CH. vi] Drainage 101 



engineer, who drained Chat Moss and other great areas, 

 laid deep drains. It is now known that both sides had 

 a good case : shallow drains are needed when the water 

 to be removed comes from above e.g., from excessive 

 rain or seepage from high land and deep drains when 

 the water is thrown up from below. Before deciding on 

 the depth of the drains, therefore, it is necessary to 

 ascertain where the water is coming from and how and 

 where it can best be intercepted. 



On clay lands the water usually comes as rain and 

 therefore shallow drains are best. The pipes are com- 

 monly 3 in. diameter and are often laid 2| to 3i ft. deep 

 and at distances of 15 to 30 ft. apart, but an intelligently 

 thought-out plan is always wanted. The cost is con- 

 siderable before the war it was about 7 per acre 

 and where it is undesirable to spend so much money a 

 mole plough often furnishes a cheap and tolerably 

 efficient substitute especially where there is a reasonable 

 fall to a ditch. This implement cuts out a 3-4 inch 

 tunnel 18 in. to 3 ft. below the surface of the soil into 

 which the water can drain. The tunnel is more per- 

 manent than might be anticipated, and lasts 15 to 20 

 years or more, especially if it does not run straight into 

 the ditch but into the old mains, or, if these cannot be 

 found and cleared, into new pipe drains discharging into 

 the ditch^. The method breaks down if large stones are 

 present; a turf drain might then be tried or even a 

 surface drain made by casting out a furrow. 



Whatever the drainage scheme it is particularly im- 

 portant that the ditches should be kept clean and the 

 outfalls of the drains open : the main drainage brook of 



^ See paper by D. T. Thring, "Mole-drainage and the renovation of old 

 pipe drains," Journ. Roy. Agric. Soc, 1914, lxxiv. 76-89. 



