50 HYDRAULICS AND ITS APPLICATIONS 



(4) Curvature with the greatest velocity al the inside of the curve. 



(5) Greater density of the fluid. 



K \periments carried out at McGill University show that an increase in 

 pressure also tends to stability of motion. 



The effect of solid boundaries in producing instability would appear to 

 be due rather to their tangential than their lateral stiffness. One very 

 remarkable instance of this effect of boundaries possessing tangential 

 stiffness, however small this may be, occurs when a film of oil is allowed to 

 form on the surface of water. Here the oil film exerts a small but appreci- 

 able tangential constraint, with the result that motion which was originally 

 stable becomes unstable. 



This results in the formation of eddies below the surfaces of the oil and 

 water, and the energy which was originally imparted by the action of the 

 wind to form and maintain stable wave motion is now applied to the 

 institution of this eddy motion, with the well-known result as to the 



stilling of the waves. 



Where one stream of liquid is in 

 contact with a second stream mov- 

 ing with a different velocity, the 

 common surface of separation is 

 found to be in a most unstable con- 

 dition. Eeynolds showed this by 

 allowing the two liquids, Carbon 

 Bisulphide and water, to form a 

 horizontal surface of separation in 



a long horizontal tube. The tube was then slightly tilted so as to pro- 

 duce a relative axial motion of the fluids, when it was found that for. 

 extremely small values of the relative velocity the motion was unstable. 



In this manner may be explained why diverging boundaries are a cause 

 of instability. In such a case as that shown in Fig. 24, the motion from 

 A to B in the parallel portion of the channel may be perfectly steady, 

 depending on the dimensions of the channel and the velocity of flow. On 

 leaving B, however, at any but the smallest velocities the water appears 

 to be projected in the form of a core of the same dimensions as the 

 channel, through the mass of dead water at C and D. Here all the 

 conditions necessary for instability are present. 



This instability attending the relative motion of fluids of different 

 density affords an explanation of the interesting phenomenon known as 

 " deadwater," which is sometimes noticed when a ship encounters a 

 surface layer of fresh water, overlying the denser sea-water. Under such 



