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On Wed, Aug 13, 2014 at 12:36 PM, Lu Abel  wrote:
> Without being an expert in technological instruments, I rather suspect that
> something as simple as crude protractors and/or parallel rules existed in
> the 16th century.   So why all these "bursts" of rhumb/compass direction
> lines when it would have been fairly simple to measure a course line with a
> protractor as navigators do today, or walk a line to a compass rose - again
> as navigators do today?


I have looked through several early 19th century editions of Bowditch,
Norie, Moore, and MacKay, and have found no mention of the use of
either protractors or parallel rulers.  What I have found are repeated
references to compasses, meaning the pivoted divider type of
mathematical instrument.  Very confusing because compass is also used
in these texts to mean the compass rose on maps, and of course the
mariner's magnetic compass.

Typical is this description from Bowditch 1807 under
Mercator'sSailing, which uses two of these definitions in one
sentence:

To find the bearing of any place from the ship:
Lay a ruler across the given place and place of the ship [on the
Mercator chart]; set one foot of the compasses in the centre of some
compass near the ruler, and take the nearest distance to the edge of
the ruler;  slide one foot of the compasses along that edge keeping
the other extended to the greatest distance from the ruler, and
observe what point of the compass it comes nearest to, for that will
be the bearing required.

Don Seltzer

   

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