TERRACING AND DRAINING 8 1 



burden of fine sand and other soil particles. These are 

 thrown down and fill the ditch, making work for the farmer 

 in opening it again. The banks of a deep open ditch 

 should not be upright nor nearly upright, for they invari- 

 ably cave in, and the earth fills the ditch. 



It is a good rule for a ditch to have a uniform grade, 

 steep enough to carry off the water without filling the 

 ditch with soil, and yet not so steep that the current will 

 cut deep into the bottom of the ditch. This grade varies 

 with the dimensions of the ditch, as you will learn in larger 

 books on drainage. For small ditches there is usually 

 from three to five inches of fall in every one hundred feet. 



EXERCISE. Walk over a field and notice the little washes just be- 

 ginning. Think of a way by which each one could be stopped. When 

 a ditch or river makes a sand-bar at a curve, is the sand-bar on the 

 inner or on the outer side of curve? Why? Watch a winding brook as 

 it flows and learn why a ditch or river tends to become more and more 

 crooked (Fig. 40) . Look at one of the ditches on a farm that you 

 know and plan how it could be improved. 



NOTE TO THE TEACHER. If a rain occurs soon after this lesson 

 has been studied, point out that the current is strong and washing is 

 possible only in those places on the school yard or an adjoining field 

 where water collects, and not where it is spread in a thin sheet. Does 

 washing occur on the bare or on the grass-covered parts of the school 

 yard? Perhaps some pupil's father has a drainage-level and will bring 

 it to school and show the class how to use it. If he will lend it, the 

 directions above will be sufficient guide for you to use it in locating 

 terrace lines. Why not have a short excursion for using the level and 

 for inspecting ditches and streams? 



