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The 1955 USAF air navigation manual explains how to operate the A-14
(AN 5851-1) bubble sextant averager:

1. Set the counter to zero by turning the counter knob. If the counter
is not zeroed, the observation will be worthless.

2. Wind the averaging device until you reach a solid stop. This not
only resets the averaging mechanism, but also removes the shutter from
the field.

3. Push lever 2 and rotate the scale drum to a stop. This operation
engages the sextant scale drum with the averaging device, and the stop
indicates that the averager has been brought down to the base line.

4. Push lever 1 to engage lever 2. This operation disengages the
averaging device from the sextant.

5. If precomputation techniques are being used, set the Hp in the
sextant.

6. Take a preliminary sight. That is, collimate the body. Then push
lever 2.

7. Again, rotate the drum down to a stop, and if the amount of
rotation is more than 2 degrees as indicated on the micrometer drum,
rotate the drum up to the sighted angle and collimate the body.

8. If the amount of rotation down to the stop is less than 2 degrees,
disengage the averaging device by pushing lever 1, rotate the drum
down approximately one full turn, push lever 2, and continue rotating
the drum down to a stop. Then proceed with the sight.

There is always a 15 degree speread between stops, which allows at
least a margin of 2 degrees on either side of the altitude of the
body. This allows sufficient spread of altitude during observation.

9. Push lever 3 to start the averaging mechanism.

10. Maintain coincidence between the bubble and the celestial body. At
the end of  two minutes, a shutter automatically moves into the field
obscuring your vision.

11. To obtain the average time, add one minute to the starting time of
the observation, or subtract one minute from the time you finish your
observation.

12. To obtain the final average altitude, combine the counter reading
with the main scale reading. (Do not read the scale drum; this
indicates the altitude of the 60th shot only.) For example, if the
counter reads 7 degrees 44 minutes, and the main scale pointer is
between 2 and 3, the final reading is 27 degrees 44 minutes. Remember,
the final main scale reading is within 2 degrees of the average
reading.


CHECKING THE SEXTANT

Like the A-10-A, the A-14 sextant should also be checked for index and
collimation error.

When checking for collimation and index error, you should use a
collimator, if it is available, or prepare an Hc Curve. These are
discussed later in this section.

With the A-14 sextant, you should also check the averaging device and
timing mechanism. This may be done as follows:

1. Prepare the sextant as though you were going to make an
observation.

2. Set the scale drum on zero.

3. Push lever three. At the same time, start a stop watch.

4. When the timing mechanism stops, stop the watch. It should have run
two minutes, and the reading on the counter dial should be 0 degrees 0
minutes. If either the timing or the counter are off an appreciable
amount, turn the sextant in.

   

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