[417] , SCIENTIFIC MANUAL. 131 



If the bottom is sandy, foot-rocks must be used, to pre- 

 vent the side-stones from sinking into the earth and stop- 

 ping up the duct ; besides, unless the bottom of the ditch 

 is firm clay, it will wash in some parts and stagnate in oth- 

 ers, forming deposits of sand and mud, and eventually fill 

 up the duct. 



Fig. 22 Box Drain with Fig. 23 Box Drain with- 

 Foot-rock. out Foot-rock. 



Pounded rock, in which the rock is broken into pieces 

 ranging from the size of a guinea egg to that of a large 

 hen egg, and thrown into the bottom of the ditch from six 

 to ten inches deep. The difficulty about this drain is the 

 danger of choking, in consequence of there being no con- 

 tinuous duct to carry off* the water. 



Tile drain^ though expensive in first cost, is by far the 

 most effective and permanent, and though the tile pipe 

 costs more than other material, the ditch for the reception 

 of the tile is less expensive than that for rock, poles or 

 plank, since the bottom of the ditch need not be larger 

 than the outside diameter of the tile used, and the top no 

 wider than is necessary to admit the operator in opening 

 the first two feet, the balance being dug with long-handled 

 narrow instruments, made for the purpose, and the tile low- 

 ered by a "tile layer," the operator standing on the surface 

 of the ground, while opening the bottom of the drain and 

 laying the tile. The following cuts show the implements 

 made expressly for opening drains for tile, the tile-layer, 

 guages and span-level : 



