No. 4.] DRAINAGE. 377 



How Water enters Underdrains. 

 Two misconceptions as to the manner in which water enters under- 

 drains appear to be not uncommon. 



1. There is a general idea that special provision for the entrance of 

 water must be made, like that, for example, between stones in drains 

 of that type, or that, in the case of tile drains, the tile should be porous 

 in order that water may find its way through. 



2. It is thought that water runs down from above into the undcr- 

 drain. 



As a matter of fact the underdrain is brought into action only when 

 some portion of the channel for water is below the water table in the 

 soil. However rapidly water is carried into the soil, the underdrain 

 will not run until the level of the hydrostatic water in the soil rises 

 above the bottom of the channel which the drain affords. As the 

 hydrostatic water rises above the drains, the water, by its natural 

 hydrostatic pressure, is forced into the channel afforded by the drain. 

 The pressure increases of course as the water rises, and before it has 

 risen much above the channel it is sufficiently great so that, practically 

 speaking, however impervious the tile, or however close the joints 

 (if laid without mortar), it is impossible to keep the water out. 



In putting in underdrains we have not to consider in the case of 

 either tiles or stones the provision of openings through which the 

 water will enter; on the contrary, we must use every care to make all 

 joints just as close as possible. The danger is not that the water will 

 be prevented from entering, but that foreign substances (fine sand and 

 silt) will gain entrance and thus obstruct the drain. 



The Location of Drains. 

 The location of drains in a field requiring drainage should in all 

 cases receive special and careful attention and study. It is not possible 

 to lay down general rules which will prove of much value. In the case 

 of large operations there must usually be a principal or main line and 

 subordinate branch lines of drains. The principal drains are usually 

 spoken of as mains, and these will naturally run through the lowest 

 part of the area to be drained, while with both mains and submains, 

 branch lines, which are usually spoken of as laterals, will be connected. 

 In planning a system of underdrainage it seems to be wise to provide 

 for a system with relatively few outlets into open water courses. 

 Thus, for example, if we have a field with a principal channel running 

 through it somewhere near the middle, and the land on either side 

 slopes gradually towards this channel, the system will be most satis- 

 factory if a main drain is put through the principal channel, and 

 the laterals connected with it, only one outlet being provided for the 

 entire system, which, of course, should be at the lower end of the 

 main. In such a case good drainage might be secured by putting a 



