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George B asked-

"I was browsing through the 1797 logbook of a local ship in our library
today. This was my first attempt at doing this, and although I understood
much of what I found I have a couple of questions.

On the left side of the page are three columns with hourly entries for
H(our), K(nots), and F(???). Is the last entry fathoms? The numbers in this
column ranged from 2 to 7, or were blank. "

===============

Yes, these were fractions of the distance between the knots, and
traditionally labelled F for "fathoms", a fathom being six feet. In which
case these should have been eighths of a knot, which would have been 48
feet. However, it was often considered more convenient, for working with a
traverse table, if knots were actually subdivided into tenths, rather than
eighths, although these would still labelled with "F". The spacing between
knots settled down, over time, at around 50 feet, though would vary, as did
the timing of the glass, some being 28 seconds, some 30 seconds, and these
were not always consistently matched. If knots were being split into
tenths, the number in that "F" column would then run from 0 to 9 rather
than 0 to 7, and the estimated subdivisions would have been 5 feet rather
than 6 feet. You just have to look at the way the numbers ran: in the case
that George B refers to these would have been in eighths of a knot of about
6 feet. Things were not always consistent, even on the same ship.

Parts of a knot, noted as fathoms, should just be thought of as "armfuls"
of line, very roughly estimated. Often, if it wasn't much over a whole
number of knots, it was taken to be that whole number, in which case an
entry of 1 in the F column would be rare. Similarly, if the reading was
estimated as just short of the next whole knot, entries of 7 (if in
eighths) or 9 (if in tenths) would be omitted, and the next-higher whole
knot would be noted instead. It was by no means an exact science, this
interpolation between whole knots.

=================

 "On the right side of the page is a column for what I think is magnetic
variation, determined either by "Azimuth at 6 AM (or PM)", or by "Amplitude
at Sun (circle with dot in center) Set". If there are three entries for a
day they are in the order Azim at 6 PM, Amp at Sun Set, Azim at 6 AM. I can
see how you could measure compass deviation by sighting along the compass
and measuring the azimuthal angle to the setting sun, but I don't see how
that matches up with these entries."

================
The bearing compass would have a pair of vanes with slots in to allow the
magnetic bearing of the Sun to be established, if it wasn't too high in the
sky. In the days when ships carried a timekeeper of some sort, the true
bearing of a low Sun could be calculated at a moment in local time, such as
(in Summer) at 6 am or 6 pm, knowing the latitude.

But this could also be done with no knowledge of the time,  by measuring
the azimuth of Sunrise or Sunset, whenever that happened to be, and
comparing it with an amplitude, taken from a table in an almanac. The
amplitude was the difference between that azimuth and due East or West, and
the moment of measurement was not taken at actual sunrise / sunset, but
when the whole Sun was clear of  the horizon be a few arcminutes, which
allowed appropriately for dip (depending on height-of-eye at the poop) and
refraction, at the moment when the centre of the true Sun would be truly
horizontal. That offset was usually just estimated by eye in terms of an
appropriate fraction of the Sun's disc, floating clear above the horizon, a
fraction with which any navigator would become familiar. The difficulty
with measurement by amplitude was the great uncertainty in refraction, when
near the horizon, depending on atmospheric conditions.

George

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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