Grade 481 



not only that they may represent its true level, but 

 also to avoid danger of the pegs being disturbed. 



MORE THAN ONE GRADE ON THE SAME DRAIN 



It very frequently happens that the surface of the 

 land to be drained is such as to make it impracticable 

 to lay out the whole of a main or of a lateral with the 

 same amount of fall throughout. Let it be supposed 

 that at the end of the 600 feet represented in Fig. 

 157, the ground continued rising backward at a slower 

 rate for 500 feet more, as the figures show it had 

 begun to do, and that in the 500 feet the rise was 

 only six inches. In order to avoid digging too deeply 

 in some portions of the line, or of placing the tile too 

 close to the surface at others, it is necessary to change 

 the grade, and the new grade will be found by divid- 

 ing the total fall .5 feet by 5, the number of 100 feet, 

 which gives .1 foot, and half this amount instead of 

 .12, is what would be added at each 50-foot station, 

 in order to get the new grade line elevations. 



DIGGING THE DITCH 



It has been pointed out that practice is required 

 in order to dig a ditch well, rapidly and easily. It is 

 further necessary to have suitable tools for the pur- 

 pose. First in importance is the ditching spade, two 

 forms of which are represented in Fig. 158. These 

 spades have blades 18 inches long, narrower than the 

 common tool, and strongly curved forward, to give 



