146 LAND DRAINAGE 



be well, in building the cement work, to set in bolts to 

 hold the frame carrying the screen or bars in place, as 

 shown in Fig. 57. 



192. Trap. A trap of galvanized iron, like that 

 shown in Fig. 58, will prove effective. A better device 

 is that of a screen of i-inch iron rods suspended by 

 hinge or chain links. The hinged trap or screen is espe- 

 cially desirable when surface water enters the tile above, 

 by way of silt-basins or otherwise. 



193. Laterals. As has already been stated, the work 

 of placing laterals does not differ materially from that of 

 laying single lines of tile or mains. The leveling is done 

 in the same way, excepting that usually the lateral con- 

 nects with the main at one of the original grade stakes. 

 This is not necessary, but it is convenient. This grade 

 stake becomes stake 1 of the lateral, and its elevation as 

 determined for the main is retained as that of stake 1 of 

 the lateral. This is desirable since by it the elevations 

 of all points in the system are referred to the same datum 

 plane, and a basis is thus given for comparing the actual 

 elevation of the lateral, or any point in the lateral, with 

 any other point in the system. 



194. Leveling for laterals. The leveling for the 

 laterals is often done immediately after that for the main 

 is completed. Sometimes it is deferred till the work of 

 digging and laying the tile of the main is nearly finished. 

 In the former case the levels for a lateral may help to 

 determine the proper depth of the main, and may thus 

 increase the efficiency of the lateral. In the latter case, 

 the danger from inaccuracies, due to disturbing grade 

 stakes along the laterals after the leveling has been done, 

 is greatly lessened. 



The depth of the outlet of the lateral is determined by 



