

1893.] on Study of Fluid Motion by means of Coloured Bands. 131 



internal motion. Of what sort ? Well, I think my illustration may 

 carry more weight if I do not tell you ; you can all, I have no doubt, 

 form a good idea. It is not fluid motion or I should feel bound to 

 explain it. You have here an ordinary looking object which behaves 

 in an extraordinary manner, which is yet very decided and clear, to 

 judge by the motion of its surface, and from the manner of the 

 motion I wish you to judge of the cause of the observed motion. 



This is the problem presented by fluids, in which there may be 

 internal motion which has to be taken into account before the motion 

 of the surface can be explained. You can see no more of what the 

 motion is within a homogeneous fluid, however opaque or clear, than 

 you can see what is going on within the box. Thus, without colour 

 bands the only visual clue to what is going on within the fluids is 

 the motion of their bounding surfaces. Nor is this all ; in most cases 

 the surfaces which bound the fluid are immovable. 



In the case of the wave on water the motion of the surface shows 

 that there is motion, but because the surface shows no wave it does 

 not do to infer that the fluid is at rest. 



The only surfaces of the air within this room are the surfaces of 

 the floor, walls, and objects within it. By moving the objects we 

 move the air, but how far the air is at rest you cannot tell unless it 

 is something familiar to you. 



Now I will ask you to look at these balloons. They are familiar 

 objects enough, and yet they are most sensitive anemometers, more 

 sensitive than anything else in the room ; but even they do not show 

 any motion ; each of them forms an internal bounding surface of the 

 air. I send an aerial messenger to them, and a small but energetic 

 motion is seen by which it acknowledges the message, and the same 

 message travels through the rest, as if a ghost touched them. It is 

 a wave that moves them. You do not feel it, and, but for the surfaces 

 of the air formed by the balloons, would have no notion of its 

 existence. 



In this tank of beautifully clear distilled water, I project a heavy 

 ball in from the end, and it shows the existence of the water by 

 stopping almost dead within two feet. The fact that it is stopped 

 by the water, being familiar, does not raise the question, Why does it 

 stop? — a question to which, even at the present day, a complete 

 answer is not forthcoming. The question is, however, suggested, and 

 forcibly suggested, when it appears that with no greater or other 

 evidence of its existence, I can project a disturbance through the 

 water which will drive this small disc the whole length of the tank. 



I have now shown instances of fluid motion of which the manner 

 is in no way evident without colour bands, and were revealed by colour 

 bands, as I showed in this room sixteen years ago. At that time I was 

 occupied in setting before you the manners of motion revealed, and ] 

 could only incidentally notice the means by which this revelation 

 was accomplished. 



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