476 STATE BOARD OF AGRICULTURE. 



above ground. Sometimes, however, where the field is cultivated the wall 

 is brought to within about twelve inches of the top of the ground. 

 A heavy cover is then placed upon the top of the wall and the soil is 

 filled in. In this case it is necessary to use some special means of 

 locating the silt basin. 



Where the wall is brought to or above the surface of the ground it 

 should have placed upon it a substantial cover, 



LEVELING. 



Leveling is a process by which we determine the heigh Is, or elevations 

 of definite points in a line or area above an arbitrarily adopted level 

 plane. This plane is called the datum plane. It is always so located 



Fig. 9. Drainage level. 



A telescope; B spindle; C spirit level; D eye-piece; E leveling head; F to adjust 

 objective;.© objective; S leveling screws. 



as to lie lower than the lowest point whose elevation we are seeking. In 

 ordinary practice of drainage leveling this plane is so established that 

 the point at which the leveling begins lies just 10 feet above it — "10 feet 

 above datum." 



It will be seen then that if the datum plane is itself level and if we 

 are able to determine the height of each stake in the line or area above 

 the datum plane, then it is an easy matter to determine the difference 

 in elevation between any stake and any other stake, or in other words 

 to determine the fall between any stake and any other stake. 



THE LEVEL. 



The drainage level, (Fig. 9) consists of a telescope mounted on a 

 spindle which is in turn mounted on a tripod. The telescope carries a 



