THE SPLASH OP A DROP AND ALLIED PHENOMENA. 201 



nomeuon thus easily brought under examination, but the apparatus 

 also affords the means of measuring the time interval between any two 

 stages. All that is necessary is to know the distance that the timing 

 sphere falls in the two cases. Elementary dynamics then give us the 

 interval required. Thus, if the sphere falls 1 foot and we then lower D 

 one-fourth of an inch, the interval between the corresponding stages 

 will be about 0.0026 second. 



Having thus described the apparatus, which I hope shortly to show 

 you in action, I pass to the information that has been obtained by it. 



This is contained in a long series of drawings, of which a selection will 

 be i)resented on the screen. The first series that I have to show repre- 

 sents the splash of a drop of mercury 0.15 inch in diameter that has 

 fallen o inches onto a smooth glass plate. (Plates II, III.) It will be 

 noticed that very soon after the first moment of impact minute rays are 

 shot out in all directions on the surface. These are afterwards over- 

 flowed or united, until, as in fig. 8, the outline is only slightly rippled. 

 Then (fig. 9) main rays shoot out, from the ends of which in some cases 

 minute droplets of liquid would split off", to be left lying in a circle on 

 the ])late, and visible in all subsequent stages. By counting these drop- 

 lets when they were thus left, the number of rays was ascertained to 

 have been generally about twenty-four. This exquisite shell-like con- 

 figuration shown in fig. 9, marks about the maximum spread of the 

 li(i[uid, which, subsiding in the middle, afterwards flows into an annulus 

 or rim with a very thin central film, so thin, in fact, as often to tear 

 more or less irregularly. This annular rim then divides or segments 

 (figs. 14, 15, 1(5) in such a manner as to join up the rays in pairs, and 

 thus passes into the twelve-lobed annulus of fig. 16. Then the whole 

 contracts, bat contracts most rapidly between the lobes, the liquid then 

 being driven into and feeding the arms, which follow more slowly. In 

 fig. 21 the end of this stage is reached, and now the arms continuing to 

 come in, the liquid rises in the center; this is, in fact, the beginning of 

 the rebound of the drop from the plate. In the case before us the drops 

 at the ends of the arms now break oft" (fig. 25), while the central mass 

 rises in a column which just fails itself to break up into drops, and falls 

 back into the middle of the circle of satellites which, it will be under- 

 stood, may in some cases again be surrounded by a second circle of the 

 still smaller and more numerous droplets that split off the ends of the 

 rays in fig. 9. The whole of the thirty stages described are accom- 

 plished in about one-twentieth of a second, so that the average interval 

 betw^een them is about one six-hundredth of a second. 



It should be mentioned that it is only in rare cases that the subordi- 

 nate drops, seen in the last six figures, are found lying m a very com- 

 plete circle after all is over, for there is generally soniQ slight disturbing- 

 lateral velocity which causes many to mingle again with the central 

 drop, or with each other. But even if only half or a quarter of the 

 circle is left, it is easy to estimate how many drops and therefore how 



