PLATE 6 



A. — A plow bottom with two sets of straight lines. 



B. — A plow bottom, the surface of which is composed of each of two surfaces. 



C— A plow bottom similar to B, but with the smfaces merging into each other 

 farther back on the moldboard. 



D. — A plow bottom, the surface of which does not contain an infinite set of straight 

 lines. ' 



