evel position, _d_, by the action of the large-headed brass
screw, _c_. A stud is affixed to the framing, and pushed firmly
into a gimlet-hole in the top of the short rod, which is pushed or
driven into the ground at the spot from whence the level is desired
to be ascertained. It need scarcely be mentioned, that the height
of the eye sight, from the guard, is to be deducted from the height
of observation, which quantity is easily obtained by having the rod
marked off in inches and feet; but it may be mentioned, that this
instrument should be used in all cases of draining on level ground,
even when one is confident that he knows the fall of the ground;
for the eye is a very deceitful monitor for informing you of the
levelness of ground. It is so light as to admit of being carried in
the pocket, whilst its rod may be used as a staff or cane.

[Illustration: Fig. 56.--SPIRIT LEVEL.]

A staff of ten feet in length, graduated in feet and inches, and held by
an attendant at the various points of observation, is necessary in the
use of the spirit-level in the field. A painted target, arranged with a
slide to be moved up and down on this staff, and held by a thumbscrew,
will be found useful.

We have made for our own use a level like the above, and find it
sufficiently accurate for drainage purposes. Small spirit-levels set in
iron can be had at the hardware shops for twenty cents each, and can be
readily attached to wood by a screw, in constructing our implement; or a
spirit-level set in mahogany, of suitable size, may be procured for a
half dollar, and any person, handy with tools, can do the rest. The
sights should be arranged both ways, with a slit cut with a chisel
through the brass or tin, and an oblong opening at each end. The eye is
placed at the slit, and sight is taken by a hair or fine thread, drawn
across the opening at the other end. Then, by changing ends, and
sighting through the other end at a given object, any error in the
instrument may be detected. The hair or thread may be held in place by a
little wax, and moved up or down till it is carefully adjusted. The
instrument should turn upon the staff in all directions, so that the
level of a whole field, so far as it is within range, may be taken from
one position.

[Illustration: Fig. 57.

STAFF AND TARGET.]

To maintain a uniform grade in the bottom of a drain so as to economize
the fall, and di