s
have developed many instances where tiles were at first half filled with
silt, and three months later were entirely clean. The muddiness of the
flow indicates what the doctors call "an effort of nature to relieve
herself," and nature may be trusted to succeed, at least, until she
abandons the effort. If we are sure that a drain has been well laid, we
need feel no anxiety because it fails to take the water from the ground so
completely as it should do, until it settles into a flow of clear water
after the heaviest storms.

In the case of art actual stoppage, which will generally be indicated by
the "bursting out" of the drain, i.e., the wetting of the land as though
there were a spring under it, or as though its water had no underground
outlet, (which is the fact,) it will be necessary to lay open the drain
until the obstruction is found.

In this work, the real value of the map will be shown, by the facility
which it offers for finding any point of any line of drains, and the exact
locality of the junctions with the mains, and of the silt-basins. In
laying out the plan on the ground, and in making his map, the surveyor
will have had recourse to two or more fixed points; one of them, in our
example, (fig. 21,) would probably be the center of the main silt-basin,
and one, a drilled hole or other mark on the rock at the north side of the
field. By staking out on the ground the straight line connecting these two
points, and drawing a corresponding line on the map; we shall have a
_base-line_, from which it will be easy, by perpendicular offsets, to
determine on the ground any point upon the map. By laying a small square
on the map, with one of its edges coinciding with the base-line, and
moving it on this line until the other edge meets the desired point, we
fix, at the angle of the square, the point on the base-line from which we
are to measure the length of the offset. The next step is to find, (by the
scale,) the distance of this point from the nearest end of the base-line,
and from the point sought. Then measure off, in the field, the
corresponding distance on the base-line, and, from the point thus found,
measure on a line perpendicular to the base line, the length of the
offset; the point thus indicated will be the locality sought. In the same
manner, find another point on the same drain, to give the range on which
to stake it out. From this line, the drains which run parallel to it, can
easily be found, or it may