nes.

A system for marking the stakes is indicated on the map, (in the _C_
series of drains,) which, to avoid the confusion which would result from
too much detail on such a small scale, has been carried only to the extent
necessary for illustration. The stakes of the line _C_ are marked _C1_,
_C2_, _C3_, etc. The stakes of the sub-main _C7_, are marked _C7a_, _C7b_,
_C7c_, etc. The stakes of the lateral which enters this drain at _C7a_,
are marked _C7a/1_, _C7a/2_, _C7a/3,_ etc. etc. This system, which
connects the lettering of each lateral with its own sub-main and main, is
perfectly simple, and avoids the possibility of confusion. The position of
the stakes should all be lettered on the map, at the original drawing, and
the same designating marks put on the stakes in the field, as soon as set.

_Grade Stakes_, (pegs about 8 or 10 inches long,) should be placed close
at the sides of the marked stakes, and driven nearly their full length
into the ground. The tops of these stakes furnish fixed points of
elevation from which to take the measurements, and to make the
computations necessary to fix the depth of the drain at each stake. If the
measurements were taken from the surface of the ground, a slight change of
position in placing the instrument, would often make a difference of some
inches in the depth of the drain.

*Taking the Levels.*--For accurate work, it is necessary to ascertain the
comparative levels of the tops of all of the grade stakes; or the distance
of each one of them below an imaginary horizontal plane. This plane, (in
which we use only such lines as are directly above the drains,) may be
called the "Datum Line." Its elevation should be such that it will be
above the highest part of the land, and, for convenience, it is fixed at
the elevation of the levelling instrument when it is so placed as to look
over the highest part of the field.

_Levelling Instruments_ are of various kinds. The best for the work in
hand, is the common railroad level, which is shown in Fig. 6. This is
supported on three legs, which bring it to about the level of the eye. Its
essential parts are a telescope, which has two cross-hairs intersecting
each other in the line of sight, and which may be turned on its pivot
toward any point of the horizon; a bubble glass placed exactly parallel to
the line of sight, and firmly secured in its position so as to turn with
the telescope; and an apparatus for raising or depressing any side