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Re: Sextants in Little Rock
From: Henry Halboth
Date: 2010 Jan 9, 07:12 -0800

Bill is certainly correct in his comment. It is a worm gear that engages the rack on the sextant arc and I debated with myself as to how it would be best described. To me, at least, a "worm and rack gear" just did not smell right, so I decided to call the "worm" a "pinion". No offence meant or taken - just hope the worms don't mind.

While on the subject, I would ask that Bill give his opinion as to the correct terminology for the friction arrangement utilized in moving the index arm where the old style "clamp" is used to secure the arm - a matter over which I also agonized. I do not believe that I have ever heard this technically described.

As I have rebuilt many an old sextant and am no slouch at using a Watchmakers' Lathe, I have very much appreciated Bill's postings and general comments.

Regards,

Henry

--- On Sat, 1/9/10, engineer@clear.net.nz <engineer@clear.net.nz> wrote:

From: engineer@clear.net.nz <engineer@clear.net.nz>
Subject: [NavList] Re: Sextants in Little Rock
To: NavList@fer3.com
Date: Saturday, January 9, 2010, 2:15 AM

I very much enjoyed reading the old salt's story.

I hope Henry won't mind my correcting his terminology for the micrometer sextants. They do use a rack, a term inherited from watch and clock makers and meaning a toothed segment of a circle, but the only real sextant that has a pinion,a gear wheel with a small number of teeth, usually fewer than 12) is the box sextant (see attachment for an interior view). Nearly all nautical micrometer sextants use a short length of a helical thread, a worm, to engage with the rack. The one exception that I know of is the Freiberger Skalensextant, that has an internal glass scale divided to degrees, viewed through a microscope having a micrometer eyepiece, which allows reading directly to one minute and estimation to 30 seconds.

Bill Morris
Pukenui
New Zealand
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