1893.] on Study of Fluid Motion by means of Coloured Bands. 137 



operandi by which internal waves can exist in fluid without any 

 motion in the external boundary. Not only is this plate moving 

 flatwise through the water, but it is followed by all the water, 

 coloured and uncoloured, enclosed in these cylindrical vortices. 

 Now, although there is no absolute surface visible, yet there is 

 a definite surface which encloses these moving vortices, and 

 separates them from the water which moves out of their way. 

 This surface will be rendered visible in another experiment I 

 shall show you. Thus the water which has only wave motion 

 is bounded by a definite surface, the motion of which corresponds 

 to the wave; but inside this closed surface there is also water, 

 so that we cannot see the surface, and this water inside is moving 

 round and round, but so that its motion at the bounding surface is 

 everywhere the same as that of the outside water. 



The two masses of water do not mix. That outside moves out 

 of the way of and past the vortices over the bounding surface, while 

 the vortices move round and round inside the surface in such a way 

 that it is moving in exactly the same manner at the surface as the 

 wave surface outside. 



This is the key to the internal motion of water. You cannot have 

 a pure wave motion inside a mass of fluid with its boundaries at 

 rest, but you have a compound motion, a wave motion outside, and a 

 vortex within, which fulfils the condition that there shall be no sliding 

 of the fluid over fluid at the boundary. 



A means which I hope may make the essential conditions of this 

 motion clearer occurred to me while preparing this lecture, and to this I 

 will now ask your attention. I have here a number of layers of cotton- 

 wool (wadding). Now I can force any body along between these layers 

 of wadding. They yield, as by a wave, and let it go through ; but the 

 wadding must slide over the surface of the body so moving through 

 it. And this it must not do if it illustrate the conditions of fluid 

 motion. Now there is one way, and only one way, in which material 

 can be got through between the sheets of wadding without slipping. 

 It must roll through ; but this is not enough, because if it rolls on 

 the under surface it will be slipping on the npper, But if we have 

 two rollers, one on the top of the other, between the sheets, then the 

 lower roller rolls on the bottom sheet, the upper roller rolls against 

 the upper sheet, so that there is no slipping between the rollers or 

 the wadding, and, equally important, there is no slipping between the 

 rollers, as they roll on each other. I have only to place a sheet of 

 canvas between the rollers and draw it through ; both the flannel 

 rollers roll on the canvas and on the wadding, which they pass 

 through without slipping, causing the wadding to move in a wave 

 outside them, and affording a complete parable of the vortex motion. 



I will now show by colour bands some of the more striking 

 phenomena of internal motion, as presented by Nature's favourite 

 form of vortex, the vortex ring, which may be described as two 

 horseshoe vortices with their ends founded on each other, 



