LAYING OUT A DRAIN OR SYSTEM 131 



170. The depth of cut. We are now ready to deter- 

 mine the depth of the ditch at the stakes where the depths 

 have not yet been determined. In column 6 of the table 

 we have the elevations of all the stakes, while in column 

 8 we have the elevations of the bottom of the ditch at all 

 the stakes. .If now the elevation of the bottom of the 

 ditch at any point is subtracted from the elevation of 

 the stake at that point, the result will be the depth of 

 the ditch. Make the proper subtractions and enter 

 results in column 9. Compare results with the values 

 recorded in column 9 of Table XVI. 



171. Grade bars. We have thus determined the 

 depth the ditch must be dug at each grade stake. It is 

 necessary to provide some simple means (1) by which 

 we may know just how deep to dig at every point, and 

 (2) by which we may finish the bottom of the ditch so 

 that the fall shall be constant from one grade stake to 

 the next, above or below. In Fig. 50 are shown what 

 are known as grade bars, more commonly called, batter 

 boards. These grade bars are set up over each grade 

 stake, and the top of each grade bar is set at the same 

 height above the proposed bottom of the ditch and hori- 

 zontally. This height is usually 5| feet. Some workmen 

 prefer to have it 6 feet, and some would probably have it 

 5 feet. 



172. Boning line and boning rod. A light strong 

 cord, drawn tight and resting on the tops of these bars, 

 will stand parallel to the proposed bottom of the ditch. 

 If, then, the cord stands above the center of the ditch, 

 and 5| feet above the desired line of bottom, the workman 

 finishing the bottom can, with a light rod bearing a 5.5- 

 foot mark, by placing the rod on the bottom of the ditch 

 at any point, and holding the rod perpendicular with the 



