MAGNETIC DISTURBANCES AT KEW. 



295 



given for H, V, and I in Table XII. Arithmetical means from the 12 monthly values 

 obtained for the c constants, and the values derived for the c and a constants from 

 the yearly and seasonal diurnal inequalities are given in Table XIII., p. 296. This 

 includes the 8- and 6-, as well as the 24- and 12-hour waves. For the c constants the 

 unit is ly for H and V, and 1' for I. 



TABLE XII. Fourier Coefficients, Amplitudes, and Phase Angles. 



19. Table XII. shows in all three elements a marked and fairly regular rise in the 

 values of both c t and c 2 from midwinter to midsummer. There was, it must be 

 remembered, an excess in the mean absolute ranges in the summer months over those 

 in the winter months, especially in the case of V ; but the difference between the 

 summer and winter values of c l and c 2 is considerably more prominent, especially 

 inH. 



This supports the conclusion, suggested by general inspection of the diurnal 

 inequalities, that the nature of the changes of larger feature which take place in the 

 course of a magnetic storm are largely dependent on the season of the year when the 

 storm occurs. Some external influence may, so to speak, pull the trigger, but the 

 nature and amplitude of the disturbance are influenced in an important degree by the 

 local time and by the season of the year. 



If " accidental " influences had been dominant in the diurnal inequalities obtained 

 for individual months, the amplitudes and phase angles in Table XII. would have 

 varied erratically from month to month. It cannot, of course, be claimed that there 

 are no erratic values. Those for c 2 and a 2 in August for V are unquestionably 

 outstanding, but, on the whole, the smoothness of the monthly values, especially in 

 summer when the amplitude is large, is not a little remarkable. 



20. Values of the a, b, c, and a constants were calculated for the seasonal 



