stribute it equally through the whole length, several
different instruments and means may be adopted. The first which we will
figure, is what is called the Span, or A Level. Such a level may be
easily constructed of common inch-board. If it be desired to note the
fall in feet, the span may conveniently be ten feet. If a notation in
rods be preferred, the span should be a rod, or half rod long.

The two feet being placed on a floor, and ascertained to be perfectly
level by a spirit-level, the plumb-line will hang in the centre, where a
distinct mark should be made on the cross-bar. Then place a block of
wood, exactly an inch thick, under one leg, and mark the place where the
line crosses the bar. Put another block an inch thick under the same
leg, and again mark where the line crosses the bar, and so on as far as
is thought necessary. Then put the blocks under the other leg in the
same manner, and mark the cross-bar. If the span be ten feet, the
plumb-line will indicate upon the bar, by the mark which it crosses, the
rise or fall in inches, in ten feet. If the span be a rod, the line will
indicate the number of inches per rod of the rise or fall.

[Illustration: Fig. 58.--SPAN, OR A LEVEL.]

This instrument is used thus: The fall of the ditch from end to end
being ascertained by the spirit-level, and the length also, the fall per
rod, or per one hundred feet, may be computed. The span is then placed
in the bottom of the drain, from time to time, to guide the workman, or
for accurate inspection of the finished cut. We have constructed and
used this level, and found it very convenient to test the accuracy of
the workmen, who had opened drains in our absence. A ten-foot span will
be found as large as can be conveniently carried about the farm.

For the accurate grading of the bottom of drains, as the work proceeds,
we have in practice found nothing so convenient and accurate as the
arrangement which we are about to illustrate.

The object is simply to draw a line parallel with the proposed bottom of
the drain, for the laborers to work under, so that they, as they
proceed, may measure down from it, as a guide to depth. Having with the
spirit-level, ascertained the fall from end to end of the drain, a short
stake is set at each end, and a line is drawn from one to the other at
the requisite height, and supported by the cross-pieces, at suitable
distances, to prevent the sagging of the line.

[Illustration: Fig. 59.--GRAD