[Ill.u.s.tration: 7

[Tau] = 0759 sec.]

[Ill.u.s.tration: 8

[Tau] = 0901 sec.]

[Ill.u.s.tration: 9]

[Ill.u.s.tration: 10]

[Ill.u.s.tration: 11]

[Ill.u.s.tration: 12

[Tau] = 295 sec.]

[Ill.u.s.tration: 13]

[Ill.u.s.tration: 14]

If we keep to the same size of drop and increase the fall to something over a yard, no great change occurs in the nature of the splash, but the emergent column is rather higher and thinner and shows a tendency to split into drops.

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 a.s.sume quite a different character. The raised rim of the previous series is now developed into a hollow sh.e.l.l of considerable height, which tends to close over the drop. This sh.e.l.l 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.

[Ill.u.s.tration: 1

[Tau] = 0 sec.]

[Ill.u.s.tration: 2

[Tau] = 0021 sec.]

[Ill.u.s.tration: 3

[Tau] = 0042 sec.]

[Ill.u.s.tration: 4

[Tau] = 0165 sec.]

[Ill.u.s.tration: 5

[Tau] = 0206 sec.]

[Ill.u.s.tration: 6

[Tau] = 0443 sec.]

[Ill.u.s.tration: 7

[Tau] = 0482 sec.]

[Ill.u.s.tration: 8

[Tau] = 0595 sec.]

[Ill.u.s.tration: 9

[Tau] = 0707 sec.]

[Ill.u.s.tration: 10]

[Ill.u.s.tration: 11]

But on other occasions the sh.e.l.l 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.

[Ill.u.s.tration: 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 sh.e.l.l (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.]

Such is the history of the building of the bubbles which big rain-drops leave on the smooth water of a lake, or pond, or puddle. Only the bigger drops can do it, and reference to the number at the side of Fig. 5 of Series IV. shows that the dome is raised in about two-hundredths of a second. Should the domes fail to close, or should they open again, we have the emergent columns which any attentive observer will readily recognize, and which have never been better described than by Mr. R.L.

Stevenson, who, in his delightful _Inland Voyage_, speaks of the surface of the Belgian ca.n.a.ls along which he was canoeing, as thrown up by the rain into "an infinity of little crystal fountains."

Very beautiful forms of the same type indeed, but different in detail, are those produced by a drop of water falling into the lighter and more mobile liquid, petroleum.

It will now be interesting to turn to the splash that is produced when a solid sphere, such as a child's marble, falls into water.

I found to my great surprise that the character of the splash, at any rate up to a height of 4 or 5 feet, depends entirely on the state of the surface of the sphere. A polished sphere of marble about 06 of an inch in diameter, rubbed very dry with a cloth just beforehand and dropped from a height of 2 feet into water, gave the figures of Series VI., in which it is seen that the water spreads over the sphere so rapidly, that it is sheathed with the liquid even before it has pa.s.sed below the general level of the surface. The splash is insignificantly small and of very short duration.[3] If the drying and polishing be not so perfect, the configurations of Series VII. are produced; while if the sphere be roughened with sandpaper, or _left wet_, Series VIII. is obtained, in which it will be perceived that, as was the case with the liquid drop, the water is driven away laterally, forming the ribbed basket-shaped hollow, which, however, is now prolonged to a great depth, the drop being followed by a cone of air, while the water seems to find great difficulty in wetting the surface completely. Part of this column of air was carried down at least 16 inches, and then only detached when the sphere struck the bottom of the vessel.

SERIES VI., VII.

_Splash of a Solid Sphere (a marble 1/2 inch in diameter falling 2 feet into water)._

[Ill.u.s.tration: SERIES VI.

When the sphere is _dry_ and _polished_.]

[Ill.u.s.tration: SERIES VII.