\fdO'^ - Old Forge Coal Mines

252 SURVEYING.
and including that at Sta. 43, we subtract from the same
H. I., viz., 166.95. Here at a turning point (T. P.) of 8.66
ft. is taken and recorded in the column of rod readings.
This reading being a foresight is viinus, and is subtracted
from the preceding H. I. This gives us for the elevation of
theT. P., 166.95 ft. - 8.66 ft. = 158.29 ft., which we record in
the column of elevations. The instrument is then moved forwards
and a backsight of 2.22 ft taken on the same T. P. and
recorded in the column. This is a plus reading, and is added
to the elevation of the T. P., giving us for the next H. T.
an elevation of 158.29 ft.+ 2.22 ft.= 160.51 ft. The next
rod reading, viz., 4.8, is at Sta. 44, and the elevation at that
station is the difference between the preceding H. I., 160.51,
and that rod reading, giving an elevation of 160.51 ft. — 4.8
ft. = 155.7 ft., which is recorded in the column of elevations
opposite Sta. 44. In a similar manner, the remaining elevations
are determined.
In checking level notes, only the turning points rod readings
are considered. It will be evident that starting from
a given bench mark, all the backsight or plus readings will
add to that elevation, and all the foresight or minus read-
ings will subtract from that elevation. If now we place in
one column the height of the B. M., together with all the
backsight or -|- readings, and in another column all the fore-
sight or — readings, and find the sum of each column, then,
by subtracting the sum of the — readings from the sum of
the -\- readings, we shall find the elevation of the last point
calculated, whether it be a turning point or a height of instrument.
Applying this method to the foregoing notes
we have the following :
-f readings. — readings.
R M. 161.42ft. 8.66 ft.
5.5 3 ft. 11.2 4 ft.
2.2 2 ft. 11.6 2 ft.
^•^Q ^*-
17 2.4 7 ft.
31.52 ft.
X 4 0.9 5 ft.
31.52 ft.