BELOW THE SURFACE[F]
Our investigation has so far been limited to what we can see from above
the surface of the liquid; nor perhaps would it occur to any one
acquainted only with so much as we have yet examined that it might be
worth while to look below the general level of the surface. The
discovery, however, that when the splash is made by a solid sphere very
remarkable phenomena, which will be described in the next chapter, take
place below the surface, led at a much later date to a similar
examination in the case of a liquid drop.
A suitable arrangement of the apparatus in the dark room is shown in the
accompanying diagram (Fig. 16).
The water into which the drop is to fall is placed in a thin glass
vessel AB, with parallel sides. (An inverted clock-shade makes a very
convenient vessel.) The water fills the vessel to the brim, and is
allowed to overflow it in a steady stream, thus presenting a surface
which, being perpetually renewed, is maintained perfectly clean. Close
behind the vessel is a plate P of finely roughened glass, on which the
light from the spark-gap F, in front of its concave reflector M, is
thrown by means of the condenser lens L taken from an optical lantern.
This provides a very uniformly illuminated background against which the
splash is viewed by means of the camera C, whose optic axis is
horizontal, either a little below the level of the liquid surface or at
that level. By having it just at the level of the surface we secure
simultaneous pictures of what is going on both above and below the
surface. There is, to be sure, a narrow band or region of confusion
stretching across the photographs in which the images obtained by
reflection, both external and internal, overlap the direct images, and
it should also be mentioned that the two pictures will not be quite in
focus together, for the optical effect of the water, through which the
part below the surface is viewed, is to bring the image forward.
The photographs of Series IV were obtained in this way from the splash
of a drop of water weighing 0.176 grams falling 40 cm. into water. (The
same splash as that of Series I-a.) The perfectly spherical form
presented by the cavity below the surface is very remarkable. In the
present case, this spherical cavity when at its deepest, as in Fig. 5,
would contain about fifty of the original drops, and in other
cases--e.g. with a drop of 1/4 the volume, falling from 177 cm.--the
cavity would contain as many as 360 of the original drops.
In Figs. 5, 6, and 7 the depth of the cavity is nearly constant, but the
diameter is steadily increasing. The spherical form, however, is still
maintained. The last figure shows the central column just beginning to
rise.
[Illustration: SERIES IV
The splash of Series I-a viewed below the surface.
1
2 T = 0
3 0.009 sec.
4 0.012 sec.
5 0.016 sec.
6 0.023 sec.
7 0.039 sec.
8 0.055 sec.
9 0.070 sec.]
There can be no doubt that the liquid of the original drop is spread out
in an excessively thin lining over the interior of this sphere. The
reader has seen for himself part of the evidence in the streaks of milk
that are carried up the inner walls of the crater when a milk-drop falls
into water (Series II); in the streaks of lamp-black that are carried
there when the drop is of milk, and it may here be mentioned that other
photographs that cannot be reproduced here have enabled me to trace the
gradual deformation of the drop into this thin layer and show that it
passes through configurations like Figs. 17, 18, and 19.
[Illustration: FIG. 17]
[Illustration: FIG. 18]
[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.