manner as to leave the mean variation over a whole 

 lunation of the simple form already described. The magnetic needle is most mobile 

 during the day hours : at certain seasons of the year, BROUN found that the 

 amplitude of the lunar diurnal variation of magnetic declination at Trevandrum was 

 five times as great during the solar day hours as during the night hours. t These 

 facts clearly show that the conductivity of the medium in which the electric currents 

 flow to produce the lunar magnetic variation depends on the position of the sun ; and 

 since it is unreasonable to suppose that the mechanisms concerned in producing 

 the lunar and solar diurnal magnetic variations are materially different, the 

 assumption of variable conductivity in SCHUSTER'S theory is confirmed in a very 

 definite and independent way ; in SCHUSTER'S discussion two barometric oscillations, 

 diurnal and semi-diurnal, were concerned, and it was necessary to explain why the 

 resulting magnetic variations, deduced on the assumption of uniform conductivity, 

 did not bear the proper ratio to one another. This might be because the conductivity 

 was not uniform, or because the ratio of the two oscillations was different in the 

 upper regions of the atmosphere from that indicated by the barometer. This latter 

 uncertainty is absent in the case of the lunar variations, where there is only a single 

 barometric oscillation, from which arise magnetic variations of other periods, 

 depending on the solar hour angle. 



7. In order to examine the effect of this variable conductivity, it is natural to 

 determine the harmonic components of the lunar diurnal variation for different lunar 



* By BROUN, CHAMBERS, FIGEE, and Moos in the investigations already cited, 

 t 'Trevandrum Observations,' vol. I., p. 121. 



2 o 2 



