1902.] between Sun-spots and Terrestrial Magnetism. 



223 



In obtaining these figures I have retained a figure in the value of 

 b beyond that recorded in Table I. 



Tables I and II will suffice to bring out one of the most important 

 points established, viz., that b is certainly different from one month to 

 another, and is, for all the elements except the vertical force, decidedly 

 larger at the equinoxes (more especially it would appear at the spring- 

 equinox) than at other seasons. This means that the equinoxes are 

 the seasons at which the amplitude of the diurnal inequality, when 

 considered absolutely, is most dependent on the sun-spot frequency. 

 When we take into account, however, the difference between the ranges 

 of the diurnal inequalities at different seasons of the year, we find 

 that winter is the season when sun-spot frequency is relatively most 

 important. This will be recognised on reference to Table III, remem- 

 bering that a represents the range corresponding to a total absence of 

 sun-spots, while a + 41*7 b is the range corresponding to a sun-spot 

 frequency of 41*7, this being, as already mentioned, Wolfer's mean 

 value for the 11 years in question. 



Table III. 

 Values of 41-76 -r- a. 



Horizontal Vertical 



Declination. Inclination. force. force. 



Winter 0'42 0'69 0-60 0-19 



Equinox 0'27 0-49 0'39 0'06 



Summer 0-20 0-35 0'26 0-07 



Table III serves also to bring out another important result, viz., that 

 the influence of sun-spot frequency on the amplitude of the diurnal 

 inequality is very much less for the vertical force than for the three 

 other elements considered. 



A recent interesting paper by Kajna'* shows that the idea of a linear 

 relationship between diurnal magnetic range and sun-spot frequency 

 has already been applied by at least two previous investigators, Kajna 

 and Wolfer. They seem, however, to have applied it only to mean 

 annual values, and to have considered declination only. Eajna, deal- 

 ing with declination data, observed at Milan over the long period 

 1836 to 1901, applies a formula of type (1) to what he calls the 

 " medie annuali dell' escursione diurna." 



The value he finds for b is 0'*047. He mentions that in an earlier 

 similar investigation, including declination data from several stations, 

 Wolfer obtained the value 0'-040. 



I am uncertain as to the precise meaning of Kajna's " medie annuali," 

 but it certainly is not quite the same thing as the mean range in 

 Table I, so that the results are not absolutely comparable. 



* 'Rendiconti del E. 1st. Lomb.,' Serie II, vol. 35 1902. 



