] = .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.]

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 canals 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 foun