e level of the drain, the ditch should be excavated a little
below the grade of the drain, and then filled to that level with a
retentive clay, and rammed hard. In all cases when the tile is well laid,
(especially if collars are used,) and a stiff earth is well packed around
the tile, silt will not enter the drain to an injurious extent, after a
few months' operation shall have removed the loose particles about the
joints, and especially after a few very heavy rains, which, if the tiles
are small, will sometimes wash them perfectly clean, although they may
have been half filled with dirt.

_Vermin_,--field mice, moles, etc.,--sometimes make their nests in the tile
and thus choke them, or, dying in them, stop them up with their carcases.
Their entrance should be prevented by placing a coarse wire cloth or
grating in front of the outlets, which afford the only openings for their
entrance.

_Roots._--The roots of many water-loving trees,--especially willows,--will
often force their entrance into the joints of the tile and fill the whole
bore with masses of fibre which entirely prevent the flow of water.
Collars make it more difficult for them to enter, but even these are not a
sure preventive. Gisborne says:

"My own experience as to roots, in connection with deep pipe draining, is
as follows: I have never known roots to obstruct a pipe through which
there was not a perennial stream. The flow of water in summer and early
autumn appears to furnish the attraction. I have never discovered that the
roots of any esculent vegetable have obstructed a pipe. The trees which,
by my own personal observation, I have found to be most dangerous, have
been red willow, black Italian poplar, alder, ash, and broad-leaved elm. I
have many alders in close contiguity with important drains, and, though I
have never convicted one, I cannot doubt that they are dangerous. Oak, and
black and white thorns, I have not detected, nor do I suspect them. The
guilty trees have in every instance been young and free growing; I have
never convicted an adult. These remarks apply solely to my own
observation, and may of course be much extended by that of other
agriculturists. I know an instance in which a perennial spring of very
pure and (I believe) soft water is conveyed in socket pipes to a paper
mill. Every junction of two pipes is carefully fortified with cement. The
only object of cover being protection from superficial injury and from
frost, the pipes