STEUCTUBES USED IN DRAINING AGRICULTURAL LAND 5 
may be made of any size, but the top opening should never be less 
than 12 inches in diameter. For very large inlets the upper section 
of the vertical riser pipe is sometimes of larger diameter than the 
lower sections and is connected with a concrete ring. Where the 
soil freezes to considerable depth, a more substantial riser pipe may 
be made by placing a large pipe around a smaller one and filling the 
Fig. 3. — Types of surface water inlets for closed drains 
intervening space with concrete. The entrance may be protected 
by a cast-iron grate of the type illustrated in Figure 3, C, or one 
made in the form of the frustrum of a cone may be substituted. 
The latter type is more expensive but will provide greater area of 
opening. The grate should be secured against removal. 
A type of inlet which will prevent debris from dropping into the 
tile line is shown in Figure 3, B. but the entrance of water at the 
