186 MESSRS. A. M. WORTHINGTON AND R. 8. COLE 



The minute but deep corrugations on the surface of the air-bubble in fig. 6 are 

 probably indications of rapid turbulent motion at the interface. 



We may here recall to the recollection of the reader that a remarkable feature of 

 the sheath enveloping a smooth sphere, entering with low velocity, was the strongly 

 accentuated radial ribs and flutings (cf. Series XXVI., figs. 1 and 3, p. 196), which are 

 specially well seen in the figs. 3, 4, 5, and 9 of Series VI. of Paper I. The present 

 photographs, taken, as they are, with the light behind the object, do not bring these 

 out very well, but traces of them may be detected with a lens in the original photo- 

 graphs, though not in the present reproductions, on the under surface of the liquid to 

 the left of the sphere in figs. 1 and 2 of Series XVII., and in figs. 1 and 2 of 

 Series XVIII., on the right side of the sphere in fig. 1 of Series XIV., and again in 

 fig. 4 of Series XVI. ; and it must not be forgotten that they are probably always 

 present. Of this fluting we shall be able presently to give a pretty complete 

 account. 



General Explanation of the Phenomena. 



The explanation which seems to give the key to the whole phenomenon was 

 suggested (1) partly by the observation of AITKEN that dust does not settle on 

 an object hotter than the air, (2) partly by the observation of QUINCKE that a film of 

 extremely small thickness spreads with great rapidity by molecular action over a 

 polished surface, e.g., of glass or mica when this is touched by a liquid, and (3) partly 

 by our own observation that, at any rate in the neighbourhood of the surface, a flow 

 once set up along any channel is comparatively persistent and determines the motion 

 that is to follow. As illustrations of this we may cite the regular disposition of jets 

 round the rim of the crater thrown up by an entering drop, which persists for a 

 comparatively long time, and is apparently due to the spontaneous segmentation of 

 the annular rim at a very early stage ; or, again, the very strongly salient ribs of flow 

 just alluded to (see Series VI. of our first paper), each of which seems to correspond to 

 one of the jets. 



This general explanation is as follows : 



When a sphere, either rough or smooth, first strikes the liquid, there is an 

 impulsive pressure between the two, and the column of liquid lying vertically below 

 the elementary area of first contact is compressed. For very rapid displacements the 

 liquid on account of its viscosity behaves like a solid. In the case of a solid rod we 

 know that the head would be somewhat flattened out by a similar blow, and a wave 

 of compression would travel down it ; to this flattening or broadening out of the head 

 of the column corresponds the great outward radial velocity, tangential to the 

 surface, initiated in the liquid, of which we have abundant evidence in many of the 

 photographs. 



Into this outward flowing sheath the sphere descend?*, and since each successive 



