RANGE AND THE PERIOD OF SOLAR SPOT FREQUENCY. 
543 
horizontal force. Two series of numbers are thus formed, such series being treated 
independently. 
By the mean diurnal range of declination or horizontal force is to be understood a 
number formed as follows :—Means of the indications at each separate hour being 
taken through a month, the difference between the greatest and least amongst these 
mean values is the monthly mean diurnal range. It should be stated that in the 
formation of these means, days of great magnetic disturbance were rejected, and also 
certain other days on which there prevailed a lesser but considerable amount of dis¬ 
turbance (not, however, defined by any strict numerical rule, but estimated according 
to a general standard formed in the examination of many thousands of photographs). 
The numbers, both for declination and horizontal force (those referring to the latter 
being corrected for temperature), for the years from 1841 to 1877 inclusive, are given 
in the following table. Until the end of the year 1847 the numbers were obtained 
from two-hourly values, and may therefore be a little small as compared with those for 
the remaining years which depend on hourly values, but no correction has on this 
account been applied. The two sets of numbers are taken as forming one uniform 
series. 
4 A 
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