)
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.
Pages:
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137