s nearly as possible, especially if the descent be small.
Workmen are very apt to work at a uniform depth from the surface, and so
give the bottom of the drain the same variations as the surface line;
and thus at one point there may be a fall of one inch in a rod; at
another, twice that fall; and at another, a dead level, or even a
hollow. On our own farm, we have found, in twelve rods, a variation of a
foot in the bottom line of a drain opened by skillful workmen on a
nearly level field, where they had no water to guide them, and where
they had supposed their fall was regular throughout.

The following sketch shows the difference between lines of tiles laid
with and without instruments. Next to guessing at the fall in our field,
may be placed a little contrivance, of which we have made use
sufficiently to become satisfied of its want of practical accuracy. It
is thus figured and described in the excellent treatise of Thomas, on
Farm Implements.

[Illustration: Fig. 54.]

"_A_ is a common square, placed in a slit in the top of the stake
_B_. By means of a plumb-line the square is brought to a level,
when a thumbscrew, at _C_, fixes it fast. If the square is two feet
long, and is so carefully adjusted as not to vary more than the
twentieth of an inch from a true level, which is easily
accomplished, then a twentieth of an inch in two feet will be one
inch in forty feet--a sufficient degree of accuracy for many
cases."

[Illustration: Fig. 55.--SQUARE AND PLUMB-LEVEL.]

We do not so much object to the principle of the above level, as to its
practical working. We find it difficult, without cross sights, to take
an accurate level with any instrument. However, those who are used to
rifle-shooting may hit tolerably near the mark with the square. Mr.
Thomas only claims that it is accurate enough "for many cases."

A proper spirit-level, such as is used by engineers of railroads and
canals, attached to a telescope, is the best of all instruments. "So
great is the perfection of this instrument," says the writer just
quoted, "that separate lines of levels have been run with it, for sixty
miles, without varying two-thirds of an inch for the whole distance." A
cheap and convenient spirit-level, for our purpose, is thus constructed.

It is furnished with eye sights, _a b_, and, when in use, is placed
into a framing of brass which operates as a spring to adjust it to
the l