138 Professor Beynolds on Fluid Motion. [June 2, 



To show the surface separating the water moving with the vortex 

 from that which gives way outside, I discharge from this orifice a 

 mass of coloured water, which has a vortex ring in it formed by the 

 surface as already described. You see the beautifully denned mass 

 moving on slowly through the fluid, with the proper vortex ring 

 motion, but very slow. It will not go far before a change takes 

 place, owing to the diffusion of the vortex motion across the bounding 

 surface ; then the coloured surface will be wound into the ring which 

 will appear. The mass approaches the disc in front. It cannot pass, but 

 will come up and carry the disc forward ; but the disc, although it does 

 not destroy the ring, disturbs the motion. 



If I send a more energetic ring it will explain the phenomenon 

 I showed you at the beginning of this lecture ; it carries the disc 

 forward as if struck with a hammer. This blow is not simply the 

 weight of the coloured ring, but of the whole moving mass and the 

 wave outside. The ring cannot pass the disc without destruction 

 with the attendant wave. 



Not only can a ring follow a disc, but as with the plane vane so 

 with the disc, if we start a disc we must start a ring behind it. 



I will now fulfil my promise to reveal the silent messenger I 

 sent to those balloons. The messenger appears in the form of a large 

 smoke ring, which is a vortex ring in air rendered visible by smoke 

 instead of colour. The origination of these rings has been carefully 

 set so that the balloons are beyond the surface which separates the 

 moving mass of water from the wave, so that they are subject to the 

 wave motion only. If they are within this surface they will disturb 

 the direction of the ring, if they do not break it up. 



These are, if I may say so, the phenomenal instances of internal 

 motion of fluids. Phenomenal in their simplicity they are of intense 

 interest, like the pendulum, as furnishing the clue to the more complex. 

 It is by the light we gather from their study that we can hope to 

 interpret the parable of the vortex wrapped up in the wave, as applied 

 to the wind of heaven, and the grand phenomenon of the clouds, as 

 well as those things which directly concern us, such as the resistance 

 of our ships. 



[O. E.] 



