1884.] on the Tioo Manners of Motion of Water. 51 



If oil be put on the surface it spreads out into an indefinitely thin 

 sheet which possesses only one of the characteristics of a solid surface, 

 it offers resistance, very slight, but still resistance to extension and 

 contraction. This, however, is sufficient to entirely alter the character 

 of the motion. It renders the water unstable internally, and instead 

 of waves, what the wind does is to produce eddies beneath the sur- 

 face. This has been proved, although I cannot show you the experi- 

 ments. 



To those who have observed the phenomena of oil preventing 

 waves, there is probably nothing more striking throughout the 

 region of mechanics. A film of oil so thin that we have no means 

 of illustrating its thickness, and which cannot be perceived except by 

 fts effect — which possesses no mechanical properties that can be made 

 apparent to our senses — is yet able to entirely prevent an action 

 which involves forces the strongest we can conceive, which upset our 

 ships and destroy our coasts. This, however, becomes intelligible 

 when we perceive that the action of the oil is not to calm the sea by 

 sheer force, but merely, as by its moral force, to alter the manner of 

 motion produced by the action of the wind from that of the terrible 

 waves upon the surface into the harmless eddies below. The wind 

 throws the water into a highly unstable condition, into what morally 

 we should call a condition of great excitement. The oil by an 

 influence we cannot perceive directs this excitement. 



This influence, though insensibly small, is however now proved of 

 a mechanical kind, and to me it seems that the phenomenon of one 

 of the most powerful mechanical actions of which the forces of nature 

 are capable, being entirely controlled by a mechanical force so 

 slight as to be otherwise quite imperceptible, does away with every 

 argument against the strictly mechanical sources of what we may 

 call mental and moral forces. 



But to return to the instability in parallel channels. This has 

 been the most complete, as well as the most definite result of the 

 colour-bands. 



The circumstances are such as to render definite experiments 

 possible. These have been made, and reveal a definite law of the 

 instability, which law has been tested by reference to all the nume- 

 rous and important experiments on the resistance in channels by 

 previous observers ; whereupon it is found that waters behave in 

 exactly the same manner whether the channel, as in Poiseuille's experi- 

 ment, is of the dimensions of a hair or whether it be the size of a 

 water main or of the Mississippi ; the only difference being that 

 in order that the motions may be compared, the velocity must be 

 inversely as the diameter of the pipe. But this is not the only 

 point explained if we consider other fluids than water. Some fluids, 

 like oil or treacle, apparently flow more slowly and steadily than 

 water. This, however, is only in smaller channels ; the critical 

 velocity increases with the viscosity of the fluid. Thus, while water 

 in comparatively large streams is always above its critical velocity, 



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