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DEPTH. | MEAN WIDTH.
-------+------+------+------+------+------+------
Inches.|13 In.|14 In.|15 In.|16 In.|17 In.|18 In.
-------+------+------+------+------+------+------
30 |1.655 |1.78 |1.91 |2.04 |2.164 |2.29
33 |1.82 |1.96 |2.10 |2.24 |2.38 |2.52
36 |1.986 |2.14 |2.29 |2.244 |2.60 |2.75
39 |2.15 |2.32 |2.48 |2.65 |2.81 |2.98
42 |2.32 |2.495 |2.674 |2.85 |3.03 |3.21
45 |2.48 |2.67 |2.865 |3.055 |3.246 |3.438
48 |2.65 |2.85 |3.056 |3.26 |3.46 |3.667
51 |2.81 |3.03 |3.25 |3.46 |3.68 |3.896
54 |2.98 |3.20 |3.44 |3.666 |3.895 |4.125
57 |3.14 |3.38 |3.63 |3.87 |4.11 |4.354
60 |3.31 |3.564 |3.82 |4.074 |4.33 |4.584
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"Along the top of the table is placed the mean widths in inches,
and on the left-hand side the depths of the drains, extending from
30 inches to 5 feet. The numbers in the body of the table express
cubic yards, and decimals of a yard. In making use of the table, it
is necessary first to find the mean width of the drain, from the
widths at the top and bottom. Thus, if a drain 3 feet deep were 16
inches wide at the top, and 4 inches at the bottom, the mean width
would be half of 16 added to 4, or 10; then, by looking in the
table for the column under 10 (width), and opposite 36 (inches of
depth), we find the number of cubic yards in each rod of such a
drain to be 1.53, or somewhat more than one and a half. If we
compare this with another drain 20 inches wide at the top, 4 inches
at the bottom, and 4-1/2 feet deep, we have the mean width 12, and
looking at the table under 12 and opposite 54, we find 2.75 cubic
yards, or two and three-quarters to the rod. In this case, the
quantity of earth to be removed is nearly twice as much as in the
other, and hence, as far as regards the digging, the cost of the
labor will be nearly double. But in the case of deep drains, the
cost increases slightly for another reason, namely, the increased
labor of lifting the earth to the surface from a greater depth."

Under the title of the "Depth of Drains," other reasons are suggested
why shallow drains are more easily wrought than deeper drains. The
widths giv