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In the U.S. Dept. of Transportation archives I found a 1949 Civil
Aeronautics Administration report on a new aircraft sextant of the "ball
drop" type, developed by the U.S. Naval Observatory. The CAA tested it
against a conventional Pioneer bubble sextant, best known by its
military designation AN5851-1 or A-14.

The ball drop sextant performed poorly. "The only possible conclusion to
be reached from the flight tests is that the ball drop sextant fails by
a great deal of attaining the accuracy necessary for celestial
navigation in the air."

Unfortunately, the report does not go into its principle of operation
much beyond "the ball drop sextant uses the line of fall of a small
steel ball to establish a vertical line".

You will have to navigate to the page. My attempts to create a clickable
link have failed, probably due to the way Javascript is used at the
site. Also, if you have Internet Explorer's security settings turned up,
it will be necessary to add the site IE's list of trusted sites.

http://dotlibrary.specialcollection.net/

Click "FAA and CAA Research Reports"
Click "CAA TECHNICAL DEVELOPMENT REPORTS"
Then look for the ball drop sextant report (the browser Search function
will find it if you enter "sextant").
The page has an small portion of the report, and a link to download a
PDF scan of the whole thing, which is about 1.2 MB.

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