DRAINAGE. 93 



benefited by draining. With the exception of actual swamps, 

 the soils which derive the greatest advantage, are, of course, 

 those which, during the spring and fall, are completely saturated 

 with water, and during the heat of summer, are baked to a hard 

 crust and broken with fissures ; but all heavy loams, friable soils, 

 which rest on impervious subsoil (or hard pan), — indeed all but 

 sands, and the lighter deep loams and gravels — are very much 

 benefited by such a removal of their excess of water as can be 

 economically effected only by tile-draining. 



THE MODE OF ACTION, AND THE EFFECT OF UNDERDRAINS. 



A thoroughly underdrained field is one which is underlaid, at 

 suitable intervals, with lines of continuous pipe drains, which 

 admit the water of the soil, and convey it to an outlet, from 

 which it is completely removed. The water which falls upon 

 the surface is at once absorbed, and settles through the ground 

 until it reaches a point where the soil is completely saturated, and 

 raises the general water level ; when this level reaches the floor 

 of the drains, the water enters at the joints and is carried off. 

 That which passes down through the land lying between the 

 drains, bears down upon that which has already accumulated in the 

 soil, and forces it to seek an outlet by rising into the drains.* For 

 example, if a barrel standing on end be filled with earth which is 

 saturated with water, and its bung be removed, the water of satura- 

 tion (that is, all which is not held by attraction in the particles of 

 earth) will be removed from so much of the mass as lies above the 

 bottom of the bung-hole. If a bucket of water be now poured upon 

 the top, it will not all run diagonally toward the opening ; it will 

 trickle down to the level of the water remaining in the barrel, 

 and this will rise and run off at the bottom of the orifice. In 

 this manner the water, even below the drainage level, is changed 

 with each addition at the surface. In a barrel filled with coarse 

 pebbles, the water of saturation would maintain a nearly level 

 surface ; if the material were more compact and retentive, a true 



* Except from quite near to the drain, it is not probable that the water in the soil runs 

 laterally toward it. 



