d depth, _in pencil,_ in the last column, opposite the
stakes marking those places. Then add together this depth and the
corresponding surface measurement in the column next preceding,
and enter the sum, _in pencil_, in the fifth column, as the depth
from the datum-line to the desired position of the drain. (In the
example in hand, these points are at Nos. 3, 7, 10, 11, and 17.)

7th. Subtract the second amount in the fifth column from the first
amount for the total fall between the two points--in the example,
"3" from "Silt-Basin." Divide this total fall, (in feet and
hundredths,) by one hundredth of the total number of feet between
them. The result will be the rate of fall per 100 feet, and this
should be entered, in the third column, opposite each of the
intermediate distances between the points.

Example:

Depth of the Drain at 20.45 feet.
the Silt-Basin
Depth of the Drain at 16.98 feet.
the Stake No. 3
----
Difference 3.47 feet.
Distance between the 186.-- feet.
two

1.86)3.47(1.865 or 1.87

1 86
----
1 610
1 488
----
1 220
1 116
----
1 040
930
----
110

8th. Multiply the numbers of the second column by those of the
third and divide the product by 100. The result will be the amount
of fall between the stakes, (fourth column.)--Example:
1.87x82=153/100=1.53.

9th. Subtract the first number of the fourth column from the first
number of the fifth column, (on the line above it,) and place the
remainder on the next line of the fifth column.--Example:
20.70-1.64= 19.06.