Two arms on a hinge is about all these things really are. They have points at both ends, but no pencil lead usually,
which makes them good for measuring distances on a chart, but that's about it.
How to use:
Simply spread the legs of the divider and put the pointy ends on either point that you are measuring
between. Then take the whole divider over to the Latitude scale and measure the spread. You want to use the
latitude scale nearest the mid-point of the spread because the scale is not constant throughout the chart. For example, a
10' spread at the bottom of the chart may be 3.5 inches, whereas at the top it may be 4 inches. And never use the longitude
scale, it's just not right man. Unless you are at the equator, 60' of longitude does not equal 60nm.
If you are attempting to measure along a trackline or measure a distance that is greater than
how far your dividers will spread, the simplest thing to do is set your spread to a known distance and walk your dividers
along the track. By walk I mean leave the forwardmost point firmly fixed on the chart and swing the back leg around until
it meets the chart, and repeat this until you reach your destination, counting the number of spreads along the way. For example,
using your latitude scale at the mid-point of the track, you set your dividers to a 10' spread. You walk your dividers along
the course and find that the distance between is 5 spreads. 5 x 10' = 50', you've just found your distance.
If the track you are walking is not exactly a whole number of spreads, on the last spread, simply
measure the exact distance along the latitude scale and add it to the previous number of spreads. For example, if you
make 5 spreads at 10', and the last spread you measure to be only 3', add 3' to 50' (5x10') to get a total of 53'.