319 
FROM SELECTED DAYS DURING THE SEVEN YEARS 1898 TO 1904. 
In 'egular Diurnal Changes. 
§ 16. Hitherto we have exclusively considered the regular diurnal changes, those 
for each month being derived by taking hourly means from about 70 days and 
eliminating any non-cyclic element. The amplitude of these periodic changes gives, 
however, a very inadequate idea of the phenomena. Even in the finest weather the 
regular diurnal change is accompanied by numerous irregular changes, and very often 
the latter so predominate that the former is but inconspicuously shown in the 
photographic trace. 1 have thus gone into the individual day’s results with some 
minuteness. My object was chiefly to ascertain the effect of different meteorological 
conditions—a subject considered later—but also partly to see whether any sort of 
proportionality existed between the size of the regular and irregular movements 
throughout the year. The results of this latter investigation are summarised in 
Table X. 
Table X. — Results from Individual Days. 
Mean of 
maximum 
and 
minimum. 
Range. 
Non-cyclic 
change. 
± 
Ratios to 
mean value for the 
24 hours. 
Ratio of the mean 
of individual 
daily ranges to 
the range 
in the diurnal 
inequality. 
Mean of 
maximum 
and 
minimum. 
Range. 
Non-cyclic- 
change. 
+ 
January . . . 
212 
203 
93 
1-05 
1-01 
0-46 
2-79 
F ebruary . . 
234 
218 
86 
1-05 
0-98 
0-39 
2-32 
March . . . 
197 
210 
85 
1-10 
1-17 
0-47 
2-52 
April .... 
153 
164 
62 
1-11 
1-19 
0-45 
2 * 22 
May .... 
135 
143 
53 
1-10 
1-17 
0-44 
2-03 
June .... 
120 
132 
45 
1-09 
1-19 
0-41 
2-33 
July .... 
109 
117 
40 
1-12 
1-20 
0-41 
2-12 
August . . . 
123 
129 
41 
1-08 
113 
0-36 
2-17 
September . . 
129 
141 
54 
1 07 
1-16 
0-45 
2-61 ' 
October . . . 
163 
196 
80 
1-06 
1-28 
0-52 
3-13 
November . . 
206 
186 
74 
1-03 
0-93 
0-37 
3-60 
December . . 
247 
213 
96 
1-02 
0-88 
0-40 
2-59 
Year .... 
169 
171 
67-5 
1-071 
1-107 
0-426 
2-54 
Winter . . .„ 
225 
205 
87-2 
1-037 
0-949 
0-403 
2-83 
Equinox. . . 
161 
178 
70-4 
1-083 
1-200 
0-473 
2-62 
Summer . . . 
122 
130 
44-9 
1-094 
1-173 
0-403 
2-16 
The first numerical column of Table X. gives the monthly and seasonal mean 
values of the arithmetic mean of the largest and smallest hourly values in individual 
days; the fourth column shows the ratio which the value thus found for the month 
or season bears to the corresponding mean value of P. 
The second column gives the monthly and seasonal mean values of the excess of 
