112 ENGINEERING FOR LAND DRAINAGE. 



flooded at every considerable rise of water in the chan- 

 nel into which they discharge. This difficulty should 

 not deter one from laying drains under such conditions 

 provided the water in the open channel recedes quickly. 

 Where the drain for some distance back has a pres- 

 sure of water or soil upon it, its action is similar to 

 that of an iron pipe, the head of drainage-water from 

 the upper levels being the force which causes the sub- 

 merged outlet to discharge. The objection to sub- 

 merged outlets is less for open soils than for retentive 

 ones, since in open soils additional head is given to the 

 drain when the soil in the various parts of the tract be- 

 comes fully saturated, or in other words the line of 

 saturation rises considerably above the floor of the 

 drains. Where the soil is retentive, the weight of the 

 soil water is but little more than will overcome the re- 

 sistance which tht soil particles offer. 



Inspecting Tile Drains, 



The most definite and satisfactory way of determin- 

 ing whether a drain has been laid as indicated by the 

 survey or not is to run over the work with the level. 

 Determine whether the tile is in the correct position 

 at the outlet point by taking a rod-reading on its grade 

 stake or some bench, by means of which the line may 

 be reproduced. Let the rodman rod up on top of the 

 tile at each station and also between each station and 

 at curves. The levelman records each reading, observ- 

 ing whether the differences correspond with the grade 

 as laid out, allowing in all cases margin enough to 

 cover the inequalities of the tile that have been used. 



