DRAINAGE 



289 



of inches from the hub, gives the location of the hub as well 

 as its number and the depth of the ditch below the top of 

 the hub. These stakes are set and numbered and the depth 

 of the ditch computed by the surveyor. 



After University of Minnesota 



Fig. 237. Details of a string target 



To dig the ditch to the proper grade from these stakes, the 

 depth of the ditch below the top of the hub should be sub- 

 tracted from 5 feet. The difference between this depth and 

 5 feet should then be measured up from the top of the hub 

 and marked on a target rod which has been driven near the 

 hub. At this mark a string approximately J^g inch in diame- 

 ter should be securely tied around the target rod and then 

 drawn across the trench and tied to a target rod on the 

 other side, a carpenter's level being used to keep the string 

 level while it is being tied to the second rod (Fig. 237). The 

 string should be drawn tight and the rod should be firm 

 enough to prevent the string from changing its position after 

 it has once been fixed. Similar strings should be placed 

 at not less than two more stakes. The line of sight across 

 these strings will be in a line which is parallel to and 5 feet 



