AND OF TERRESTRIAL MAGNETISM. 267 



depends more on the character of the month in which that pulse falls than on the 

 amplitude of the primary dtsfcnrlMHMM with which it is associated. 



On the whole, Table VIII. suggests no special development of the 27-day period at 

 any particular season. If, tin- example, we take the three months clustering round 

 each equinox (i.e., hVhnuuy to April, and August to October), the mean of the ratios 

 in column 17 is 0'653 as compared with 0'635 from the other six months. A very 

 similar conclusion follows if we take the ratios in which the second member is the 

 character of the representative disturbed day in the month subsequent to the primary 

 pulse. 



When a sufficiently long series of years is available, it will be possible to replace 

 the ratios in columns 15 to 17 by others sufficiently smooth to show the real nature 

 of the annual variation, if such exists. The investigation might then be extended to 

 the second and third subsequent pulses, and to the previous pulses. When this is 

 done, in the case both of selected disturbed and selected quiet days, results of interest 

 may be expected. 



17. The primary object of S.M. was to investigate the nature of the connection, 

 if any, between sunspots and the daily range of H (horizontal force). Use was made 

 of the Greenwich projected sunspot areas. The 5 days of largest spot area in each 

 month of 1890 to 1900 formed the selected days, and the mean H ranges at Kew 

 were found for days previous and subsequent to the selected days. Denoting by n 

 the representative selected day of large sunspot area, the H range showed a marked 

 pulse with its crest at day rt + 4. Moreover, when curves were drawn having time 

 for abscissae, the ordinate being in the one case sunspot area and in the other H range, 

 the rise of the latter curve to a maximum and its subsequent decline closely resembled 

 the course of the former curve, but .with a lag of about 4 days. 



If we take the H trace for an individual highly disturbed day, it may be difficult 

 even for an expert to recognise the influence of the regular diurnal variation. But if 

 a number of such days are combined, a regular diurnal inequality emerges, which in 

 the case of H differs little from that characteristic of quiet days, except in being of 

 larger amplitude. Even on days of character " 2," the H range owes an appreciable 

 amount to the regular diurnal inequality, and on the average day especially in a 

 quiet year the regular diurnal inequality is the principal contributor. Thus there 

 were strong d priori reasons for regarding the relation described above as involving 

 the regular diurnal inequality rather than magnetic disturbance. This view was 

 supported by an examination of the Kew "character" figures for days previous and 

 subsequent to the selected days of the 1 1 years. The mean " character " figure of 

 each column was derived from 5x12x11, or 660 days. Of the 660 days occurring 

 in column + 4, where the crest of the pulse in the H ranges appeared, 86 were of 

 " character " " 2." Out of 660 average days of the 11 years, 82 had a "character" "2" ; 

 thus the excess of days of " character " " 2 " in column n + 4 was only 4, and of the 

 31 columns from n 15 to n+15, 10 showed an excess larger than this, the excess 



2 M 2 



