the cause of the evil to which they were
respectively applied. The idea 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 six feet high and four 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 twenty inches or two
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 horse-shoe tile, which may be a
tolerable secure conduit in a drain of two feet, in one of four
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 horse-shoe tile. When the clay expands with moisture, it
necessarily presses on the tile and breaks it through the crown,
its weakest part.(9) When the Regent's Park was first drained,
large conduits were in fashio