ld upright in the ditch, with its arm directly over the grade
stake. The earth below it is removed, little by little, until it will
touch the top of the stake and the bottom of the ditch at the same time.
If the ground is soft, it should be cut out until a flat stone, a block of
wood, or a piece of tile, or of brick, sunk in the bottom, will have its
surface at the exact point of measurement. This point is the bottom of the
ditch on which the collar of the tile is to lie at that stake. In the same
manner the depth is fixed at _C11_ (4.19,) and _C12_ (4.41,) as the rate
of fall changes at each of these points, and at _C15_ (3.89,) and _C17_
(4.17,) because (although the fall is uniform from _C12_ to _C17_,) the
distance is too great for accurate sighting.

[Illustration: Fig. 29 - BONING ROD.]

Fig. 29 - BONING ROD.

Having provided _boning-rods_, which are strips of board 7 feet long,
having horizontal cross pieces at their upper ends, (see Fig. 29,) set
these perpendicularly on the spots which have been found by measurement to
be at the correct depth opposite stakes 10, 11, 12, 15, and 17, and fasten
each in its place by wedging it between two strips of board laid across
the ditch, so as to clasp it, securing these in their places by laying
stones or earth upon their ends.

As these boning-rods are all exactly 7 feet long, of course, a line
sighted across their tops will be exactly 7 feet higher, at all points,
than the required grade of the ditch directly beneath it, and if a plumb
rod, (similar to the boning-rod, but provided with a line and plummet,) be
set perpendicularly on any point of the bottom of the drain, the relation
of its cross piece to the line of sight across the tops of the boning-rods
will show whether the bottom of the ditch at that point is too high, or
too low, or just right. The manner of sighting over two boning-rods and an
intermediate plumb-rod, is shown in Fig. 31.

[Illustration: Fig. 30 - POSITION OF WORKMAN AND USE OF FINISHING SCOOP.]

Fig. 30 - POSITION OF WORKMAN AND USE OF FINISHING SCOOP.

Three persons are required to finish the bottom of the ditch; one to sight
across the tops of the boning-rods, one to hold the plumb-rod at different
points as the finishing progresses, and one in the ditch, (see Fig. 30,)
provided with the finishing spade and scoop,--and, in hard ground, with a
pick,--to cut down or fill up as the firs