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    Re: Margetts' 1790 Longitude Tables
    From: Frank Reed
    Date: 2018 Jan 26, 10:55 -0800

    Ed, here's a pdf copy:

    Seemingly a great idea, these graphical tables had few fans. I speculate that there are several reasons for this:

    • Prohibitive expense. Cutting plates to print graphics like this was very expensive in that period, and since there was no other financial support, the tables had to pay their own way, so George Margetts put them on the market at a high price.
    • Relative ease of standard paper methods developed around this time. Using the streamlined version of Lyons' method which evolved into the methods of Mendoza Rios and Bowditch in one direction and the methods of Turner and Thompson (also copied by Bowditch) in another direction, it really wasn't much work to clear lunars without these exotic, expensive graphics.
    • Mediocre accuracy. There's some inherent reading inaccuracy when looking up values in these graphical tables, and also there may be some reduced quality in the calculations that produced them (I haven't looked into this in enough detail to say so firmly).

    Frank Reed
    Clockwork Mapping / ReedNavigation.com
    Conanicut Island USA

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