ON “QUIET” DAYS DURING THE ELEVEN YEARS 1890 TO 1900, ETC. 
355 
cyclic eftect in V is considerably dependent on the season of the year, being greater 
in summer than in winter. We have already seen that an appreciable though small 
fraction of the effect in V is ascribable to secular change and instrumental causes ; 
and if these causes are as potent in winter as in summer, the difference between tlie 
phenomena observed in summer and winter is all the more noteworthy. 
In the case of W, N and T, I have conhned myself to mean seasonal values (see 
Table X.). The equivalent in force of 1' in D is 5‘3 y approximately. Hence it will 
be seen that the mean non-cyclic effect in W, though much less than in H, is larger 
than in D. The non-cyclic effect in N approaches closely that in H. 
In T the non-cyclic effect appears to be but little larger than in D, and it also 
shows the peculiarity of being positive in winter but negative in summer. The 
secular cliange and the instrumental causes described in dealing with V would tend 
to produce a slight (algebraic) reduction in the results obtained for T. 
Solar Diurnal Inequality. 
§ IS. By the diurnal inequality is meant the 'periodic change taking place in the 
value of an element throughout the 24 hours. In the case of a magnetic element on 
quiet days, there is as we have seen an appreciable non-cyclic effect, in virtue of 
which the value recorded at the second midnight of a day exceeds that recorded at 
the first midnight by a quantity, say N, which may be positive or negative. To 
obtain tbe proper periodic change, N must be eliminated, for unless this is done a 
diurnal inequality is olitained which is not periodic, which presents a discontinuity 
between two successive hourly values, and which is j^artly dependent on whidi hour 
of the 24 we select as the first hour of our day. It we assume N to result from 
some cause operating uniformly throughout the day, and count oui' day from mid¬ 
night, 0 hours, to midnight, 24 hours, the necessary elimination is efiected by 
applying to each hourly value the correction 
N (12 — 7i)/^4, 
where 'n is the hour counted from midniglit (U hours). 
This correction brings of course the values answering to iiours 0 and 24 into 
harmony, and leaves unaltered tlie mean value M of the element for the day as 
given by 
M = * 5|- [0] + i[:i4] + [1] + . . . + ['.iS]}, 
where [/i] represents the value of the element at hour n. When dealing with hourly 
values from a single day, or as in the present case with hourly means from 5 days a 
month, we really do not know how the non-cychc efi’ect comes in, and our method 
of elimination may introduce a factor not present in diurnal inequalities based on all 
days of the month. 
Another source of slight uncertainty may be noticed. The magnetic elements 
2 z 2 
