ON DROPS. 



547 



case corresponds to that of a simple pendulum started with a blow so 

 violent as to break the string. 



But the liquid star and the complicated pattern on the smoked 

 glass show that the splash is not a simple spreading out of the drop 

 equally in all directions, to return again. 



Fig. 7. 



Fig. 8. 



In order to observe the form of the drop at any given instant dur- 

 ing the splash, it is necessary to make use of the electric spark, and to 

 take advantage of the fact that drops of the same size falling from 

 the same height will all behave in the same way. 



It will be necessary to let a drop, say of mercury, fall on a plate 

 in comparative darkness, and to produce a strong spark at the instant 

 the bottom of the drop comes in contact with the plate, and so illumine 

 it ; the observer will then see the drop in the form it has at that m- 

 stant. 



A second drop must be let fall in the same way, and be illumined 

 by the spark not at the first moment of contact, but a shade later, say 

 ^i_ second later, when the drop will have spread itself out slightly on 

 the plate; and similarly we must illuminate a third drop a shade later 

 than the second, and so on. The observer can, after a little practice, 

 draw from memory on each occasion the drop in the form in which he 

 has seen it. It will be seen that the pi'ocess consists in isolating con- 

 secutive phases of the splash from those that precede and follow, and 

 which take place in darkness, and so do not confuse what has been 

 seen as they would do in continuous daylight. 



The device adopted by the writer for so timing the appearance of 

 the spark as to illumine the drop at any desired phase of the splash 

 consisted essentially in breaking the current of an electro-magnet at 

 the instant the drop began to fall*, the magnet, thus ceasing to act, 

 releases a spring which immediately begins to pull the terminal wire 

 of a strong electric current out of the other terminal, which is a cup 

 of mercury, and the strength of the spring and the depth of immersion 

 of the wire in the mercury are so adjusted that the wire leaves the 

 surface of the mercury, and the required spark is produced at the 

 instant the drop reaches the plate. 



