FLOW OF WATER IN CHANNELS AND PIPES. 6 1 



Now all pipes must be laid with at least a certain 

 gradient to provide power to overcome friction. 

 Such a gradient is known as the hydraulic gradient, 

 and the formula to find it is 



This will give us the required fall in feet per mile, 

 w r hich roughly pans out at 10 ft. per mile in ordinary 

 cases, viz. a gradient of i in 528. Perhaps the hy- 

 draulic gradient would warrant a little further ex- 

 planation. Say we have a reservoir and a pipe line 

 from it. Say at any point on the pipe line the pres- 

 sure recorded was 50 ft. Then if this line and the 

 reservoir were plotted correctly on a section, vide 

 fig. 48, the line joining the top water level of the re- 

 servoir and a point vertically 50 ft. above the pipe 

 line at the point in question, would be the hydraulic 

 gradient. This, of course, assumes the pipe to be 

 running full, but full only ; no pressure at the top of 

 its circumference. So if a pipe has not a gradient 

 equal to the hydraulic gradient, there would be a 

 point on the pipe where we should have no pressure, 

 and its distance in feet from the reservoir for any 

 pipe would be 



d = diam. in ft. 



