Magnetic Disturbances at Greenwich. 307 



(t. e. those o£ sudden commencement) . . .the irregularities in 

 the hourly distribution shown ... might be fortuitous. But 

 of the . . . not-S storms no less than . . .71 per cent, are ascribed 

 to the 8 hours noon to 7 p.m., and there is a most pro- 

 nounced maximum at 1 p.m. "; adding : Li If the phenomenon 

 is a true physical one it indicates a very dominant influence 

 which has nothing to do with the Sun's rotation period, and 

 which would have to be dealt with in any complete theory, 

 or in any estimate of probabilities/' 



The question of the diurnal variation of magnetic distur- 

 bances has been treated from a variety of standpoints. Some, 

 e.g. Sabine, have taken a disturbance as existing at any hour 

 when the observed value of a magnetic element has differed 

 by more than a certain amount from the mean monthly value 

 for that hour, and have treated positive and negative distur- 

 bances separately, finding diurnal variations for each as well 

 as for the two combined. Others have taken the difference 

 between the mean hourly values for all days in a month and 

 those for a few specially quiet days, considering the amplitude 

 of the departure at any given hour as an estimate of the 

 tendency to disturbance at that hour. Van Bemmelen * has 

 specially considered the incidence of the tiny undulatory 

 movements not unusual at times when large movements are 

 altogether absent. 



In all cases "disturbances " have shown a diurnal variation, 

 but the times of maxima and minima have varied according to 

 the particular standpoint adopted, and for any given definition 

 of disturbance different results have been obtained at different 

 parts of the Earth. A good deal of information on these 

 points is given in w T ell-known treatises, such as Balfour 

 Stewart's article in the 9th Edition of the Encyclopcedia 

 Britannica and Mascaras Magnetisme Terrestre. 



There was thus nothing surprising in the existence of a 

 diurnal inequality in the figures of Mr. Maunder's Table I.; 

 but some features of the inequality, especially the sudden- 

 ness of the rise to a maximum at 1 p.m., led me to suspect 

 that the phenomena were not wholly due to ^Nature. 

 Accordingly, on the appearance of Mr. Maunder's second 

 paper I examined its figures with interest, and found that 

 they also showed a conspicuous diurnal inequality, but that 

 it differed markedly in some details from the inequality I had 

 found in the first paper. Meantime Mr. Maunder read the 

 third paper referred to above, in which he too recognized 

 the existence of a diurnal inequality, and also noticed the 

 difference between his first two papers. Having seen a 



* Natuurkundig Tijdschrift voor Nederlandsch-Indi'e, 1902, p. 71. 



