June 14, 1894J 



NA TURE 



163 



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 

 i; 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 back- 

 wards 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 sur- 

 faces over which it has to glide, or which surfaces fold on them- 

 selves, the internal motions are of this purely wave character. 

 The colour bands, however much they may be distorted, cannot 

 be relatively displaced, twisted, or curled up, and in this case 

 motion in water once setup 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 

 establish 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 w ith the direction of motion of the 

 waves, the motion only becomes evident in thickening and bend- 

 ing 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. They are owing to the slight differences in the density 

 of the fluids, as is indicated by the extreme slowness of the 

 motion. Of such kind are the waves in the air, that cause the 

 clouds which make the mackerel sky, the vapour in the tops of 

 the waves being condensed and evaporated again as it descends 

 showing the results of the motion. 



The distortional motions, such as alone occur in simple wave 

 motion, or where the surfaces of the fluid do not fold in on them- 

 selves, or wind in, are the same as occur in any homogeneous 

 continuous material which completely fills the space between 

 the surfaces. 



If plastic material is homogeneous in colour it shows nothing 

 as to the internal motion ; but if I take a lump built of plates, 

 blue and white, say a square, then I can change the surfaces to 

 any shape without folding or turning the lump, and the coloured 

 bands which extend throughout the lump show the internal 

 changes. Now the first point to illustrate is that, however I 

 change its shape, if I bring it back to the original shape the 

 colour bands will all come back to their original positions, and 

 there is no limit to the extent of the change that may thus be 

 effected. I may roll this out to any length, or draw it out, and 

 the diminution in thickness of the colour bands shows the 

 extent of the distortion. This is the first and simplest class 

 of motion to which fluids are susceptible. By this motion alone 

 elements of the fluid may be, and are, drawn out to in- 

 definitely fine lines, or spread out in indefinitely thin sheets, 

 but they will remain of the same general figures. 



By reversing the process they change back again to the original 

 form. No colour band can ever be broken, even if the outer sur- 

 face be punched in till the punch head comes down on the table ; 

 still all the colour bands are continuous under the punch, and 

 there is no folding or lapping of the colour bands unless the 

 external surface is folded. 



The general idea of mixture is so familiar to us that the vast 

 generalisation to which these ideas afford the key, remains un- 

 noticed. That continued mixing results in uniformity, and that 

 uniformity is only to be obtained by mixing, will be generally 

 acknowledged, but how deeply and universally this enters into 

 all the arts can but rarely have been apprehended. Does it 

 ever occur to any one that the beautiful uniformity of our textile 



NO. 1285, VOL. 50] 



fabrics has only been obtained by the development of processes 

 of mixing the fibres ? Or, again, the uniformity in our con- 

 struction of metals ; has it ever occurred to any one that the 

 inventions of Arkwright and Cort were but the application of 

 the long-known processes by which mixing iseftected in culinary 

 operations? Arkwright applied the draw-rollers to uniformly 

 extend the length of the cotton sliver at the expense of the 

 thickness ; Cort apphed the rolling-mill to extend the length of 

 the iron bloom at the expense of its breadth ; but who invented 

 the rolling-pin by which the pastrycook extends the length at 

 the expense of the thickness of the dough for the pie-crust ? 



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

 — to obtain some particular shape, but chiefly to obtain a uni- 

 form 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 

 obtained is a byword ; 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 restriction 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 we 

 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 physical process of diffusion completes the ad- 

 mixture. 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 them with a spoon these colour bands can be 

 drawn r.nd folded so fine that the difl"usion will be instantaneous, 

 and the fluid become uniformly tinted. .A.11 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 

 has never been seen before by any one but Mr. Foster and 

 myself, namely, the complete process of the formation of a 

 cylindrical vortex sheet resulting from the motion of a solid 

 surface. To make it visible to all I am obliged to limit the 

 colour band to one section of the sheet, otherwise only those 

 immediately in front would be able to see between the convolu- 

 tion of the spiral. But you will understand that wnat is seen 

 is a section, a similar state of motion extending right across the 

 tank. From the surface you see the plane vane extending half- 

 way down right across the tank ; this is attached to a float. 



I now institute a colour band on the right of the vane out of 

 the tube. There is no motion in the water, and the colour 

 descends slowly from the lube. I now give a small im|)ulse to 

 the float to move it to the right, and at once the spiral form is 

 seen from the tube. Similar spirals would be formed all across 

 the tank if there were colours. The float has moved out of the 

 way, leaving the revolving spiral with its centre stationary. 



