DRAINAGE 79 



ance to the entrance of water into the? pipe, the resistance of 

 the walls of the pipe to the flow of the water, which varies 

 largely with the roughness of the inside of the pipe, the 

 obstructions at joints and bends, and the amount of sedi- 

 ment deposited, etc. 



Poncelet's Formula. One of the more generally used 

 formulas which have been proposed for the flow of water in 

 tile drains, is Poncelet's formula. The usual way of stating 

 this formula is as follows : 



For mean velocity: 



In which 



d = diameter of tile in feet. 



h = head, or difference in elevation between outlet and upper end, 



in feet. 

 I = length of drain hi feet. 



Modification of Formula. Under varying conditions 

 which are encountered, certain modifications of the formula 

 will be found necessary. Thus in open soil where the water 

 is free to enter the tile line, it is recommended by Mr. C. G. 

 Elliott, formerly Chief of the Drainage Investigations, of the 

 United States Department of Agriculture, that }/ of the 

 depth of the soil over the drain at its head be added to the 

 quantity dh, making the formula read: 



In which 



k = the depth of the soil over the drain at its head. 



Mr. Elliott also recommends that an increase in the 

 head be made in the case of mains which have a com- 

 paratively large number of laterals, on account of the drop 

 of these laterals into the main. This drop in the submains 



