CHAP. XI. 



CLASSES OF DRAINS. 



867 



and as the stratification generally lies in sheets at an angle to the 

 surface, the water passing in or between the strata, immediately below 

 the bottom of the drain, nearly comes to the surface before reaching 

 the next lower drain ; but as water seeks the lowest level in all 

 directions, if the strata be cut longitudinally by a drain directed down 

 the steeps, the bottom of which cuts each stratum to the same distance 

 from the surface, the water will flow into the drain at the intersecting 

 point of each sheet or layer, on a level with the bottom of the drain, 

 leaving an uniform depth of dry soil." In favour of the second mode 

 of running the drain across the slope, Mr. G. Stephens, author of a 

 treatise on drainage and agriculture, has the following : " A drain 

 made across the slope or declivity of a field, or any piece of land, will 

 undoubtedly intercept more water than when it is carried straight up 

 the bank or rising ground ; this principle holds good in every case, 

 whether the drain be made to receive surface or subterraneous water. 

 Drains winding across the slope or declivity of a field, whatever their 

 number or depth may be, their effect upon tenacious or impervious 

 substrata will be much greater than if they were made straight up and 



Fig. 372. Plan of Drainage. 



down the slope ; and when the soil is mixed with thin strata of 

 fine sand, which is the case nine times out of ten, the effect will be 

 increased in proportion, and accordingly a much less number will 

 answer the purpose." 



Drains are ranged under three classes, "main-drains, sub-drains, 

 and small, or furrow-drains." The main-drains are those into which 

 all the other drains deliver the water, and which lead the united flow 

 to the point of outfall. The position of these mains should be 

 invariably along the lowest part of the field, or principal hollow. 

 Where the length of the small or furrow drains is such as to give them 

 a great quantity of water to deliver, it will be advisable to divide the 

 length, and allow the first half to deliver into a "sub-main" this 

 communicating directly with the "main." Where there are minor or 

 secondary hollows in the field to be drained, " sub -mains " should be 

 placed in these hollows. 



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