262 



tailed conditions under which they are to be laid are known. Errors 

 resulting from too small tile are most serious in their effects and the 

 only safe plan is to be sure they are large enough even though the 

 first cost is somewhat greater. 



In regard to the fall of drains, it may be said that the best rule 

 is to have ail the fall possible. A foot to the hundred feet is desirable 

 if it can be had. Cases will sometimes occur where three inches or 

 even less must be accepted and in these instances the bottom of the 

 ditch must be carefully leveled and graded so that no sags will occur. 

 Jt is important, also, to have a uniform fall, that is, not to change 

 from one grade to another in the same line of tile. In changing 

 from a steep to a less steep grade the velocity of the water is checked 

 and there is danger of sediment being deposited where the change is 

 made. Of course there is no objection to changing from a small 

 grade to a greater one for in this case the velocity will be increased 

 and the sediment will be the better carried onward. (N. H. C. 

 B. 118.) 



Depth. Depth of drains is also a variable distance depending 

 largely upon soil characteristics. In some cases drains have been 

 laid four feet deep with indifferent results, while drains of two and 

 one-half feet deep on the same land have been successful. Four feet 

 is considered deep drainage, three feet medium and two and a half 

 feet shallow drainage. In general three feet is a safe depth, but on 

 some soils containing very thick and close hardpans it is safer to lay 

 the tile at two and a half feet, if such a depth will keep them above 

 this impervious subsoil. Drains should be placed as deep as they will 

 receive the water readily, with four feet the limit in clay and alluvial 

 soils. 



Aeration. Aeration of the soil is one office of the underdrain, 

 and is of great benefit to very close soils. In treating such soils it has 

 been found beneficial to provide surface vents to the drains for the 

 purpose of inducing a more rapid circulation of air through the 

 drains and soil. In this way some refractory soils have been drained 

 and greatly improved in texture. Shallow drains in such cases serve 

 the purpose better than deep ones. (F. B. 187; Ark. Ex. Sta. 

 B. 104.) 



Distance Apart. This depends chiefly upon the nature of the 

 soil and the depth of the drains. Drains should be sufficiently close 

 together to remove the surplus water and lower the water midway be- 

 tween them. Water flows much more freely through coarse, sandy 

 and loamy soils than through close clays, hence the more open the 

 soil and subsoil the farther apart can the drains be placed. 



To secure efficient drainage the individual lines should be 

 placed sufficiently near to each other for the effect of one line to 

 reach that of another on either side so as to bring all the soil within 

 the active range of the drains. The distance apart will depend upon 

 the closeness of the soil, or, in other words, upon its retentive char- 

 acter. Soils are spoken of as open or close with respect to their 

 drainage properties, all variations in each class being recognized 

 and requiring drainage treatment according to their several charac- 



