Measuring directions with a rolling plotter...
(1) Align parallel plotter with the line of interest
(2) Roll plotter to nearest meridian and place center of plotter's scale on that meridian.
(3) Read line direction from the plotters scale, where the meridian crosses it.

General notes
This example is the same as last example, but with a Weems plotter, rather than parallel rulers. Read in that section about the trick of using dividers to get the alignment, or in this case, a first step toward the alignment, since we can fine tune that one using this tool (see Video Notes, below).

Meridians are the lines of longitudes, vertical lines on most charts. This measurement can also be made on a parallel of latitude (horizontal lines on the chart) using the diagrams on the two edges of the plotter. These plotters work very well when you have good chart table area. It is sometimes difficult to get to use them near the edge of a chart, so we always have parallel rulers at hand if needed.

These plotters are very convenient, but you must use true bearings — as opposed to using parallel rulers and the compass rose, in which case you can choose which scale, true or magnetic, you prefer. When using these tools, you must have the local magnetic variation in mind at all times and make the corrections whenever you are relating to compass courses or bearings.

Video Notes
As in last example, the finger traces out the direction we wish to measure. And, as explained in the last example, we use the dividers to align the plotter with the charted line.

In this case, though, we fine tune that alignment using the thin black line printed on the plotter itself. You could use the edge of the plotter or this line, but we have found that using this line is more accurate. To do this, look through the plotter and micro-adjust the plotter orientation until the line on it coincides exactly with the line on the chart.

Then carefully roll the plotter to the nearest meridian line and align the center of the plotter's compass rose scale with the meridian as shown.

Read the heading from where the meridian crosses the scale. Note that the diagram reminds you with small arrows that if looking NW use the inner scale and when looking SE use the outer scale (see last frame of video). The bearing we want in this example is 320 T, in the NW quadrant. The reciprocal is 140 T, which is in the SE quadrant.

As cautioned in the last example, we must be even more careful here to get the right direction. Generally keeping some rough numerical estimate in mind will solve this problem.