134 Professor Osborne Reynolds [June 2, 



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 themselves, 

 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 

 the elements of the fluid may be, and are, drawn out to an indefinitely 

 fine line, or spread out in an indefinitely thin sheet, but they will 

 remain of the same general figure. 



By reversing the process they change back again to the original 

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

 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 fabrics 

 has only been obtained by the development of j>rocesses of mixing 

 the fibres. Or, again, the uniformity in our construction of metals ; 

 has it ever occurred to any one that the inventions of Arkwright 

 and Cort were but the application of fthe long-known processes by 

 which mixing is effected in culinary operations ? Arkwright applied 

 the draw-rollers to uniformly extend the length of the cotton sliver 

 at the expense of the thickness ; Cort applied 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 pastry-cook extends the 

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



