162 DR. S. CHAPMAN ON THE LUNAR DIURNAL MAGNETIC VARIATION, 



his later paper, SCHUSTER suggested that the lunar magnetic variations might throw 

 light on these questions : " It is much to be desired that some systematic attempt 

 should be made to investigate the lunar influence on the magnetic changes, for we 

 possess at present only the vaguest information as to how the different components 

 are affected. It is quite possible that the effects may depend on a tidal disturbance 

 of the upper regions of the atmosphere. If so, we may expect to get a valuable test 

 of our theory by their investigation " (loc. cit. , p. 181). 



BALFOUR STEWART also has said that "it is impossible to refrain from associating 

 the lunar diurnal magnetic variations either directly or indirectly with something 

 having the type of tidal action, but in what way this influence operates we cannot 

 tell" (loc. cit., 146). 



These references to tidal action seem to have been suggested largely by a discovery 

 of BROUN'K, that the amplitude, of the lunar magnetic variation is greater at perigee 

 than at apogee in the inverse cube ratio of the moon's distance at these epochs, " as 

 in the theory of the tides," as he briefly concluded. The semi-diurnal character of the 

 lunar magnetic variation agrees with this tidal hypothesis. 



In 1912 VAX BEMMBLBN,* in an important paper on the lunar magnetic variations, 

 referred them (by methods similar to those used by SCHUSTER in the two papers 

 cited) to the lunar atmospheric tide as computed from the Batavian barometric 

 records; he showed that the electrical conductivity of the atmospheric layers in 

 which circulate the currents responsible for the production of the lunar magnetic 

 variations is of the same order as that calculated by SCHUSTER from the solar diurnal 

 variations. Since the lunar barometric variation is clearly a tidal effect, the 

 hypothesis of a tidal origin of the lunar magnetic variations is thus supported. 



The most direct confirmation of this hypothesis, however, would naturally be 

 obtained from evidence such as that adduced by BROUN, concerning the influence of 

 lunar distance upon the amplitude of the lunar magnetic variations. BROUN'S result 

 coincided remarkably closely with the theoretical value, but further examination 

 indicates that this was due to a happy, accident ; the ratio of the moon's mean 

 distance in the half lunations centred at perigee and apogee respectively is I'OO : 1'07, 

 and the inverse cube of this is 1'23 ; the observed values of the ratio of the mean 

 amplitudes of the lunar magnetic variation during these epochs varied between 1'15 

 and 1'34, with a mean value of 1'24. 



FiGEE,t making a similar investigation from the Batavian magnetic records, 

 dissented from BROUN'S conclusion that the amplitudes vary as the tidal theory 

 would predict. 



If a result contrary to that of BROUN could be definitely established, the above 

 theory of the lunar magnetic variations, which is so attractive in many ways, would 

 become almost untenable. The matter being of some importance, and BROUN'S and 



* VAN BEMSIELEN, ' Meteorologische Zeitschrift,' vol. 5, p. 218, 1912. 



t 'Batavian Magnetical and Meteorological Observations,' vol. 26, Appendix, 1903. 



