ON IMPACT WITH A LIQUID SURFACE. 195 



stage the ensheathiiig film was almost complete. This photograph is practically 

 identical with fig. 6, Series XIII., Plate 3. 



Passing now to the " rough " splash, we have, in fig. 1, of Series XXII., Sheet 8 

 (see Plate 3), a " rough " splash in air ; in fig. 2, the same in vacuo; in fig. [3], a later 

 stage in vacuo. The liquid in each case was water, and the height of fall, 13 '5 centims. 

 Figs. [4] and [5] show the collapse of the vacuous column formed behind a rough 

 sphere falling into Alexandra oil. In each of these last there is an appearance of 

 folding at the surface, which was not observable when air was present. And we see 

 the l>eginning of the same kind of thing on the left-li.tml side in fig. 2. 



Tli is difference, which is probably due to negative pressures induced by vortical 

 motion near the interface, appears, however, to be quite a secondary matter, and does 

 not prevent us from asserting that the presence of the air has no material influence 

 on the early course of the splash, except, as already explained, in respect of the 

 extent to which its pressure serves to relieve the liquid from a cohesive tension under 

 which it would cavitate. 



EJ'I " i-iiii< ,,ts with a Viscid Liquid. 



It appeared probable that experiments made with water thickened by successive 

 additions of glycerine would throw light on the part played by viscosity in the 

 transaction. With a mixture consisting of water 51 vols., glycerine 2 vols., no change 

 of any kind was perceptible in the splashes observed. When the glycerine was 

 increased to 6 vols. in 51 of water, the disturbance set up was still extremely similar, 

 as will be seen by comparing the figures of Sheet 9, which represent the splash of a 

 smooth serpentine sphere in the glycerine mixture at the heights specified, with the 

 corresponding figures in the case of water, Sheet 4, Series XIV. and XVI. The only 

 noticeable difference is the rather greater salience of the ribs in some of the glycerine 

 figures, and the greater reluctance in the jets to segment into droplets. 



We then experimented with pure glycerine. Series XXVI. of Sheet 10, reproduced 

 in part on page 196, gives the splash in pure glycerine of the same polished serpentine 

 sphere, 257 centims. in diameter, falling 75 centims. In all cases the radial ribs are 

 seen in the negatives of the photographs to be very pronounced. Even at so early a 

 stage as fig. 1 the fluting is well developed. The two photographs taken of stage 3 

 had each of them an isolated jet, probably owing to the fact that when working with 

 so sticky a liquid it was difficult to avoid contaminating the cloth on which the sphere 

 was each time re-polished after washing in water, with the result that the sphere 

 behaved as if locally rough. The relatively great length of this jet brings out very 

 well the part played by the viscous drag in hindering the flow of that portion of the 

 liquid sheath which has remained in contact with the sphere. In the last figure 

 No. 4, of this series the droplets just visible in the centre, below the level of the 

 general surface, correspond to those of figs. 6 and 7 of Series XIII. at a much higher 



2 c 2 



