[Illustration: FIG. 19] It appears possible that the study of this remarkable spherical excavation may afford a clue that will lead to a solution of the very difficult hydro-dynamical questions involved, and the matter is still being investigated. FOOTNOTES: [F] The information conveyed in this chapter was first published in a communication to the Mathematical and Physical Section of the British Association at Leicester in 1907. CHAPTER VII THE TWO KINDS OF SPLASHES OF SOLID SPHERES In the present chapter will be described the splash that follows the entry of a _solid_ sphere falling vertically into a liquid from a small height, and I should like to persuade the reader, if possible before he begins to read, or at any rate afterwards, to make a very simple experiment. Let a few child's marbles be taken--not glass "marbles," for these are seldom round enough or smooth enough, but what are sold in the toy-shops as "stone" marbles--and let one of these be well rubbed and polished with a dry handkerchief, and then dropped from a height of about 30 cm., or, say, 1 foot, into a deep bowl or basin of water, the bottom of which may be conveniently protected from breakage by a few folds of fine copper gauze. If the polishing has been good, and the surface of the sphere has not been dimmed by subsequent handling with hot or greasy fingers, it will be observed that the splash is singularly insignificant, the sphere slipping noiselessly into the liquid with very little disturbance of the surface. But if the same sphere be fished out of the water, and again let fall from the same height without being first dried, or, better still, if another marble be taken, which has been previously roughened with sand-paper, the resulting splash is totally different. There is now a noise of bubbles, which may be seen rising through the liquid, and a tall jet is seen to be tossed into the air. (1) THE SPLASH OF A ROUGH SPHERE.