August 17, 19 16] 



NATURE 



511 



The great difference in amplitude between the Ant- 

 arctic diurnal inequalities from all and from quiet 

 days suggested a comparison between inequalities 

 from highly disturbed days, on the one hand, and quiet 

 days on the other. To secure a demonstrably impar- 

 tial selection, I took for each month the five inter- 

 national quiet days selected at De Bilt and the live 

 davs which had the largest "character " figures on the 

 international list. " Day " in this connection means 

 a period of twenty-four hours commencing at Green- 

 wich midnight. Thus Greenwich civil time has been 

 used in the curves in Fig. 3, which embody the results 

 obtained for the two sets of days in the Antarctic. 

 When comparing Antarctic results in Figs. 2 and 3, 

 it must be remembered that i ih. on the former 

 answers to oh. on the latter. 



-K All or ordinctry days 



Year 



o o Quiet ddy5 



Kew 



Antarctic 



Fig. 2. — Diurnal variation. 



Fig. 3 is confined to the four midwinter months, 

 May to August. 



Large as was the difference between the all and 

 quiet-day vectors in Fig. 2, it is quite eclipsed by the 

 difference between the disturbed and quiet-day vectors 

 in Fig. 3. In the latter figure the amplitude of the 

 disturbed-day vector averages about four times that of 

 the quiet-day vector. In fact, the vector for the dis- 

 turbed winter day averages about the same as the 

 vector of the ordinary summer day. 



While opinions may differ as to what the pheno- 

 mena shown by Figs. 2 and 3 really imply, it can 

 scarcely be questioned that they have an important 

 bearing on theories which attempt to account for the 

 diurnal variation. A difference in t>'pe between simul- 

 taneous diurnal inequalities at different places is a 

 natural enough consequence of difference of geograph- 



NO. 2442, VOL. 97] 



ical position. But the influence of disturbance is out 

 of all proportion greater in the Antarctic, and pre- 

 sumably also in the Arctic, than in the temperate 

 latitudes of Europe, and no mathematical formula 

 which contains only geographical co-ordinates and 

 sun's position can adequately meet the case of diurnal 

 inequalities the ratio of the amplitudes of which at 

 different places varies from day to day according to 

 the prevalence of disturbance. 



The 2'j-Day Period. 



A remarkable feature in magnetic disturbance is the 

 so-called 27-day period. This seems to have been first 

 noticed by J. A. Broun * in 1858, but the phenomenon 

 for some reason was practically overlooked until re- 

 discovered by W. Maunder' in 1904 in Greenwich 

 magnetic storms, and about the same time or a little 

 earlier by A. Harvey * in Toronto disturbances. 



All I think we are really entitled to say is that if a 

 certain day is disturbed, days from twenty-five to thirty 

 days later have more than the usual chance of being 



Autarctic 

 ■winter 



-K Disturbed days (5a month) 

 •« Quiet days - « ■• 



Fig. 3. — Dinmal variation. 



disturbed, and this probability is greater for the 

 twenty-seventh day than for the twenty-sixth or 

 twenty-eighth. 



If we confine our attention to large magnetic dis- 

 turbances an obvious difficulty arises. Large disturb- 

 ances are rare, and if all but large disturbances are 

 disregarded, a very inadequate supply of data remains. 

 If, on the other hand, we count a large number of 

 disturbances as magnetic storms, numerous chance 

 ref>etitions in twenty-seven, or any other specified 

 number of days, must be expected ; and in the absence 

 of any precise definition of what constitutes a storm — 

 and none commands general respect — claims as to 

 repetitions in twenty-seven days naturally fail to carry 

 conviction. There are, however, ways of testing the 

 existence of the period less exposed to criticism, and 

 those I have tried point to the real existence of a 

 27-day period in a certain sense of the term. 



The first thing is to get what will be generally 

 accepted as an impartial measure of disturbance, so 

 that days may be selected as representative of dis- 



S Ph'losopkieal Magazint, August, 1858. 

 ' R.A.S. Notices, vol. Ixv.. pp. 2 and 538, etc. 



* Proceedings of the Royal Astronomical Society of Canada, igos-s, 

 P- 74- 



