5th. Add this depth to the first number of the line next preceding it, and enter the sum obtained on the first line of the fifth column, as the depth of the drain below the datum-line.
6th. Having reference to the grade of the surface, (as shown by the figures in the sixth column,) as well as to any necessity for placing the drain at certain depths at certain places, enter the desired 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 the Silt-Basin | 20.45 feet. |
| Depth of the Drain at the Stake No. 3 | 16.98 feet. |
| —— | |
| Difference | 3.47 feet. |
| Distance between the two | 186.— feet. |
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: 1.87×82=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.
Then, from this new amount, subtract the second number of the fourth column, for the next number of the fifth, and so on, until, in place of the entry in pencil, (Stake 3,) we place the exact result of the computation.