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Yesterday, I posted a message about Lewis and Clark's measurements of
magnetic variation, in the early part of their great journey, 200 years
ago.

I have had second thoughts about part of that message, which is repeated below-

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

>3 Dec 1803. still at the same Kaskaskais camp.
>
>Moulton records-
>
>"Sun's magnetic Azamuth by Circumpherenter- at 9h9m59sA.M.S.43deg45'East".
>
>Decoded, this should read "at 9h 9m 59s am [the Sun's azimuth was] South
>43deg 45'East", which works out as an observed azimuth (measured clockwise
>from North) of 136deg 15'.
>
>From an equal-altitude observation bracketing noon on that day, we can
>deduce that the Local Mean Time at the moment of the Sun compass
>observation was 9h 24m 45.1s am. From that, the Sun azimuth was 141.9deg.
>So the magnetic variation from this observation is 5.6deg W., or less by
>1.3 deg than the measurement of the previous day of 6.9deg W. Perhaps this
>indicates the limit of accuracy of reading that compass, even on land.
>
>As usual, however, L&C did not deduce a variation from their observation.

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

I was calculating out that observation at the same time as I was putting
the message together, and got myself confused between local mean time and
local apparent time. No excuses are offered...

Here is an amended version of that offending passage-

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

3 Dec 1803. still at the same Kaskaskais camp.

Moulton records-

"Sun's magnetic Azamuth by Circumpherenter- at 9h9m59sA.M.S.43deg45'East".

Decoded, this should read "at 9h 9m 59s am [the Sun's azimuth was] South
43deg 45'East", which works out as an observed azimuth (measured clockwise
from North) of 136deg 15'.

From an equal-altitude observation bracketing noon on that day, we can
deduce that the Local Apparent Time at the moment of the Sun compass
observation was 9h 34m 50s am. From that, the Sun azimuth was 143.7deg.
So the magnetic variation from this observation is 7.4deg W., half a degree
greater than the figure of 6.9deg W, obtained the day before. Perhaps this
indicates the limit of accuracy of reading that compass, even on land.

As usual, however, L&C did not deduce a variation from their observation.

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

To review their magnetic variations, then, the following values were obtained-

Near Cairo, 7.2deg West.

At Kaskasia, 6.9deg West, and 7.4deg West.

To me these seem to be very reasonable reasults, showing no more scatter
than one would expect from any compass measurements, and showing little change
in the magnetic variation over that small fraction of their great journey. I
do not have any comparable magnetic-survey figures for that part of the
American continent at that date. Does anyone else?

Sorry about that mistake, though.

George.



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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
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