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    Re: Intended High Resolution Chronometer
    From: Brad Morris
    Date: 2014 Feb 26, 15:37 -0500

    Hi Frank

    Yup, it was a fun little deal

    You wrote
    rotating the micrometer on a sextant which in turn rotates the instrument's index arm. To me, it's intriguing that this geared micrometer concept was not applied to sextants decades earlier.

    In two words "gear lash".  Gears that fit together so precisely that there is no lost motion are termed master gears.  The minute you use them, friction creates wear, which creates gear lash.  Unlike a vernier sextant, the micrometer sextant will read differently for the same altitude, dependent on the direction you turn the drum in.  That's gear lash or lost motion.  A vernier sextant does not depend upon a mechanical gear train so there is no equivalent lost motion.

    Inherently, vernier sextants are superior to micrometer sextants in this characteristic.  The altitude indicated will be the same, independent of direction. 

    Its my theory that 1800's navigators understood that the mechanism had this weakness (in addition to the inability to cut precision gears in mass manufacture) and resisted what was viewed as a lesser instrument

    Brad

    On Feb 26, 2014 3:27 PM, "Frank Reed" <FrankReed{at}historicalatlas.com> wrote:

    Hi Brad, you wrote:
    "The photo was meant to be good hearted fun."

    And that's why I started my previous post with this sentence:
    "Thanks for the photo, Brad. Here's a little fun with it..."

    So thanks, again. It was fun. :) Your intentions were not mis-understood.

    To elaborate a little more on my last point. I had written, "Turning back to sextants and their ability to read fine angles, that, after all, is ALSO exactly what happened to sextants in the early twentieth century! A micrometer is a "minute hand" geared to the main motion of the index arm. It took over a hundred years for that simple concept to be applied to the sextant itself."

    It doesn't look like a minute hand on a clock, but that's literally what the micrometer is. The next time, you push the minute hand on a clock forward, and the hour hand rotates by some small but perceptible angle, you can compare it to rotating the micrometer on a sextant which in turn rotates the instrument's index arm. To me, it's intriguing that this geared micrometer concept was not applied to sextants decades earlier.

    -FER

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