FLOW OF WATER IN CHANNELS AND PIPES. 63 



be made in laying a water supply pipe unless the 

 theory of flow in them is understood. Let us con- 

 sider an example such as might occur in practice. 

 Referring to fig. 49 we have a reservoir main which 

 delivers the water into a tank as shown, following 

 the contour of the ground as near as possible. Let a 

 valve be placed where shown and let V V V, etc., be 

 imaginary vertical pipes. Water will now stand in 

 them to the same level as the water in the tank, as 

 shown by the horizontal line. Open the valve, how- 

 ever, and the water level will sink in these pipes to 



Main, 



FIG. 49. 



points which are always on the hydraulic gradient and 

 marked H H H, etc. This position is however only 

 true for a certain position of the valve. Open it more 

 and the line H H will drop, close it and it will rise ; 

 that is, the level of the hydraulic gradient will vary 

 according to the draw off. Hence the water in the 

 tank will not always rise at the other end of the 

 pipe line to the same level as that of the reservoir, 

 and hence it is very important that the highest point 

 of the pipe line after it leaves the reservoir should 

 be lower than the lowest level in that reservoir by 

 an amount equal at least to the values found in equa- 



