FROM SELECTED DAYS DURING THE SEVEN YEARS 1898 TO 1904. 
305 
Diurnal Inequality. 
§ 6. Table III. shows the mean diurnal inequality—after elimination of the non- 
cyclic effect—for the individual months and the year, and also for three seasons, 
viz., Winter (November to February), Equinox (March, April, September and 
October) and Summer (May to August). Each hourly value depends on approxi¬ 
mately 70 readings, and as these were taken to at least the nearest 10, sometimes 
the nearest 5 volts, the table gives the results to the nearest 0T volt. Distinct 
maxima and minima appear in heavy type. 
The ranges in the penultimate column represent the excess of the largest mean 
hourly value over the smallest. The last column gives the sum of the 24 hourly 
differences from the mean for the day to the nearest volt. 
The diurnal inequalities for the individual months and the year are illustrated by 
the curves in fig. 2 which are all drawn to a common scale. They pass through all 
the observational points and thus enable the reader to judge for himself of the 
sufficiency of the data. The curve for the year and those for a majority of the 
months, notably April, June, August and October, possess a smoothness which leaves 
little to be desired. A few isolated observational points catch the eye, e.g., 1 p.m. in 
January and in September. I have satisfied myself that the cause is not arithmetical 
error, but if the number of year’s data were largely increased the outstanding values 
for individual hours would very probably disappear. In the four months November 
to February there is a slight flattening or depression of the curve shortly after the 
evening maximum which looks hardly natural. Whether this would disappear if 
a larger number of year’s data were employed is, however, more doubtful. It seems 
to arise from a slight variability in the hour of maximum in the same month of 
different years. 
The double daily period, though most prominent in summer, is shown clearly in every 
instance. December is the month showing the nearest approach to a single maximum 
and minimum, November and January coming next; but even in these three months 
there is not the slightest doubt as to the existence of an afternoon minimum, while 
the forenoon maximum exceeds that of the late afternoon in January and February, 
and is practically equal to it in December. The early morning minimum is the 
principal one only in the four midwinter months and in August and September. The 
equinoctial months show the afternoon minimum just about as prominently as the 
midsummer months. 
The morning minimum occurs near 4 a.m., its time varying very little throughout 
the year. The afternoon minimum is only a little more variable, its time of occurrence 
falling usually near 2 p.m. The hours of maxima are rather more variable, the day 
interval between the two being less in the winter than in the summer months. 
When, as in the present case, there is a well-marked double maximum and minimum, 
the amplitude of the range is probably not so satisfactory a measure of the activity of 
VOL. CCVI.-A. 2 II 
