EXPERIMENT STATION BULLETINS. 



499 



A light strong cord drawn tight and resting on the tops of these grade 

 bars will stand parallel to the proposed bottom of the ditch. If then, 

 the cord extends above the center of the ditch, and 5.5 feet above the 

 desired line of bottom, the workman finishing the bottom can, with 

 a light rod bearing a 5.5 ft. mark, by placing the rod on the bottom of 

 the ditch at any point and holding the top of the rod against the line, 

 tell when he has brought the bottom to the proper depth. 



HEIGHT OP GRADE BAR. 



The height of the grade bar above any stake is found by subtracting 

 the depth of the ditch at that stake from the height the line is to stand 

 above the bottom of the ditch (5.5 ft. in our practice.) 



Turn again to Table V and determine the height of the grade-bar at 

 each stake. This will be found in each case by subtracting the depth 



Fig. 22. Grade bars. 



of the ditch as recorded in your column 9 from 5.5 ft. Record your re- 

 sults properly in your column 10 and compare your results with those 

 in column 10 of Table VI. 



THE WORK. 



1. The grade bars should be put up. This is sometimes put off till 

 after the digging is well under way. The objection to this plan is that 

 the grade stakes are likely to be disturbed by the workmen when the 

 digging begins. 



The grade bar should be 4 to 6 ft. long, usually %-inch stuff with one 

 straight edge. With each bar there must be two stakes preferably 

 %-inch X 4-inch to 6-inch and suflSciently long to stand higher, when 

 they are driven into the ground, then the height of the bar at that stake. 

 The two stakes should be driven firmly into the ground one on each 

 side of and out about one foot from the ditch and so that the two stakes 

 and the grade stake shall stand in a straight line, at right angles to 

 the ditch. 



