When, however, we double the volume of the drop and raise the height of fall to 52 inches, the splash of Series IV. is obtained, which is beginning to assume quite a different character. The raised rim of the previous series is now developed into a hollow shell of considerable height, which tends to close over the drop. This shell or dome is a characteristic feature of all splashes made by large drops falling from a considerable height, and is extremely beautiful. In the splash at present under consideration it does not always succeed in closing permanently, but opens out as it subsides, and is followed by the emergence of the drop (Fig. 8). In Fig. 9 the return wave overwhelms the drop for an instant, but it is again seen at the summit of the column in Fig. 10. SERIES IV. _The Splash of a Drop, followed in detail by Instantaneous Illumination._ Diameter of Drop, 1/4 inch. Height of Fall, 4 ft. 4 in. [Illustration: 1 [Tau] = 0 sec.] [Illustration: 2 [Tau] = .0021 sec.] [Illustration: 3 [Tau] = .0042 sec.] [Illustration: 4 [Tau] = .0165 sec.] [Illustration: 5 [Tau] = .0206 sec.] [Illustration: 6 [Tau] = .0443 sec.] [Illustration: 7 [Tau] = .0482 sec.] [Illustration: 8 [Tau] = .0595 sec.] [Illustration: 9 [Tau] = .0707 sec.] [Illustration: 10] [Illustration: 11] But on other occasions the shell or dome of Figs. 4 and 5 closes permanently over the imprisoned air, the liquid then flowing down the sides, which become thinner and thinner, till at length we are left with a large bubble floating on the water (see Series V.). It will be observed that the flow of liquid down the sides is chiefly along definite channels, which are probably determined by the arms thrown up at an earlier stage. The bubble is generally creased by the weight of the liquid along these channels. It must be remembered that the base of the bubble is in a state of oscillation, and that the whole is liable to burst at any moment, when such figures as 6 and 7 of the previous series will be seen. [Illustration: SERIES V. _The Splash of a Drop, followed in detail by Instantaneous Illumination._ The Size of Drop and Height of Fall are the same as before, but the hollow shell (see figs. 4 and 5 of the previous Series) does not succeed in opening, but is left as a bubble on the surface. This explains the formation of bubbles when _big_ rain-drops fall into a pool of water.]