SECTION II 

 CHAPTER III. 



Motion of Viscous Fluids Stream Line and Eddy Motion Vortices Conditions regulating 

 the two modes of Motion Reynolds's Researches Critical Velocity in Parallel Pipes In 

 Converging Pipes Motion of a Fluid Equations of Motion Flow against Viscous 

 Resistance Between Parallel Plates Through Circular Tube Genesis of Eddy 

 Formation. 



ART. 14. MOTION OF Viscous FLUIDS. 



WHEN a rigid body moves, the motion at any point in its mass can be 

 accurately determined once the motions of any three points, not in the 

 same straight line, are fixed, In this case, when the boundary conditions 

 are known, the determination of the internal motion is simple. But with 

 a fluid such as water the motion of the mass of fluid as a whole does not 

 necessarily give the internal motion. While giving the mean motion, it 

 does not fix the absolute motion of each particle of the fluid relatively to 

 any fixed point. This is quite apart from intermolecular motion, which 

 gives rise to diffusion in a mass of water even when apparently 

 motionless. 



By the earlier experimentalists the motion of water in mass was im- 

 perfectly understood. On the assumption of a motion simple and 

 analogous to that of a rigid body, it was impossible to reconcile the 

 results experimentally obtained by many observers, while the very trans- 

 parency, or uniform opacity of most fluids made it impossible to see the 

 internal motion. Probably the first indication of there being more than 

 one kind of fluid motion was obtained from the appearance of the free 

 surface of flowing water. Where not otherwise disturbed this may have 

 two appearances, corresponding to different modes of motion. In the one 

 objects are reflected from the surface without distortion, while in the 

 other an irregular motion of the surface may be noted, and reflection is 

 accompanied by distortion. Where motion occurs in a passage having 

 solid lateral boundaries, however, even this indication is absent, and the 

 introduction of floating particles of solid matter does not help to any 

 extent in showing the nature of the action which is taking place. 



Matters remained in this state for many years, and it was left for 



