52 



HYDRAULICS AND ITS APPLICATIONS 



opened, allowing water to flow slowly through the tube. A little water, 

 coloured with aniline dye, is introduced into the mouthpiece through a 

 fine tube supplied from the vessel B. 



At first this coloured water is drawn out into a single stream tube, 





(a) 



extending through the whole length of the tube, as shown in Fig. 26, the 

 whole appearing to be motionless unless a slight motion of oscillation is 

 given to the water in the supply tank, when the stream line sways gently 

 from side to side, but without in the least losing its definition. As the 

 valve A is further opened the velocity through the tube increases, and the 

 stream tube is drawn out more and more, still retaining its definition 



until at a certain velocity eddies 

 begin to be formed intermit- 

 tently near the end of the tube 

 more remote from the mouth- 

 piece (Fig. 27 a). 



The formation of these eddies 

 is very clearly denoted and is 

 accompanied by the almost in- 

 stantaneous diffusion of the 

 colour band. As the velocity is 

 still further increased the point 

 of initiation of eddy motion 

 advances towards the mouth- 



Loss of Head 

 It 



FlG - 28 - piece, this point being fixed for 



any one velocity. Finally the 

 motion throughout becomes unsteady (Fig. 27 6). 



Any initial disturbance of the water tends to reduce the velocity at 

 which the motion changes from steady to sinuous, and which is termed 

 the Critical Velocity. 

 Having determined the nature of the two manners of motion possible 



