102 
DR. C. CHREE: SOME PHENOMENA OF SUNSPOTS AND OF 
hitherto prevailed amongst magneticians as to the reality of a 27—2 8-day period of a 
kind. There is undoubtedly a period, in the sense that a day which follows 27 or 28 
days after a disturbed day is more likely to be itself distubed than is the average day. 
But whether there is a period in the sense in which the term is applied to solar and 
lunar phenomena, which go through ,a regular cycle in a fixed period of time, and 
continually persist in doing so, is of course an entirely different matter. The present 
investigation does not throw, and was not intended to throw, light on this further 
question. 
§ 23. Data which relate to so long a period as 24-hours are not naturally very well 
adapted for determining the length of a period with any high precision. For such a 
purpose one wants data relating to a much shorter interval of time. Such data could 
be obtained in the present case by following the recent example of Prof. Bidlingmaier,* 
of Wilhelmshaven Observatory, in assigning character figures to individual hours of 
the day. It might prove possible in this way to estimate the period to a fraction of 
an hour, and investigate whether it is the same in all years, or depends in any way 
on the solar latitude of the greatest sunspot distribution, which is known to vary 
throughout the sunspot cycle. In this way light might be thrown on the cause of 
the phenomenon, whether of solar, lunar, or terrestrial origin. 
Though we can hardly expect to make a very exact estimate of the length of the 
period shown by the 11-year figures in Table XI., it may be worth recording the 
results of several rough estimates which are fairly accordant. It will be best to 
employ not the actual data in Table XI., but the sums of the character figures for 
the 660 days from which these data were derived. These sums contain each three 
significant figures. They had the following values for the columns specified :— 
Column . . . 
n + 24:. 
n + 25. 
71 + 26. 
n + 27. 
77 + 28. 
71 + 29. 
77+30. 
77 + 31. 
77 + 32. 
77 + 33. 
Sum .... 
430 
467 
545 
620 
607 
556 
523 
476 
464 
440 
Calculations were made on the followiner lines :— 
o 
Basis (i) 
Basis (ii) 
From day n + 26 to day 7i + 27 rise of 75 per diem, 
„ n + 2^ „ 71 + 29 fall „ 51 
From day 7i + 25 to day 7i + 27 mean rise of 76’5 per diem, 
„ n-t-28 „ n + ^l „ fall „ 43-6 
If we assume the rates of change between day 7i + 27 and the summit, and between 
the summit and day 28, 
period— 
to be first as in (i), second as in (ii), we find for the 
On basis (i) 27'30 days, 
„ (ii) 27-25 „ . 
* ‘ Veroffentlichungen des k. Observatoriums in Wilhelmshaven,’ Blatt 1-4, 1910, 1911. 
