If Fig. 19 is reached without the top of the column having separated, then the splash follows the course shown in Figs. 20-a to 23-a, in which it will be observed that the disappearance of the first column is very quickly followed by the rise of a secondary column very different in shape, which itself subsides again, but has not yet (in 23-a) formed, as it ultimately will, a second "cake" on the top of the first. Thus the second ripple follows late after the first. [Illustration: SERIES I--(_continued_) Alternative (_a_). 20-a 0·240 sec. 21-a 0·242 sec. 22-a 0·248 sec. 23-a 0·253 sec.] If, however, the summit of the primary column succeeds in breaking off (as in Fig. 18-b), or even in very nearly breaking off, then the impact of this newly-formed drop forms a second slight crater on the top of the first cake, and we have the series (18-b to 24-b), in which it will be observed that the rim of the secondary crater spreads rapidly outwards, so that a second well-marked circular ripple in this case _quickly_ follows the first. The secondary column that is thrown up in Fig. 23-b is very like that which emerged at a much earlier stage in the (_a_) series. The photographs of this (_b_) series show very beautifully the manner in which the advancing edge of the ripple degenerates into smaller ripples travelling with greater speed. [Illustration: SERIES I--(_continued_) Alternative (_b_). 18-b 0·214 sec. 19-b 0·237 sec. 20-b 0·242 sec. 21-b 0·244 sec.] [Illustration: SERIES I--(_continued_) Alternative (_b_). 22-b 0·261 sec. 23-b 0·257 sec. 24-b 0·311 sec.] It will be readily understood that if the splitting off of the head of the primary column happens to take place a little earlier, or on the other hand is nearly, but not quite, complete when it descends below the surface, then subsequent configurations will differ somewhat from either of the sub-series here shown. Since any figure photographed might belong to either sequence, the disentanglement of the two series required careful consideration and long experimenting. The reappearance of the original drop at the head of the rebounding column, of which the explanation has been given in this chapter, is easily verified by naked-eye observation. Let the reader when he next receives a cup of tea or coffee to which no milk has yet been added, make the simple experiment of dropping into it from a spoon, at the height of fifteen or sixteen inches above the surface, a single drop of milk. He will have no difficulty in recognizing that the column which emerges carries the white milk-drop at the top only slightly stained by the liquid into which it has fallen.