DRAINAGE. 1 03 



was, that this tile, standing on narrow feet, and pressed by 

 the weight of the refilled soil, sank into the floor of the 

 drain ; whereas, in fact, the floor of the drain rose into the 

 tile. Any one at all conversant with collieries is aware that 

 when a strait work (which is a small subterranean tunnel 

 6 feet high and 4 feet wide or thereabouts) is driven in 

 coal, the rising of the floor is a more usual and far more 

 inconvenient occurrence than the falling of the roof : the 

 weight of the two sides squeezes up the floor. We have 

 seen it formed into a very decided arch without fracture. 

 Exactly a similar operation takes place in the drain. No 

 one had till recently dreamed of forming a tile drain, the 

 bottom of which a man was not to approach personally 

 within 20 inches or 2 feet. To no one had it then occurred 

 that width at the bottom of the drain was a great evil. For 

 the convenience of the operator the drain was formed with 

 nearly perpendicular sides, of a width in which he could 

 stand and work conveniently, shovel the bottom level with 

 his ordinary spade, and lay the tiles by his hand ; the 

 result was a drain with nearly perpendicular sides, and a 

 wide bottom. No sort of clay, particularly when softened 

 by water standing on it or running over it, could fail to 

 rise under such circumstances ; and the deeper the drain 

 the greater the pressure and the more certain the rising. 

 A horseshoe tile, which may be a tolerably secure conduit 

 in a drain of 2 feet, in one of 4 feet becomes an almost 

 certain failure. As to the longitudinal fracture not only 

 is the tile subject to be broken by one of those slips which 

 are so troublesome in deep draining, and to which the 

 lightly-filled material, even when the drain is completed, 

 offers an imperfect resistance, but the constant pressure 

 together of the sides, even when it does not produce a 

 fracture of the soil, catches hold of the feet of the tile, and 

 breaks it through the crown. Consider the case of a drain 

 formed in clay when dry, the conduit a horseshoe tile. 

 When the clay expands with moisture, it necessarily presses 

 on the tile, and breaks it through the crown, its weakest 



