egrees and minutes shown on the arc
at the line which the vernier zero has passed, and the sum will be the
angle measured by the instrument.

Now in measuring the altitude of the sun or other celestial body,
exactly the same process is gone through except that the sextant is held
vertically instead of horizontally. You look through the telescope
toward that part of the sea directly beneath the celestial body to be
observed. You then move the sliding limb until the image of the
celestial body appears in the horizon glass, and is made to "kiss" the
horizon, i.e., its lowest point just touching the horizon. The sliding
limb is then screwed down and the angle read. More about this will be
mentioned when we come to Celestial Navigation.

Every sextant is liable to be in error. To detect this error there are
four adjustments to be made. These adjustments do not need to be learned
by heart, but I will mention them:

1. The mirror must be perpendicular to the plane of the arc. To prove
whether it is or not, set the vernier on about 60 deg., and look slantingly
through the mirror. If the true and reflected images of the arc
coincide, no adjustment is necessary. If not, the glass must be
straightened by turning the screws at the back.

2. The horizon glass must be perpendicular to the plane of the arc. Set
the vernier on zero and look slantingly through the horizon glass. If
the true and reflected horizons show one unbroken line, no adjustment is
necessary. If not, turn the screw at the back until they do.

3. Horizon glass and mirror must be parallel. Set the vernier on zero.
Hold the instrument vertically and look through the line of sight and
horizon glass. If the true and reflected horizons coincide, no
adjustment is necessary. If they do not, adjust the horizon glass.

4. The line of sight (telescope) must be parallel to the plane of the
arc. This adjustment is verified by observing two stars in a certain way
and then performing other operations that are described in Bowditch,
Art. 247.

Do not try to adjust your sextant yourself. Have it adjusted by an
expert on shore. Then, if there is any error, allow for it. An error
after adjustment is called the Index Error.

Put in your Note-Book:

How to find and apply the IE (Index Error):

Set the sliding limb at zero on the arc, hold the instrument
perpendicularly and look at the horizon. Move the sliding limb forward
or backward slowly until the true horizon and refle