Sunday in 1888, 1892, 1896. 
    Tuesday in 1894. 
    Thursday in 1886, 1897. 
    Friday in 1887. 
    Saturday in 1893 and 1895.

Since, in Chart 11, the curves are drawn from Sunday to Sunday, it is obvious that the real symmetry of the curve is brought out in those years only which are characterized by a Sunday maximum; and, accordingly, in Chart 12 I have depicted the curves in a more suitable form.

Chart 12 A is obtained by combining the data of 1888, 1892, and 1896:  the years of a Sunday maximum.  Curve 12 B represents the results of 1894, the year of a Tuesday maximum—­multiplied throughout by three in order to render the curve strictly comparable with the former.  Curve 12 C represents 1886 and 1897—­the years of a Thursday maximum—­similarly multiplied by 1.5.  In Curve 12 D we have the results of 1887—­the year of a Friday maximum—­again multiplied by three; and in Curve 12 E those of 1893 and 1895—­the years of a Saturday maximum—­multiplied by 1.5.  Finally, Curve 12 F represents the combined results of all nine years plus (the latter half of) 1891; and this curve shows that, on the whole period, there is a very strongly marked Sunday maximum.

I hardly think that these curves call for much comment.  In their general character they display a notable concord among themselves; and it is significant that the most regular of the five curves are A and E, representing the combinations of three years and of two years, respectively, while the least regular is B, which is based upon the records of one year only.  In every case we find that the maximum which opens the week is rapidly succeeded by a minimum, which is itself succeeded by a secondary maximum,—­usually very secondary, although in 1894 it nearly equals the primary maximum,—­followed again by a second minimum—­usually nearly identical with the first minimum,—­after which there is a rapid rise to the original maximum.  The study of these curves fortunately amplifies the conclusion drawn from our study of the annual rhythm, and suggests that, in at least part of the year, the physiological condition of man requires sexual union at least twice a week.

As to Curve 12_F_, its remarkable symmetry speaks for itself.  The existence of two secondary maxima, however, has not the same significance as had that of our secondary maximum in the preceding curves; for one of these secondary maxima is due to the influence of the 1894 curve with its primary Tuesday maximum, and the other to the similar influence of Curve C with its primary Thursday maximum.  Similarly, the veiled third secondary maximum is due to the influence of Curve E.  Probably, any student of curves will concede that, on a still larger average, the two secondary maxima of Curve F would be replaced by a single one on Wednesday or Thursday.