if, HYDRAULICS AND ITS APPLICATIONS 



shown in a series of experiments by Dr. Hele Shaw 1 , who, by producing 

 flow between two parallel glass plates at an extremely small distance apart, 

 produced a state of affairs in which viscosity was the predominating 

 factor. In ordinary cases, however, this is not so, and the effect of the 

 inertia of the fluid is more marked than that of cohesion. On this 

 account it is always necessary for easy flow to design any pipe or passage 

 for conveying fluids with curves having as gradual a curvature as 

 possible. 



If the stream line be imagined to form the axis of a tube of finite 

 sectional area having imaginary boundaries, and such that its area, at 

 different points in its length, is inversely proportional to the velocity at 

 those points, this is known as a stream tube. 



If the motion at a fixed point varies, and if it is still possible to find a 

 definite motion for these points such that the motion of the fluid relative 

 to the points may remain constant, the latter may be reduced to steady 

 motion by considering the motion relative to these moving points. The 

 fluid may then be said to have a continuous, steady, but unequal 

 motion. 



In almost all the cases of fluid motion which are of practical importance 

 in hydraulics,, the motion is found to be unsteady. 



ART. 15. VORTICES. 



A mass of fluid, rotating about some axis in the fluid itself and forming 

 a closed circuit, is termed a vortex or eddy. This axis may be straight, 

 curved, or may return on itself, in which latter case we have a vortex 

 ring. Where a vortex of the former type is formed, it may be shown that 

 the motion is unstable unless the ends of the axis are in contact with 

 some solid surface. A stable vortex, whether with a rectilinear or a 

 circular axis, may be projected through the surrounding fluid with 

 surprisingly small loss of energy. 



An instance of this may be found in the case of the ordinary smoke 

 ring, or of a similar vortex ring in water. If this vortex ring be coloured 

 by the admission of aniline dye to the generating box, and if the ring be 

 then projected through a tank of clear water, it is seen to travel with a 

 motion of uniform rotation about its circular axis through the surround- 

 ing liquid, its outer layers moving, relatively to the axis, in the opposite 

 direction to that of its own motion of translation. Eelatively to the 

 surrounding water the motion at the outer layers is very small, so that 



1 "Trans, inst. Naval Architects," 18978, 1900. 



