Since the origination of these ripples is an interesting phenomenon from a physical point of view, as throwing light on the dispersion of waves travelling with different velocities, special precautions were taken to secure the most favourable conditions, and in order to clean the surface after the arrival of each drop, which inevitably brings down a little adherent lamp-black, a continuous slow stream of fresh water was maintained which swept the contaminated surface-liquid away over the edge of the vessel. The effect of this precaution is seen by a comparison of the photographs No. 11 and No. 11-a. In the first the surface was kept quite clean in the way described; in the second it had only been cleaned by skimming it with a fine wire-gauze dish. The beginning of the descent of the first central column seems to be marked by the appearance of a slight depression round its base, which has just not begun in No. 11-a, and has just begun in No. 11, and goes on increasing in Figs. 12 and 13. [Illustration: SERIES I-a--(_continued_) Running water. Scale reduced to 6/10. 9 0·087 sec. 10 0·014 sec. 11 0·139 sec. 11-a 0·139 sec.] [Illustration: SERIES I-a--(_continued_) Running water. Scale 6/10. 12 0·163 sec. 13 0·185 sec. 14 0·207 sec. 15 0·227 sec.] The same feature marks the beginning of the descent of the secondary central column, which is still rising in Fig. 17, is just poised in Fig. 18, and thence onwards shows a gradually increasing central depression. These last four figures carry us to a rather later stage than was reached in Series I. It should be noticed that in this Series the water-drop used was of smaller diameter than that of Series I, weighing ·13 grams as against ·2 grams. By employing the smaller drop, we diminish irregularities due to oscillations of form set up on release, for the smaller drop is more spherical when lying on the dropping cup than the larger; a few photographs taken for comparison with the full-sized drop showed, however, extremely little difference in the splashes at this height of fall. [Illustration: SERIES I-a--(_continued_) Still water. Scale 9/10. 16 0·247 sec. 17 0·266 sec. 18 0·294 sec.] [Illustration: SERIES I-a--(_continued_) Still water. Scale 9/10. 19 0·285 sec. 20 0·311 sec. 21 0·321 sec.] CHAPTER V HIGHER FALLS--BUBBLE-BUILDING