CHAP. xi. LAYING DOWN DRAINS. 871 



friction in the interior, and this, taken along with the contraction of 

 channel, gives the water a much faster current in circular than in any 

 other kind of drains. Even in ordinary main drains, tiles of a round 

 or a reversed elliptical section should always be preferred to those that 

 are flat-bottomed. The flow of water in any kind of under-drains, but 

 especially in main ones, is also greatly dependent on the accuracy 

 with which the levels are kept ; and a very slight fall, if the bottom is 

 cleaned out with a very great nicety, will give a good run, when a 

 considerable fall with inaccurate levels will fail to secure this end. In 

 cases where main drains have a long stretch without a discharging 

 outlet, and run some risk of being flooded during heavy rains, it will 

 be found conducive to their safety to provide at different points, if 

 possible, a line of pipes branching off into some adjoining open ditch, 

 which, while too shallow to receive the discharge from the main drain 

 itself, may yet be low enough to catch the overflow water that is likely 

 to do harm if not allowed an exit. Suppose, for instance, that a main 

 drain 4 feet 9 inches deep is provided with a branch pipe at intervals, 

 laid 3J feet under the surface, it will be obvious that the vertical 

 pressure on the former can never exceed 15 inches ; and when the 

 nature of the ground permits of it, may be so arranged as to be a great 

 deal less. To catch any solid matter which may be likely to get into 

 the main drains, it is always a safe provision to form, at intervals of 

 180 to 200 yards, small sediment wells, with brick on bed. A depth of 

 a foot below the level of the drain will be sufficient in ordinary cases ; 

 and the building being carried up to within about 18 inches of the 

 surface, a flat closely-fitting stone with a ring in it will cover up the 

 opening. Wherever there is an overflow branch tile, there should be 

 one of these wells in the main ; and those that may be necessary at 

 other points can have a mark set up near them, that the cover may 

 easily be reached, and the well cleaned out at pleasure. In practice 

 this will be found to add greatly to the efficiency of the drains ; and 

 the expense of providing wells such as have been described is very 

 trifling. 



" Another provision well calculated to promote the continued 

 efficiency of outfalls and main drains is that of providing some sort of 

 protection at the various mouths. A grating is sometimes placed 

 before the vent, but the water is necessarily obstructed by it to an 

 injurious extent ; and, whenever drains are to be finished in the best 

 possible manner, a light cast-iron valve, working on pivots, should be 

 furnished for each outlet, in preference to a grating. By a number 

 being placed on the valve, and a reference made to it in the drainage 

 book, there is less fear of the outlet being forgotten than would other- 

 wise be the case. How often, even on well-drained land, do we see 

 the entire efficiency of the drains hazarded by the outfalls being 

 neglected. The principal main drain is perhaps run into an open 

 ditch ; but, beyond putting a turf or two round the tile, no special 

 building is provided to mark the place. As the ditch grows up from 

 want of attention, the drain mouth is concealed, and by-and-by it 



