128 ENGINEERING FOR LAND DRAINAGE. 



Simple as this task may at first seem, it has occupied 

 the time and attention of many experimenters who are 

 justly noted by reason of their researches in this de- 

 partment of practical science. 



When the flow of water through pipes is considered, 

 the resistances to gravity are, first, resistance to en- 

 trance of water into the pipe; second, the resistance 

 offered by the walls of the pipe with which the water 

 comes in contact. The first will vary with the kind of 

 opening through which the water enters the pipe, the 

 second with the kind of pipe and its length and the 

 head which produces flow. 



Many of the formulas deduced agree with each other 

 in the results obtained from them sufficiently near for 

 practical purposes, yet there is a wide difference in their 

 simplicity and availability for use. There is one which 

 represents at a glance the corrections that must be ap- 

 plied to the gravity formula, so that it will express the 

 velocity of water in pipes. It is known as Weisbach's 

 formula and is expressed as follows: 



where e = coefficient of resistance to entrance of water 

 into pipe ; 



c = coeflScient of friction of pipe; 



/ = length of pipe in feet; 



d = diameter of pipe in feet. 



The numerator of the second number of this equation 

 is the theoretical velocity of falling bodies. The de- 



