174 PRACTICAL DRAINAGE 



the soil can be regulated by the operator. In making 

 drains this machine begins at the outlet end and moves 

 toward the higher land leaving a gopher-like hole through 

 which the drainage water passes. Such drains can be made 

 25 feet apart for about $5 an acre. These will last 5 or 6 

 years in the right kind of soil. If any of them happen to 

 become clogged, new ones may be made between the others. 



The type of covered drain to use depends on a number 

 of factors. In wet brush land where rock and lumber are 

 scarce and where tile cannot be had, rush and straw may 

 be used to good advantage, although usually less ef- 

 ficiently than some of the more permanent types. 



Brown and Hart (3) found lumber drains to be very 

 effective in a swamped soil that would not remain firm 

 enough to hold tile. Rock properly placed in the trench 

 has long been used to keep open the water channel. 



These various unusual types of drains are unimportant 

 in comparison with tile. The most common kinds are 

 clay tile, either porous or vitrified. Many types of clay 

 tile are to be had. These are so well and favorably known 

 that further discussion seems unnecessary here. Cement 

 tile is being used to some extent, but its use on alkali land 

 is attended with some risk which is explained below. 



Cement Tile for Alkali Land. The ease with which 

 cement tile can be made in some localities has encouraged 

 its use for drainage. This has often resulted in failure, 

 because it has been found that under certain conditions 

 the cement is attacked and destroyed by some of the al- 

 kali salts. This observation has led to considerable study 

 on 'the relation of soluble salts to cements and their de- 

 terioration. 



Burke and Pinckney (4) found that to cause weakening 

 it was necessary for salt solutions to penetrate the concrete. 



