AT KEW OBSERVATORY, 1890 TO 1900. 
235 
frequency for quiet clays and for other days, it must be exceedingly small. Assuming 
Wolfer’s frequencies an appropriate measure of the phenomenon, this seemed 
practically conclusive against any theory which postulates a direct relationship 
without time lag between sunspot frequency and magnetic disturbance. 
It is clear, however, as I fully recognised at the time, that the result does not 
necessarily militate against any theory which supposes a day or more to elapse 
between the phenomenon on the sun and the corresponding phenomenon on the 
earth. Since the paper was written, Arrhenius and others have advanced theories 
—and evidence regarded as favourable to the theories—which postulate the transfer 
from the sun of electrified matter or ions at a rate very slow compared to that of 
light. Arrhenius considers two days as about the time required to reach the earth. 
Again, there are a considerable number of days for which Wolfer has no sunspot 
data, and in some months this was the case for more than one of the Astronomer 
Royal’s quiet days. I accordingly decided to repeat the investigation, at the same 
time widely extending its scope and replacing Wolfer’s frequencies by the Greenwich 
daily values of sunspot areas. These, being based on data from India and Mauritius, 
as well as Greenwich, are seldom lacking for more than 2 or 3 days in the year. 
Sunspot areas, as given at Greenwich, are of two kinds, projected areas and corrected 
areas. The former are the areas as seen and measured in photographs, expressed 
as millionths of the sun’s apparent disc ; the latter are corrected for foreshortening. 
I have made use of the projected areas, principally because these are collected and 
presented in a convenient table in the annual Greenwich volumes. The corrected 
areas are of course the more correct measure of the state of spottedness of the visible 
hemisphere, but if the action exerted by the sun is a species of bombardment, as 
seems postulated by the views of Arrhenius, Maunder, and others, the projected 
area should be the more appropriate for the present purpose. 
§ 26. The first results I shall discuss relate to the quiet and disturbed days. The 
days of each month were arranged in three approximately equal groups, according 
to sunspot areas. Normally the first and third groups contained respectively the 
10 days of largest and the 10 days of least sunspot area, the second group containing 
the intermediate days. It was then investigated into which groups the quiet and 
the disturbed days fell.* This was done for each month of the 11 years. If the 
sole cause of magnetic disturbance lay in simultaneous solar action dependent on 
sunspots, what we would expect to find would be all the disturbances falling under 
Group I. days, and all quiet days under Group III. 
In carrying the scheme into execution a difficulty was encountered, viz., that in 
1890, 1899, and 1900 there were months—21 in all—in which more than 10 days 
were wholly free from spots. It being impossible to get equal groups in these 
* The more natural process would have been to compare the mean sunspot areas for the quiet and the 
disturbed days with the mean area for the month; but this would have entailed the calculation of the 
monthly means, which do not seem to be given in the Greenwich volumes. 
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