466 THE HOME, FARM AND BUSINESS CYCLOPAEDIA. 



cording to the peculiarities it may present ; so that it is scarcely possible 

 to give a rule invariably correct when applied to practice. 



MAIN DRAINS. 



Having ascertained what depth and distances asunder the ordinary sur- 

 face-water under-drains in any particular field ought to be, the carrying 

 out of the works will next demand attention. In laying down the main 

 and furrow drain lines a levelling instrument should always be used, if 

 there is the slightest doubt about there being a good declivity on the 

 surface. This is more particularly necessary in the formation of the 

 outfalls. In very flat ground, it is sometimes a most difficult matter to get 

 sufficient declivity to insure a self -cleansing action. As the usefulness of 

 most drainage works depends very largely on the proper formation of the 

 discharging outlets, no reasonable amount of expense should be spared in 

 making these as complete as possible. If they are to be open, the 

 sides of the cutting should be formed at an angle of 45 ; and if the 

 material is very liable to slip, an angle of 40, or even less, will 

 be sufficiently steep. All the excavated soil should be thrown well 

 back from the edges, as it will have a tendency, if left too near, to. 

 make the sides give way. When the cutting is too deep to be left open, 

 horse-shoe tiles, or circular pipes of large calibre, should be laid in it. It 

 is rarely advisable to lay several open tiles abreast or atop, as the sides 

 obstruct the water, and silt is deposited, with considerable injury to the 

 drain. To save a little expense many drainers commit the serious mistake 

 of not going far enough down the fall to secure a deep and effective outlet ; 

 and others put in pipes that are much too small for the work they have 

 to perform. 



It requires some skill to arrange the main drains in a field, so as to 

 catch every hollow in it, without adding considerably to the cost ; and it 

 also needs much care to keep the levels properly, in cutting the drains. 

 When the main drains are very long, they should have overflow branches 

 at various points. Suppose, for example, a drain four feet deep runs par- 

 allel with an open ditch which is fully three feet in depth, then it is obvi- 

 ous that, if a few branches be carried into the latter, the pressure on the 

 main during great floods, supposing it incapable of venting the water, will 

 never be more than from a foot to a foot and a half. This can only be done, . 

 of course, when there is an open ditch, into which the overflow branches 

 may be carried. At important junctions, and in places where there are 

 sharp curves, there should also be sediment wells in the main drains, each 



