208 THE SPLASH OF A DROP AND ALLIED PHENOMENA. j 



configuration viewed may be .said to oscillate sliglitly about the mean 

 for Avhicli the apparatus is adjusted. Kow this is due both to small , 

 imperfections in the timing- apparatus and to the fact thai the splashes ' 

 themselves do actually vary within certain limits. The reasons are not j 

 very far to seek. In the first place the rate of demagnetization of the I 

 electro-magnets varies slightly, being partly dependent on the varying 

 resistance of the contacts of crossed wires and partly on the tempera- 

 ture of the magnet, which is affected by the length of time for which 

 the current has been running. But a much more important reason is 

 the variation of the slight adhesion of the drop to the smoked watch 

 glass that has supported it, and consequently of the oscillations to 

 which, as we shall see, the drop is subjected as it descends. Thus the 

 drop will sometimes strike the surface in a flattened form, at others in 

 an elongated form, and there will be a difference, not only in the time 

 of impact, but in the nature of the ensuing splash; consequently some 

 judgment is required in selecting a consecutive series of drawings. 

 The only way is to make a considerable number of drawings of each 

 stage and then to pick out a consecutive series. Now, whenever judg- 

 ment has to be used there is room for error of judgment, and moreover 

 it is impossible to put together the drawings so as to tell a consecutive 

 story, without being guided by some theory, such as I have already 

 sketched, as to the nature of the motion and the conditions that 

 govern it. You will therefore be good enough to remember that this 

 chronicle of the events of a tenth of a second is presented by a fallible 

 human historian, whose account, like that of any other contemporary 

 observer, will be none the worse for independent confirmation. That 

 confirmation I am fortunately able in some measure to supply. When 

 I endeavored eighteen years ago to photograph the splash of a drop 

 of mercury, I was unable to obtain plates sufficiently sensitive to 

 respond to the very short exposures that were required, and conse- 

 quently abandoned the attempt. But in recent years plates of exqui- 

 site sensitiveness have been produced, and such photographs as those 

 taken by Mr. Boys' of a flying rifle bullet have shown that difficulties 

 on the score of sensitiveness have been practically overcome. Within 

 the last few weeks, with the valuable* assistance of my colleague at 

 Devonport, Mr. B. 8. Cole, I have succeeded in obtaining i)hotographs 

 of various splashes. Following Professor Boys's suggestion, we 

 employed Thomas's cyclist plates, or occasionally the less sensitive 

 "extra-rapid" plates of the same makers, and as a developer, eikono- 

 gen solution of triple strength, in which the plates were kept for about 

 forty minutes, the development being conducted in complete darkness. 

 A few preliminary trials with the self-induction spark produced at 

 the surface of mercury by the apparatus that you have seen at work 

 showed that the illumination, though ample for direct vision, was not 



•Professor Boys' article on photographs of flying bullets was reprinted in th^ 

 §l^ith8ouiau Report for 1893. 



