main-drains on this farm was observed not discharging the
water freely, as it hitherto had done, after a heavy fall of rain;
and the land adjoining it showed unmistakable signs of wetness. The
drain was opened, and traced to the point of obstruction, which was
found to be convenient to a _small poplar tree_, the rootlets of
which made their way into the tiles, at the depth of five and a
half feet, and completely filled them, in the direction of the
stream, for several yards. We have some of the tiles (horse-shoe)
in our museum here, as they were then lifted from the drain,
showing clearly the formidable nature of the obstruction. Another
serious case of obstruction has come to my knowledge, occasioned by
frogs or toads getting into the tiles of the main-drain in large
numbers, on account of the outlet being insufficiently protected.
In this case, a large expenditure had to be incurred, to repair the
damage done.

I have not observed any case of obstruction from the roots of our
cultivated plants. It has been said by some that the rootlets of
mangold will reach the drains under them; and, particularly, where
the drains contain most water in rapid motion. I took up the tiles
from a drain on this farm, in '54, which had been laid down (by a
former occupier), about the year '44, at a depth not exceeding
two-and-a-half feet, and not one of these was obstructed in the
least degree, although parsnips, carrots, cabbages, mangolds, &c.,
had been grown on this field. Obstructions may occur through the
agency of _mineral_ springs; but very few cases of this nature are
met with, at least in this country. I would anticipate this class
of obstruction, if from the nature of the land there was reason to
expect it, by increasing the fall in the drains and having _traps_
more frequent, where the main outlets are at a distance to render
them necessary. In my opinion, the roots of trees are the great
intruders to be guarded against, and more particularly the
_soft_-wooded sorts, such as poplars, willows, alders, &c. The
distance of a drain from a tree ought always to be equal to the
height of the latter.

_Tiles flattening in the drying process._ With this subject, I am
not practically familiar. In most tile-works, the tiles, after
passing through the moulding-