datum-line

The numbers above the vertical lines indicate the stakes; the figures in brackets between these, the number of feet between the stakes; and the heavy figures at the left of the vertical lines, the recorded measurements of depth from the datum-line to the surface of the ground, which is indicated by the irregular line next below the datum-line.

Draining for Profit, and Draining for Health 1865

The topmost line, from "Silt Basin" to "17," is the horizontal datum-line.

Draining for Profit, and Draining for Health 1865

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.

Draining for Profit, and Draining for Health 1865

The attendant then reads the elevation, and the operator records it as the distance below the _datum-line_ of the top of the grade-stake.

Draining for Profit, and Draining for Health 1865

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.

Draining for Profit, and Draining for Health 1865

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.

Draining for Profit, and Draining for Health 1865