1883.] 



On the Motion of Water. 



85 



The transparency or uniform opacity of most fluids renders it 

 impossible to see the internal motion, so that, broadly distinct as are 

 the two classes (direct and sinuous) of motion, their existence would 

 not have been perceived, were it not that the surface of water, where 

 otherwise undisturbed, indicates the nature of the motion beneath. 

 A clean surface of moving water has two appearances, the one like 

 that of plate glass in which objects are reflected without distortion ; 

 the other like that of sheet glass, in which the reflected objects appear 

 crumpled up and grimacing. These two characters of surface corre- 

 spond to the two characters of motion. This may be shown by 

 adding a few streaks of highly coloured water to the clear moving- 

 water. Then, although the coloured streaks may at first be irregular 

 -they will, if there are no eddies, soon be drawn out into even colour 

 bands ; whereas if there are eddies, they will be curled and whirled 

 about in the manner so familiar with smoke. 



3. Connexion between the Leading Features of Fluid Motion. — 

 These leading features of fluid motion are well known, and are 

 supposed to be more or less connected, but it does not appear that 

 hitherto any very determined efforts have been made to trace a 

 definite connexion between them, or to trace the characteristics of the 

 circumstances under which they are usually presented. 



Certain circumstances have been definitely associated with the 

 particular laws of force. Resistance as the square of the velocity is 

 associated with motion in tubes of more than capillary dimensions, and 

 with the motion of the bodies through the water at more than 

 insensibly small velocities, while resistance as the velocity is associated 

 with capillary tubes and small velocities. 



The equations of hydrodynamics, although they are applicable to 

 direct motion, i.e., without eddies, and show that then the resistance is 

 -as the velocity, have hitherto thrown no light on the circumstances on 

 which such motion depends. And although of late years these equations 

 have been applied to the theory of the eddy, they have not been in the 

 least applied to the motion of water, which is a mass of eddies, i.e., in 

 sinuous motion, nor have they yielded a clue to the cause of resistance 

 varying as the square of the velocity. Thus, while as applied to waves 

 and the motion of water in capillary tubes the theoretical results 

 agree with the experimental, the theory of hydrodynamics has so far 

 failed to afford the slightest hint why it should explain these pheno- 

 mena, and signally failed to explain the law of resistance encountered 

 by large bodies moving at sensibly high velocities through water, or 

 that of water in sensibly large pipes. 



This accidental fitness of the theory to explain certain of the 

 phenomena, while entirely failing to explain others, affords strong 

 presumption that there are some fundamental principles of fluid 

 motion of which due account has not been taken in the theory ; and 



