SOILS. 25 



ments along this line are by no means complete, yet 

 much work has been done which bears directly upon 

 the theory and practice of land drainage, some of the 

 results of which will be briefly outlined in this chapter. 



Soil Structure, 



Soils and subsoils are composed of solid grains of 

 variable sizes which touch each other at certain points 

 on their surfaces. The comparative size of soil grains 

 varies greatly, and is an important and interesting sub- 

 ject for investigation. 



According to a mechanical analysis made in 1891 at 

 the Maryland Experiment Station, the diameter of soil 

 grains for the several materials named is as follows : 



Gravel 2 to i millimeters 



Medium sand i to .5 ** 



Fine sand 25 to .1 ** 



Very fine sand I to .05 ** 



Silt 05 to .01 *' 



Fine silt 01 to .005 



Clay 005 to .0001 



Note, — Millimeter = .03937 in. 



This is only comparative, as by the method used for 

 analysis the absolute size could not be obtained, but a 

 reliable method for classifying soils with reference to 

 the size of soil grains was hit upon and used. 



The empty space between these grains has also been 

 determined for a variety of soils, the following being 

 the result of some of the experiments : 



ii 



