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Points on the same curve are supposed to have the same average number of auroras in the year, and this average number is shown adjacent to the curve.
The total number of auroras in the year is taken as 100, and t denotes the time, in months, that has elapsed since the middle of January.
Putting t=o, 1, &c., in succession, we get the percentages of the total number of auroras which occur in January, February, and so on.
These show how the frequency of visible auroras diminished as cloud increased from o (sky quite clear) to 10 (sky wholly overcast).
Whilst daylight is the principal cause of the diurnal inequality, it is not the only cause, otherwise there would be as many auroras in the morning (forenoon) as in the evening (afternoon).