V] THE SPLASH AND THE BUBBLE 393 



annulations which these tentacles exhibit, the web-like film which 

 sometimes (when they stand a httle way apart) conjoins their bases, 

 the thin annular film of tissue which surrounds the little organism's 

 mouth, and the manner in which this annular "peristome" con- 

 tracts*, like a shrinking soap-bubble, to close the aperture, are 

 every one of them features to which we may find a singular and 

 striking parallel in the surface-tension phenomena of the splash "f". 



Some seventy years ago much interest was aroused by Helmholtz's 

 work (and also Kirchhoff's) on "discontinuous motions of a fluidj"; 

 that is to say, on the movements of one body of fluid within another, 

 and the resulting phenomena due to friction at the surfaces between. 

 What Kelvin§ called Helmholtz's "admirable discovery of the law 

 of vortex-motion in 'a perfect fluid" was the chief result of this 

 investigation; and was followed by much experimental work, in 

 order to illustrate and to extend the mathematical conclusions. 



The drop, the bubble and the splash are parts of a long story; 

 and a "falling drop," or a drop moving through surrounding fluid, 

 is a case deserving to be considered. A drop of water, tinged with 

 fuchsin, is gently released (under a pressure of a couple of milh- 

 metres) at the bottom of a glass of water || . Its momentum enables 

 it to rise through a few centimetres of the surrounding water, and 

 in doing so it communicates motion to the water around. In front 

 the rising drop thrusts its way through, almost like a sohd body; 

 behind it tends to drag the surrounding water after it, by fluid 

 friction^ ; and these two motions together give rise to beautiful vorti- 

 coid configurations, the Strdmungspilze or Tintenpilze of their first 

 discoverers (Fig. 119). Under a higher and more continuous pressure 



* See a Study of Splashes, p. 54. 



t There is little or no difference between a splash and a burst bubble. The craters 

 of the moon have been compared with, and explained by, both of these. 



X Helmholtz, in Berlin. Monatsber. 1868, pp. 215-228; Kirchhoflf, in CreUe'a 

 Journal, lxx, pp. 289-298, lxxi, 237-273, 1869-70. 



§ W. Thomson, in Proc. R.S.E. vi, p. 94, 1867. 



II See A. Overbeck, Ucber discontinuirliche Fliissigkeitsbewegungen, Wiedemann'' s 

 Annalen, ii, 1877; W. Bezold, Ueber Stromungsfiguren in Fliissigkeiten, ibid. 

 XXIV, pp. 569-593, 1885; P. Czermak, ibid, l, p. 329, 1893; etc. 



% The frictional drag on the hinder part of the drop is felt alike in the ship, the 

 bird and the aeroplane, and tends to produce retarding vortices in them all. It is 

 always minimised in one way or another, and it is autoraaticaUy minimised in the 

 present instance, as the drop thins off and tapers down. 



