THE SPLASH CONTINUED
I have stated that the addition of the milk to the water made but little
difference in the character of the resulting splash. It does, however,
make certain differences in detail, as will be gathered from an
examination of the next Series I-a, which shows the effect of letting
the water-drop fall from the same height into water instead of into
milk. Such a splash is difficult to photograph unless the illumination
is from behind. As shown in this way, the early figures of the crater
might be unintelligible to the reader had he not already studied the
same crater lighted up from the side. Sometimes, though the front of the
crater is hardly visible directly, yet every lobe on it can be clearly
traced in the inverted image seen by reflection.
The most noticeable difference between the two splashes is perhaps the
very much greater number of ripples seen with the splash in pure water.
This is partly because, with the illumination behind, such ripples are
more easily visible, but arises chiefly from the fact that ripples are
not so readily propagated over the surface of milk on account both of
its smaller surface-tension and its greater viscosity. The first
appearance of outward-spreading ripples is in No. 6, just round the
subsiding crater.
[Illustration: SERIES I-a
Water into water (40 cm. fall). Scale 9/10.
1 T = 0
2 0.004 sec.
3 0.013 sec.
4 0.018 sec.]
[Illustration: SERIES I-a--(_continued_)
5 0.026 sec.
6 0.042 sec.
7 0.058 sec.
8 0.073 sec.]
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.]