116 



KNOWLEDGE. 



[May 1, 1900. 



the one picked out? More than this, if necessary the 

 experiment must be capable of repetition, with an 

 exactly similar drop falling from exactly the same 

 height, and illuminated at exactly the same stage. It 

 must then be possible, after this particular stage has 

 been sufficiently examined, to be able to arrange that a 

 later stage, say one-thousandth of a second after the other, 

 may be studied in the same manner as the earlier one, 

 and in this way to follow step by step the course of the 

 whole series of changes. 



In his early observations, as no photographic plates 

 were then available sufficiently sensitive to respond to 

 the very short exposures that were required. Prof. 

 Worthington had perforce to study the different stages 

 by eye and make drawings of them. Though he was 

 enabled by this means to trace with marvellous accuracy 

 the complete spla-sh, yet the application of the photo- 

 graphic method in more recent years has made it possible 

 to confirm, to extend, and in some cases correct, the 

 results of ocular observations. Following Prof. Boys' 

 suggestion, who had in his popular flying bullet experi- 

 ments used sensitive plates, Prof. Worthington and his 

 colleague, Mr. R. S. Cole, employed Thomas's cyclist 

 plates, with excellent results. To give an idea of the 

 results of an extended series of experiments on splashes 

 of a great many kinds, which have all been photo- 

 graphed and examined in the same systematic way, it 

 will be best to select two typical instances for de- 

 scription. The two cases chosen, which are shown 

 graphically in the accompanying illustrations, are, first, 

 the splash of a drop of mercxu'y 4.83 millimetres in 

 diameter falling through fifteen centimetres upon a jjlate 

 of glass; and second, of a large stone sphere 3.2 centi- 

 metres in diameter, falling through a height of 14 

 centimetres into water mixed with milk, contained m 

 a glass bowl about one foot deep and nine inches in 

 diameter. 



The first set of pictures were obtained by allowing 

 the drop of mercui-y to fall upon the naked photographic 

 plate itself. The illuminating spark was produced 

 vertically above the plate, and consequently the figures 

 only show a horizontal section of the di'op in various 

 stages. Very soon after the first instant of impact 

 minute rays are shot out in all directions. These are 

 afterwards united, and then main rays shoot out (see 

 Fig. 3), from the ends of which, in some cases, minute 

 droplets of liquid split off, to be left lying in a circle 

 on the plate and visible in all subsequent stages. Figs. 

 4, 4a and 5 show how the central mass contracts but 

 leaves long arms or rays which contract more slowly. 

 In Fig. 5a the thin film has torn open in the middle 

 and yielded an annulus, which in turn would separate 

 into a ring of drops surrounded by a second circle of 

 the still smaller and more numerous droplets that split 

 off the ends of the rays. It must be remembered that 

 the interval of time during which all the stages shown 

 in the figui-e ai-e passed through is very small, being, 

 as the numbers indicated show, only about the one- 

 seventy -fifth pai-t of a second. 



Before refen-ing to the second series of photographs 

 it is necessai-y to point out that the foi-m of the splash 

 in this case depends very much upon the condition of 

 the surface of the sphere. When a polished sphere of 

 marble, rubbed very di-y with a cloth just beforehand, 

 is dropjjod into water, the water spreads over the sphere 

 so rapidly that it is sheathed with the liquid even befoie 

 it has passed below the general level of the surface. 

 The spla,sh is insignificantly small and of short duration. 



But if the sjjhere be roughened with sand-paper or left 

 wet, the water is driven away laterally, forming a ribbed 

 basketrshaped hollow, which, however, is now prolonged 

 to a great depth, the drop being followed by a cone of 

 air, while the water seems to find great difficulty in 

 wetting the surface of the sphere completely. The first 

 photograjjh shows a highly polished sphere just before 

 the impact with the liqiiid. The beginning of the rise 

 of the sheath can easily be made out in the second 



1. — Actual size, 4'83 mm. id diameter. 



2.-(l=0.) 



5. — (<=0O63.) 



5a.— (<=0094.) 



. • • 



4.-U--^0032.) 



• • • « 



6.— {/=-0]34.) 



Instantaueous Shadow Pliotograplis (life size) of the Splash of a 

 Drop of Mercury falling 15 cm. on to Glass. 



photograi^h, while the want of symmetry in the fourth 

 stage depicted is due to the sphere having been rough 

 on the right side and polished on the left. This photo- 

 graph shows at a glance the great difference between 

 a " rough " and a " smooth '' splash. The puckering of 

 the surface, which is strongly marked in the fifth photo- 

 graph, indicates that the lines of flow near the surface 

 of the liquid when once detennined are very persistent. 

 The general surface of the milk-and-water in the next 

 stage is very level, while the volume of the column 

 which can be very clearly seen is scarcely more than one- 

 tenth that of the sphere. This proves that there is an 

 instantaneous general rise of level even at a great dis- 



