Compass:
No, this is not the kind that spins and tells you where North is. This is the kind that is a
lot like a pair of dividers, but more stable and with a pencil lead in one leg. What makes them different primarily is that
a compass has a geared wheel between the two legs so that the spread can be adjusted without squeezing the legs together.
This allows the spread not to slip as easily and also allows it to be more precisely adjusted by rolling the wheel instead
of squeezing the legs.
How to Use:
There are several ways to use a compass, depending on what it is that you are trying to accomplish.
The simplest way is to measure distances, in which case they are used exactly like dividers, and rather than explaining that
all again, just go to the page that describes how to use dividers.
The pencil lead on the compass allows you to actually mark off distances or speeds, depending
on how you use it. If you want to find a location by using multiple ranges, simple spread the legs to the known distance as
per the latitude scale, then, from the object the range was taken from, swing a slight arc and drag the pencil end across
the chart in the region where you expect your position to be. (There will be a diagram at right eventually). Repeat this step
for the other objects and ranges. What you should end up with are arcs that cross one another. The point where the arcs intersect
ought to be your position, barring any errors.
The pencil lead also allows you to mark off certain distances along a trackline. Every watch will
have to mark down DR positions on the chart (see the page on DR positions). The simplest way to do this is spread your compass
to measure the ship's speed (if the speed is 20kts, spread to 20'). From your last known position, measure off and mark (with
the pencil) every spread. As each spread is equivalent to the distance the ship travels in one hour, each tick mark that you
make will be an approximation of where the ship will be at each consecutive hour.
When using Radar Plotting sheets or a Maneuvering Board, the compass is even more useful as
it allows you to swing an arc that becomes your required CPA. (See the page on these topics for more info, it's kind of an
in depth thing to explain.)
