304 DR. CHARLES CHREE: SOME PHENOMENA OF 



They were selected in a somewhat arbitrary way, and many more were taken from 

 some years (e.g., 1892 and 1896) than from others (1890, 1893, and 1900). They are 

 also in all probability not equally representative of the different seasons of the year. 

 It is thus impossible to say what the exact significance is of the difference in type 

 which is not very striking between the disturbed -- quiet and the ordinary ~*~ quiet 

 day inequalities of declination shown in (C), fig. 3. 



31. The V difference curves in fig. 2 of the present paper are of a very suggestive 

 character. Anyone, I think, looking at the three sets of V curves in this figure, if 

 knowing only that two were fundamental and one derived, would naturally select the 

 quiet day and the difference curves as the fundamental ones. The quiet day curve 

 has for its essential features the deep depression about noon, the late forenoon fall, 

 the early afternoon recovery. The difference curve has for its essential features the 

 early morning minimum, the smooth rise to the afternoon maximum, and the smooth 

 evening fall. The disturbed day curve seems to represent a struggle, only partially 

 successful, to throw off the midday depression and to approach an ideal represented 

 by the difference curve. 



The difference in type between disturbed and quiet day curves seems much less in 

 H and N than in V or W. There may conceivably be directions in space at each 

 station, the components along which of the diurnal force system are unaffected in type 

 by disturbance. If such directions exist, their discovery and utilisation might lead 

 to valuable results. 



32. A conclusion to which my investigations point is that, if circumstances allow, 

 much knowledge might be gained by dividing the days at each station into, say, 

 three not overlapping groups, one representing quiet days, one highly disturbed days, 

 and the third intermediate days, diurnal inequalities being formed from these groups 

 separately. With our present knowledge there is no means of securing that the 

 choice of the different groups will lead to equality in the degree of disturbance in the 

 same month at different stations, or at the same station in different months. Any 

 very complicated system of selection would be pretty certain to break down. The 

 best plan, until further knowledge is gained, would probably be to specify a given 

 number of days for each of the extreme groups. Judging by the international 

 results, already referred to, the choice in the case of quiet days would lie between 

 5 and 10 days a month. In most months there are at least 10 moderately quiet days 

 at stations in temperate latitudes, and if the first group were limited to 5 days it 

 would mean including in Group B (intermediate days) a number of days more 

 appropriate to Group A. On the other hand, it is often difficult in high latitudes to 

 get even 5 moderately quiet days a month, and to get 10 is difficult in some months 

 even in temperate latitudes. Then some stations may be willing to tabulate 5 but 

 not as many as 10 days a month, and it would be a pity to lose their co-operation 

 entirely. 



For the Group C of highly disturbed days, 5 promises to be the best number. 



