I42~ ENGINEERING FOR LAND DRAINAGE. 



If the sum of these equal gravity, no flow takes place. 

 It may be observed in this connection, by way of ex- 

 planation, that head is considered as divided into two 

 parts. One part overcomes friction, and is called fric- 

 tion head, the other generates velocity and is called 

 velocity head. The v of the formula is the mean 

 velocity of actual flow from the tile which is under con- 

 sideration, and is the difference between the friction 

 head and the velocity head. 



Considering the fact that friction is greater for small 

 tiles than for large, and greater for long drains than 

 for short ones, and that the velocity decreases as the 

 square root of the head, it is easy to see that a drain 

 of certain size may be laid with so little grade and 

 have such a length that the discharge will be little or 

 nothing. These facts will appear to some extent when 

 the formula is used, yet it will be best to observe in a 

 general way the following limits: 



Table 6. 



LIMIT OF SIZE OP TILE TO GRADE AND LENGTH, 



Si7e of Tile Minimuin Grade Limit of Length 



xn Inches per loo Feet m Feet 



2 10 6oo 



5 05. 2000 



/.*....••■•.*.• •V^S .••••••...».■. <tuQ\J\j 



8 .05 3000 



10 04 4000 



II..... , .04 41^00 



12 .04 S300 



