No. 4.] 



DRAINAGE. 



381 



When the grade exceeds 3 inches in 100 feet the diameter should be 

 squared and the result divided by 3. This gives the following results : — 



Wheeler has given rules which accord closely with the rules given by 

 Chamberlain. 



(c) Method of Determining Area. — Wheeler, however, has pointed 

 out that owing to the fact that water more slowly finds its way to the 

 drains in compact soils, the area satisfactorily served by a main of 

 any given size is greater in such soils than in those of more open char- 

 acter. 



Laying out and Construction of Drains. 



In all cases where the grade must be flat it will be wise to employ the 

 services of an engineer with accurate leveling instruments. Such 

 services will cost something, but the exi:)ense of expert services would 

 be amply justified in the more accurate and better work which can be 

 done. At the outset, beginning where the main drain will discharge 

 into the open water course, stakes should be put in at each end of 

 each line of drains, and at all intermediate points where the direction 

 or grade changes. Beside each stake drive a grade peg or hub, level 

 with the surface of the ground. On each of the reference stakes first 

 driven mark the depth at which the drain is to be laid below the top 

 of the grade peg. AVhen ready to begin excavation drive a pair of 

 stakes, one on either side of each peg and a sufficient distance apart 

 to clear the ditch when it is opened. Across each pair of stakes nail a 

 batter board, the top of which should be at some uniform distance 

 above the proper grade at the bottom of the ditch, — 6 feet is a con- 

 venient distance. For example, if the figure on a given reference stake 

 indicates that the ditch is to be excavated 3.8 feet at that point, the 

 top of the batter board should be 6 — 3.8, or 2.2 feet above the top 

 of the hub. If the figure on the reference stake is 4.2 feet, then the 

 height of the top of the batter board should be 1 . 8 feet above the top 

 of the hub. When the batter boards along any given line are placed 

 the excavation may begin. It is best in almost all cases to begin at the 

 outlet or lower end of the line, and in most cases it is best to begin 

 laying the tiles also at the lower end. A convenient means of de- 

 termining whether the ditch is just the proper depth at any point is to 

 stretch a light and very strong cord over the tops of the batter boards, 

 immediately above what will be the center of the bottom of the ditch. 

 The workman who finishes the grading should be provided with a 

 measuring rod 6 feet in length, and should excavate until the distance 

 from the cord above his head to the bottom of the ditch is exactly 6 

 feet at every point. There is one rather serious danger connected with 



