zon part
of the horizon glass. Now, move the sliding limb along the arc gradually
until you see the other lighthouse in the reflected horizon of the
horizon glass. When one lighthouse in the true horizon is directly on
top of the other lighthouse in the reflected horizon, clamp the sliding
limb. If any additional adjustment must be made, make it with the
tangent screw No. 9.

Now look through the reading glass No. 8. You should see that the arc is
divided into degrees and sixths of degrees in the following manner:

11 deg. 10 deg.
| |
-------------+-----------------|
| |
----|--+--+--+--+--+--|--+--+--+
| | | | | | | | | |
----+--+--+--+--+--+--+--+--+--+

Now, as every degree is divided into sixty minutes, one-sixth of a
degree is 10 minutes. In other words, each of the divisions of a degree
on this arc represents 10 minutes.

Now on the vernier in the sliding limb, directly under the arc, is the
same kind of a division. But these divisions on the vernier represent
minutes and sixths of a minute, or 10 seconds.

To read the angle, the zero point on the vernier is used as a starting
point. If it exactly coincides with one of the lines on the scale of the
arc, that line gives the measurement of the angle. In the following
illustration the angle is 10-1/2 degrees or 10 deg. 30':

10 deg. 9 deg.
----------------+-----------------------+
| |
---------|---+---+---|---+---+---|---+---+---|
| | | | | | | | | |
-+---+---+---+---+---+---+---+---+---+---+---+
| | ^
----+---+---+
0

If however, you find the zero on the vernier has passed a line of the
arc, your angle is more than 10 deg. 30' as in this:

11 deg. 10 deg.
------------+-----------------------+
| |
------------|---+---+---|---+---+---|
| | | | | | |
------------+---+---+---+---+---+---+
| | | ^
------------+---+---+---+
0

You must then look along the vernier to the left until you find the
point where the lines do coincide. Then add the number of minutes and
sixths of a minute shown on the vernier between zero and the point where
the lines coincide to the number of d