2d. Enter the distances between each two stakes in the second column, placing the measurement on the line with the number of the _upper_ stake of the two. 3d. In the next to the last column enter, on the line with each stake, its depth below the datum-line, as recorded in the field book of levels, (See page 105.) 4th. On the first line of the last column, place the depth of the lower end of the drain, (this is established by the grade of the main or other outlet at which it discharges.) 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 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