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THE POPULAR SCIENCE MONTHLY 



less fine radial striae where the smoke has been swept away. These 

 may be seen very well by holding the glass plate up to the light if it 

 has not been too tliickly smoked. 



The marks thus made are very beautiful and. symmetrical, and it 

 will be found, if the glass be uniformly smoked, that the same-sized 

 drops of the same liquid falling from the same lieight will produce 

 almost exactly similar marks : while if the height be changed the 

 mark on the lampblack will be somewhat changed; and it is a fair 

 inference, if eacli drop makes almost exactly the same complicated, 

 symmetrical mark, that the splash of each drop takes place in almost 

 exactly the same way. 



The glimpse that may be caught of the drop in the way described 

 is obtained when the drop is really almost stationary, having flat- 

 tened itself out on the plate, and being on the point of contracting 

 aoain to its orioinal form. 



That a drop if so flattened out will recover itself, is seen on press- 

 ing down a drop of mercury with the finger, or a drop of water with 

 a piece of black-lead or other substance to which it does not adhere. 



Fig. 5. 



Fig. 6. 



On removing the pressure the drop springs back to its old form ; the 

 force which causes this being exerted by the curved surface of the 

 liquid at the edge of the flattened drop, on the liquid within. The 

 flatter the drop becomes the greater is the curvature of the edge, and 

 the greater the corresponding pressure tending to restore it to its 

 original globular form. The extent to which a drop that has fallen 

 on a plate will spread out depends on the velocity with which it strikes 

 the plate, i. e., on the height of fall ; so that as long as the drop returns 

 to the globular form the whole phenomenon of the splash may be re- 

 garded as an oscillation similar to that of a pendulum ; the velocity of 

 the liquid outward being checked, overcome, and finally reversed by 

 the ever-increasing pressure of the curved edge, just as a pendulum 

 has its velocity checked, overcome, and finally reversed, by the action 

 of gravity. 



It is only when the height of fall is very great that the liquid flics 

 off in all directions and the splash ceases to be an oscillation ; this 



