20 FOEESTRY WORK 



the tiles in the main. It is a mistake to drain too deeply 

 and so lower the water-level too much. 



By Cipillary attraction a drain will draw water from a 

 depth below the drain almost equalling the depth of the 

 drain itself. The size of the main- drain tiles and pipes 

 depends on the area to be drained. The old rule-of- 

 thumb — viz., 1 inch per acre — holds good where there is 

 no great excess of water. To arrive at this the diameter 

 of the pipe is squared and multiplied by '7854 to find its 

 area. For instance, the area of a 3-inch circle is 3 x 3 x 

 •7854 = 7-0686 in. Therefore, a 3-inch pipe will drain 7 

 acres and a 4-inch pipe 12| acres. Where the pipes are 

 likely to become choked with sand or iron ore, it is cheaper 

 to use pipes of a larger size, rather than have to lift and 

 clean smaller ones every few years. The pipes in small 

 drains should never be less than 2| inches. 



In cutting the drains for a nursery or any agricultural 

 land, the methods are different from those employed 

 when making open drains in woods (see Chapter IV.). 



When pipes are to be laid, it is not necessary to slope the 

 sides, but to cut them perpendicular, allowing only 

 enough room between them for a man to move along 

 while grading the bottom or laying the pipes; 15 inches 

 is usually enough. 



In the case of very deep drains or when the pipes are 

 very large, the drains must be made wider. 



The tm-f and first spade depth of soil are thrown out 

 with an ordinary digging spade, but below that a draining 

 spade, or " gvaff," is used. It is a long, narrow, and 

 tapering tool, very strongly made to stand leverage. 



