

1893.] on Study of Fluid Motion by means of Coloured Bands. 133 



motion. The sphere shows, however, one of the rarest phenomena in 

 Nature — the internal state in almost absolute internal rest. The 

 forms resemble nothing so much as stratus clouds, as seen on a summer 

 day, though the continuity of the colour bands is more marked. A 

 mass of coloured water once introduced is never broken. The discon- 

 tinuity of clouds is thus seen to be due to other causes than mere 

 motion. 



Now, having called your attention to the rarity of water at rest, I 

 will call your attention to what is apt to be a very striking phenomenon, 

 namely, that when water is contained, like this, in a spherical vessel of 

 which you cannot alter the shape, it is impossible by moving the 

 vessel suddenly to set up relative motion in the interior of the water. 

 I may swing this vessel about and turn it, but the colour band in the 

 middle remains as it was, and when I stop shows the water to be at 

 rest. 



This is not so if the water has a free surface, or if the fluid is 

 of unequal density. Then a motion of the vessel sets up waves, 

 and the colour band shows at once the beautifully lawful character 

 of the internal motion. The colour bands move backwards and 

 forwards, showing how the water is distorted like a jelly, and as the 

 wave dies out the colour bands remain as they were to begin with. 



This illustrates one of the two classes of internal motion of water 

 or fluid. Wherever fluid is not in contact with surfaces over which 

 it has to glide, or which surfases fold on themselves, the internal 

 motions are of this purely wave character. The colour bands, how- 

 ever much they may be distorted, cannot be relatively displaced, 

 twisted, or curled up, and in this case motion in water once set up 

 continues almost without resistance. That wave motion in water 

 with a free surface, is one of the most difficult things to stop is 

 directly connected with the difficulty of setting still water in motion ; 

 in either case the influence must come through the surfaces. Thus it is 

 that waves once set up will traverse thousands of miles, establishing 

 communication between the shores of Europe and America. Wave 

 motion in water is subject to enormously less resistance than any 

 other form of material motion. 



In wave motion, if the colour bands are across the wave they 

 show the motion of the water ; nevertheless, their chief indication is 

 of the change of shape while the fluid is in motion. 



This is illustrated in this long bottle, with the coloured water 

 less heavy than the clear water. If I lay it down in order to es- 

 tablish equilibrium, the blue water has to leave the upper end of the 

 bottle and spread itself over the clear water, while the clear water 

 runs under the coloured. This sets up wave motion, which continues 

 after the bottle has come to rest. But as the colour bands are parallel 

 with the direction of motion of the waves, the motion only becomes 

 evident in thickening and bending of the colour bands. 



The waves are entirely between the two fluids, there being no 

 motion in the outer surfaces of the bottle, which is everywhere glass. 



