20 ENGINEERING FOR LAND DRAINAGE. 



With respect to their drainage we speak of them as 

 ope7i and retentive^ the terms expressing, not their 

 power of retaining certain quantities of water, but the 

 readiness or rapidity with which water moves among 

 the particles when a means of drainage is offered. Be- 

 tween the very open soils and the very retentive ones 

 there are numberless degrees of difference which must 

 be expressed by qualifying terms if they are properly 

 described. 



This may be illustrated by a few examples which 

 will serve to show the bearing of these terms. A soil 

 or subsoil composed largely of sand or gravel offers but 

 slight resistance to the movement of water among the 

 particles, so that a single drain as an outlet relieves the 

 soil of drainage-water for a very considerable distance 

 from it. There are instances of drainage districts in 

 which the opening of a single deep channel has drained 

 to a considerable degree of thoroughness certain lands 

 lying a mile each side of it. There are other localities 

 where the effect of such drain does not extend one hun- 

 dred feet, and still others where forty feet is the limit. 

 Upon these natural conditions of the soil largely de- 

 pend the means that should be employed to properly 

 drain it. 



The relation of the soil to the subsoil should also be 

 carefully observed. The subsoil may be compact and 

 retentive, while the soil directly above it may be open, 

 or the opposite conditions may exist. The subsoil 

 may not be parallel to the surface in its general con- 

 formation, but a retentive clay may crop out near the 

 surface at some points, and recede in others, thus form- 

 ing pockets or basins underneath the surface where 



