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    Re: Sextant design. [was: Another Davis...}
    From: Rodney Myrvaagnes
    Date: 2003 Mar 3, 16:01 -0500

    You replied to my posting directly to me. If you intended it to go to
    the list, you must resend it.
    Some listservers pass through the "reply to" field without offering you
    the choice of the "from"or the "reply to" address when you start a
    reply. I usually hit "reply all" and delete whichever "To:" address I
    don't want to avoid this problem.
    I send this canned message whenever I receive an off-list reply to a
    list post, because I can't tell the intent. I don't want to forward
    them to the list myself because the sender might have meant it to be
    In this case I think you clearly meant it to go to the group, so I will
    take the liberty of sending it with reply.
    On Mon, 3 Mar 2003 19:06:59 +0000, George Huxtable wrote:
    >This thread having diverged significantly from its original subject, I have
    >taken the liberty of naming a new branch (if a thread can have a branch: a
    >fibre, perhaps?).
    >We have rehearsed these matters before, and no-doubt will again. Perhaps
    >it's a good time for another airing, as there may now be many new members
    >who haven't heard it all before, and who have ideas of their own.
    >>On Mon, 3 Mar 2003 09:48:22 +1100, Peter Fogg wrote:
    >>>Not having personal experience of plastic sextants, having early been warned
    >>>away from them, but reading with interest these postings; my feeling is that
    >>>they have been effectively damned with faint praise ...
    >And Rodney Myrvaagnes replied-
    >>You may choose to read it that way, but wrongly? A Davis, used with
    >>care for index error between sights, will get you on soundings wherever
    >>you want to go. I have a nice but heavy Kelvin-Hughes so I have no need
    >>of the Davis, but if I had no sextant I would buy a Davis if budget
    >>mattered at all.
    >Rodney hit the nail on the head when he added-
    >>If you are navigating from a boat less than 50 feet long, the motion of
    >>the boat may well outweigh any difference in sextants.
    >This, I think, is the essence of the argument, especially for those like me
    >with a 26-footer. A precise metal sextant is good for taking lunars, if
    >there's a sufficiently steady platform underfoot. Indeed, that's the job
    >such sextants were originally designed for. But for simple
    >altitude-navigation, mariners would have been satisfied with their wooden
    >octants, and a plastic sextant is the modern equivalent. Good enough to do
    >the job, but no more than that. But with some significant advantages to
    >take into account: cheap, light, and robust. You can drop it, and it just
    >bounces.  So for small boat owners, I think of a plastic sextant as the
    >appropriate instrument.
    >Once, in roughish weather, I lost my balance, dropped my first Ebbco, and
    >landed on it with my seaboot. That was too much, and it succumbed.
    >Rodney added-
    >>What I would not buy (again) is the $50 EBBCO I learned on. That was
    >>exceedingly unpleasant (i.e. horrible) to use. I trust George's is a
    >>better model.
    >Ebbco sextants have evolved somewhat over the years, but still have some
    >weaknesses. I would accept that the telescope admits some distracting
    >extraneous light, in certain circumstances, and the handle, if gripped
    >tightly, distorts the frame enough to shift the Sun image significantly
    >sideways (but not up-and-down). Both could be greatly improved by a bit of
    >very simple redesign. The shades, on early instruments, were terrible. It
    >would be interesting to know just what Rodney disliked so much.
    It was the sunlight coming in around the telescope that bothered me
    most. I bought it in 1979, so I expect they have done some work on it
    >Last year, I found Ebbco sextants on sale in France under the Plastimo label.
    >This brings me to a couple more general questions about sextants.
    >1. Why does a split-horixon mirror have a piece of plain glass alongside
    >the mirrored part, for the horizon to be seen through? Why not just chop
    >that part off? What good does that plain glass do? I've asked that question
    >before, and ask it again in the hope of getting further answers.
    >2. About the layout of the mirror adjusting screws (for each mirror). Why
    >are these arranged at the corners of a delta configuration (if it's true
    >that they generally are)?
    >Whay not, instead, at the three vertices of a letter L, arranged so that
    >the arms of the L are parallel to the edges of the mirror. These would
    >allow adjustment, by the screws at the ends of the arms of the L, of the
    >mirrors, side-to-side and up-and-down, independently, by each screw. The
    >third screw could be dispensed with and replaced by a fixed pip. Perhaps
    >some sextants are indeed designed that way.
    Rodney Myrvaagnes       NYC                                                 J36 Gjo/a
    "Wanting to meet a writer because you like his work is like wanting to meet a 
    duck because you like pate."
    Margaret Atwood

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