Surface Drainage 465 



Even through a clay loam* it may require 24 hours 

 for 1.6 inches of water to percolate through a stratum 

 of soil 14 inches deep when the surface is kept under 

 2 inches of water, and since the rate of percolation is 

 somewhat nearly proportional to the length of the 

 column, 2 days would be required for the same flow 

 through 28 inches, and about 13 days through 15 feet, 

 the distance the water would have to travel with 

 underdrains placed only 30 feet apart. But the sub- 

 soils of the lands in question are much closer than 

 the loam cited, so that the best which has yet been 

 done for such soils is to plow them in narrow lands, 

 with the dead furrows extending along the slope of 

 the fields in such a way that the excess of water may 

 be quickly led away into the streams or open ditches. 



It is true that the tillage and heavy cropping of 

 such soils, especially during dry seasons, tend to cause 

 the clay subsoils to shrink into cuboidal blocks, and 

 thus facilitate underdrainage ; but the long years 

 which some of those lands have been under such 

 treatment without marked amelioration appear to 

 leave little hope of ever bringing them under thorough 

 drainage in this way. 



There are other flat sections of country, with more 

 open soils and subsoils, where sufficiently deep open 

 ditches may be provided to serve as outlets for under- 

 drains, and lands be thus thoroughly reclaimed. Such is 

 the case in Illinois, and Fig. 151 represents six square 

 miles of land treated in this way. In this figure the 

 double lines represent deep open ditches, the single lines 



*The Soil, p. 171. 

 DP 



