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  _THE ROMANCE OF SCIENCE_

  THE SPLASH OF A DROP


  BY
  PROF. A.M. WORTHINGTON, M.A., F.R.S.


  _Being the reprint of a Discourse delivered at the Royal Institution
  of Great Britain, May 18, 1894._

  PUBLISHED UNDER THE DIRECTION OF THE GENERAL
  LITERATURE COMMITTEE.


  LONDON:
  SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE,
  NORTHUMBERLAND AVENUE, CHARING CROSS, W.C.;
  43, QUEEN VICTORIA STREET, E.C.
  BRIGHTON: 129, NORTH STREET.
  NEW YORK: E. & J.B. YOUNG & CO.
  1895.




THE SPLASH OF A DROP




INSTANTANEOUS PHOTOGRAPHS OF THE SPLASH OF A WATER-DROP FALLING ABOUT 16
INCHES INTO MILK.

[Illustration: Time after contact =.0262 sec.]

[Illustration: Time after contact =.0391 sec.]

[Illustration: Time after contact =.101 sec.]




THE SPLASH OF A DROP


The splash of a drop is a transaction which is accomplished in the
twinkling of an eye, and it may seem to some that a man who proposes to
discourse on the matter for an hour must have lost all sense of
proportion. If that opinion exists, I hope this evening to be able to
remove it, and to convince you that we have to deal with an exquisitely
regulated phenomenon, and one which very happily illustrates some of the
fundamental properties of fluids. It may be mentioned also that the
recent researches of Lenard in Germany and J.J. Thomson at Cambridge, on
the curious development of electrical charges that accompanies certain
kinds of splashes, have invested with a new interest any examination of
the mechanics of the phenomenon. It is to the mechanical and not to the
electrical side of the question that I shall call your attention this
evening.

The first well-directed and deliberate observations on the subject that
I am acquainted with were made by a school-boy at Rugby some twenty
years ago, and were reported by him to the Rugby Natural History
Society. He had observed that the marks of accidental splashes of
ink-drops that had fallen on some smoked glasses with which he was
experimenting, presented an appearance not easy to account for. Drops of
the same size falling from the same height had made always the same
kind of mark, which, when carefully examined with a lens, showed that
the smoke had been swept away in a system of minute concentric rings and
fine striae. Specimens of such patterns, obtained by letting drops of
mercury, alcohol, and water fall on to smoked glass, are thrown on the
screen, and the main characteristics are easily recognized. Such a
pattern corresponds to the footprints of the dance that has been
performed on the surface, and though the drop may be lying unbroken on
the plate, it has evidently been taking violent exercise, and were our
vision acute enough we might observe that it was still palpitating after
its exertions.

A careful examination of a large number of such footprints showed that
any opinion that could be formed therefrom of the nature of the motion
of the drop must be largely conjectural, and it occurred to me about
eighteen years ago to endeavour by means of the illumination of a
suitably-timed electric spark to watch a drop through its various
changes on impact.

The reason that with ordinary continuous light nothing can be
satisfactorily seen of the splash, is not that the phenomenon is of such
short duration, but because the changes are so rapid that before the
image of one stage has faded from the eye the image of a later and quite
different stage is superposed upon it. Thus the resulting impression is
a confused assemblage of all the stages, as in the photograph of a
person who has not sat still while the camera was looking at him. The
problem to be solved experimentally was therefore this: to let a drop of
definite size fall from a definite height in comparative darkness on to
a surface, and to illuminate it by a flash of exceedingly short duration
at any desired stage, so as to exclude all the stages previous and
subsequent to the one thus picked out. The flash must be bright enough
for the image of what is seen to remain long enough on the eye for the
observer to be able to attend to it, and even to shift his attention
from one part to another, and thus to make a drawing of what is seen. If
necessary the experiment must be capable of repetition, with an exactly
similar drop falling from exactly the same height, and illuminated at
exactly the same stage. Then, when this stage has been sufficiently
studied, we must be able to arrange with another similar drop to
illuminate it at a