3.20.
MEASURING ORDINATES
After the curve is divided into stations, you begin to measure the ordinates. Take the string and stretch it
from Sta. 0 to Sta. 2, holding it on the inside of the head of the rail. The midpoint of the string will be opposite
Sta. 1. Measure with the ruler between the head of the rail and
the string at Sta. 1, and enter the distance in eighths of an inch
in your notebook beside the station number. If there is no
distance, write down 0. Next stretch the string between Sta. 1
and Sta. 3, measure at Sta. 2--the midpoint of the string, and
record the result. Continue to measure at each station all
around the curve until the far end of the string is at the last
station you have laid off. Record the result at each station.
When you are on the curve itself, you will get some
result at each station. Say, for instance that at Sta. 19 you
measured 1 1/2 inches between the string and the head of the
rail. Since 1 1/2 inches is 12/8 inches, you would enter +12 in
your notebook. Inches and eighths should be converted to
eighths of an inch and recorded in that way, as the sketch
illustrates.
If you find that the string at its midpoint is pressed
against the rail, especially on or near tangents, this shows
either that the rail is absolutely straight or that it has curved
inward at that point. Here the offset blocks are used, as shown
in the sketch. Use one block at each end of the string, and
place it between the string and the head of the rail. This keeps
each end of the string exactly 1 inch from the rail. Now
measure
at
the
midpoint of the string. If you measure 1 inch, the track is
absolutely straight, and you enter 0 in your
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