Not, however, being confident as to the soil
having already become so porous as to dispense entirely with surface
drains, Mr. Hammond had drawn two long water furrows diagonally across the
field. On examining these, it appeared that very little water had flowed
along any part of them during these 12 hours of rain,--no water had escaped
at their outfall; the entire body of rain had permeated the mass of the
bed, and passed off through the inch pipes; no water perceptible on the
surface, which used to carry it throughout. The subsoil is a brick clay,
but it appears to crack very rapidly by shrinkage consequent to drainage."
*Obstructions.*--The danger that drains will become obstructed, if not
properly laid out and properly made, is very great, and the cost of
removing the obstructions, (often requiring whole lines to be taken up,
washed, and relaid with the extra care that is required in working in old
and soft lines,) is often greater than the original cost of the
improvement. Consequently, the possibility of tile drains becoming stopped
up should be fully considered at the outset, and every precaution should
be taken to prevent so disastrous a result.
The principal causes of obstruction are _silt, vermin_, and _roots_.
_Silt_ is earth which is washed into the tile with the water of the soil,
and which, though it may be carried along in suspension in the water, when
the fall is good, will be deposited in the eddies and slack-water, which
occur whenever there is a break in the fall, or a defect in the laying of
the tile.
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