1893.] on Study of Fluid Motion by means of Coloured Bands. 135 



In all these processes the object, too, is the same throughout — to 

 obtain some particular shape, but chiefly to obtain a uniform texture. 

 To obtain this nicety of texture it is necessary to mix up the material, 

 and to accomplish this it is necessary to attenuate the material, so 

 that the different parts may be brought together. 



The readiness with which fluids are mixed and uniformity ob- 

 tained is a by-word ; but it is only when we come to see the colour 

 bands that we realise that the process by which this is attained is 

 essentially the same as that so laboriously discovered for the arts — -as 

 depending first on the attenuation of each element of the fluid — as I 

 have illustrated by distortion. 



In fluids, no less than in cooking, spinning and rolling — this 

 attenuation is only the first step in the process of mixing — all involve 

 the second process, that of folding, piling, or wrapping, by which 

 the attenuated layers are brought together. This does not occur in 

 the pure wave motion of water, and constitutes the second of the 

 two classes of motion. If a wave on water is driven beyond a certain 

 height it leaps or breaks, folding in its surface. Or, if I but 

 move a solid surface through the water it introduces tangential 

 motion, which enables the fluid to wind its elements round an axis. 

 In these ways, and only in these ways, we are released from the re- 

 striction of not turning or lapping. And in our illustration, we may 

 fold up our dough, or lap it — roll it out again and lap it again ; cut 

 up our iron bar, pile it, and roll it out again, or bring as many as wo 

 please of the attenuated fibres of cotton together to be further drawn. 

 It may be thought that this attenuation and wrapping will never 

 make perfect admixture, for however thin each element will 

 preserve its characteristic, the coloured layers will be there, however 

 often I double and roll out the dough. This is true. But in the 

 case of some fluids, and only in the case of some fluids, the physi- 

 cal process of diffusion completes the admixture. These colour 

 bands have remained in this water, swelling but still distinct ; this 

 shows the slowness of diffusion. Yet such is the facility with 

 which the fluid will go through the process of attenuating its 

 elements and enfolding them, that by simply stirring with a spoon 

 these colour bands can be drawn and folded so tine that the diffu- 

 sion will be instantaneous, and the fluid become uniformly tinted. 

 All internal fluid motion other than simple distortion, as in wave 

 motion, is a process of mixing, and it is thus from the arts we get 

 the clue to the elementary forms and processes of fluid motion. 



When I put the spoon in and mixed the fluid you could not see 

 what went on — it was too quick. To make this clear, it is necessary 

 that the motion should be very slow. The motion should also be 

 in planes, at right angles to the direction in which you are looking. 

 Such is the instability of fluid that to accomplish this at first 

 appeared to be difficult. At last, however, as the result of much 

 thought, I found a simple process which I will now show you, in 

 what I. think is a novel experiment, and you will see, what I think 



