202 THE SPLASH OF A DROP AND ALLIED PHENOMENA. 



many arms tlieie Lave been. It may be mentioned that sometimes tbe 

 .surface of tlie central lake of liquid (figs. 14, 15, 10, 17) was seen to be 

 covered with beautiful concentric ripples, not sbown in the figures. 



The question now naturally presents itself. Why should the drop 

 behave in this manner? In seeking the answer it will be useful to ask 

 ourselves another question. What should we have expected the drop 

 to do? Well, to this I suppose most people would be inclined, arguing 

 from analogy with a solid, to reply that it would be reasonable to expect 

 the drop to flatten itself, and even very considerably flatten itself, and 

 then, collecting itself together again, to rebound, perhaps as a column 

 such as we have seen, but not to form this regular system of rays and 

 arms and subordinate drops. 



Now this argument from analogy with a solid is rather misleading, 

 for the forces that operate in the case of a solid sphere that flattens 

 itself and rebounds, are due to the bodily elasticity which enables it 

 not only to resist, but also to recover from any distortion of shape or 

 shearing of its internal j^arts i)ast each other. But a liquid has no 

 ])ower of recovering from such internal shear, and the only force that 

 checks the s[)read, and ultimately causes the recovery of shape is the 

 surface tension, which arises from the fact that the surface layers are 

 always in a state of extension and always endeavoring to contract. 

 Thus Ave are at liberty when dealing with the motions of the drop to 

 think of the interior liquid as not coherent, provided we furnish it with 

 a suitable elastic skin. Where the surface skin is sharply curved out- 

 ward, as it is at the sharp edge of the flattened disk, there the interior 

 licjuid will be strongly pressed back. In fact the process of flattening 

 and recoir is one in which energy of motion is first exi)ended in creat- 

 ing fresh licjuid surface and subsequently recovered as the surface 

 contracts. The transformation is, however, at all moments accom- 

 panied by a great loss of energy as heat. INloreover, it must be remem- 

 bered that the energy expended in creating the surface of the satellite 

 drops is not restored if these remain permanently separate. Thus the 

 surface tension explains the lecoil, and it is also closely connected with 

 the formation of the subordinate lays and arms. To explain this it is 

 only necessary to remind you that a liquid cylinder is an unstable con- 

 figuration. As you know, any fine jet becomes beaded and breaks into 

 drops, but it is not necessary that there should be any flow of liquid 

 along the jet; if, for exanqfle, we could realize a rod of liquid of the 

 shape and size of this ruler and liberate it in the air, it would not retain 

 its cylindrical shape, hut Avould segment or divide itself up into a row 

 of drops regularly disposed according to a definite and very sinq)le 

 numerical law, viz, that the distances between the centers of contigu- 

 ous drops would be equal to the circumference of the cylinder. This 

 can be shown by calculation to be a consequence of the surface tension, 

 and the calculation has been closely verified by experiment. If the 

 liquid cylinder were liberated on a plate, it would still topple into a 



