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    Re: The lost expedition of La Perouse
    From: Alexandre Eremenko
    Date: 2005 Jun 8, 15:10 -0500

    Do you know by chance when exactly and who
    made micrometer drum common?
    (I suspect that this could be Heath of London
    sometimes in 1930-s. Maybe you know more precisely?)
    In fact, micrometer drum was proposed in XVIII century
    possibly by Halley or even earlier. I can check when I
    come back to the US in August and will have access to my
    papers. I am curious why this invention had to wait until
    1930-s to become common.
    The reason is unlikely to be in some advanced wormscrew
    technology because a wormscrew was used in the arc
    division machines since XVIII century. So why did not they use
    it in sextants if the idea was known and technology available?
    On Wed, 8 Jun 2005, George Huxtable wrote:
    > What you would notice first is that there was no micrometer drum. Just a
    > fine-ajustment screw, but then you had to read the angle directly off the
    > engraved arc, with the help of a Vernier scale and a magnifying lens.
    > You would find a longer telescope with smaller mirrors and a more
    > restricted field of view. Somewhat heavier, being larger than a modern
    > instrument, and made of brass, not aluminium.
    > But otherwise, you could pick up an eighteenth-century sextant, and hardly
    > notice the difference between that and your own.
    > For further information, I recommend "Taking the Stars", by listmember
    > Peter Ifland, beautifully illustrated.
    > ================================================================
    > contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ================================================================

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