MOTION OF VISCOUS FLUIDS 



51 



circumstances the resistance to motion is greatly increased. The 

 phenomenon has been investigated hy V. W. Ekman (Norwegian Polar 

 Expedition Eesearches, 1893. No. 15), who found that the resistance is due 



Fio. 25. 



to the formation and maintenance of a train of waves at the surface of 

 separation of the fluids, as shown in Fig. 25. 



More recent experiments l tend to show that the foregoing conclusions 

 as to the effect of curvature of the path in affecting the manner of 

 motion are not justified, and that, as shown at the impact of a steady 

 jet on a plane surface, at the efflux of a jet from a sharp-edged orifice, 

 and in motion in a free vortex, curved motion with the velocity greatest 

 at the inside, not at the outside of the curve, tends to steady motion. 

 Generally speaking, wherever the velocity of flow is increasing and the 

 pressure diminishing, as in the case where lines of flow are converging, 

 there is an overwhelming tendency to steady motion. In a tube with 

 converging boundaries, it is this which leads to steadiness of flow, 

 and it is because this effect is sufficiently pronounced to overcome the 

 tendency to sinuous motion which all solid boundaries, of whatever form, 

 produce, that the motion in such 

 tubes is steady for very high 

 velocities of flow. 



ART. 17. CRITICAL VELOCITY. 



The experiments by which 

 Professor Reynolds demonstrated 

 the nature of the two modes of 

 motion of water were carried out 

 on glass tubes of various diameters 

 up to 2 inches, and about 4 feet 

 6 inches long. These were fitted with bell-mouth entrances, and were 

 immersed horizontally in a tank of clear water having glass sides 

 (Fig. 26). In carrying out the experiment the water in the tank is 

 allowed to come absolutely to rest, and the valve A is then slightly 



1 By the Author, "Memoirs, Manchester Lit. and Phil. Soc.," Vol. 55, 1911, No. 13. 



2 



FIG. 26. 



