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On 21 March, under this threadname, I described Lewis and Clarks
measurements of variation of the compass, as they travelled up the
Mississippee, from (modern) Cairo to the Missouri junction, just above St.
Louis. That posting corrected some earlier errors.

Hans Heynau has pointed out yet another error, sorry to admit. It relates
to the Polaris azimuth observation of 2 Dec 1803.

I said (in part)-

>2 Dec 1803.
>
>Vol 2 of the Moulton edition of the Lewis & Clark journals (from which all
>this >information was extracted) records-
>
>"...3 miles W. of Kasskassais made the following observations-
>
>By circumpherenter- Azamuth of pole Star 7deg 47' 00" at 8h 11m 45s p.m.
>pr chronometer."
>
>There's a description of the "circumpherenter" on page 413 of Moulton vol
>2; it appears >to be a 6-inch diameter compass (presumably equipped with
>sighting vanes) and adjusted >using a spirit level. It would need such a
>vaned sighting instrument to observe the >azimuth of Polaris, which would
>have an altitude of about 38deg.
>
>How the travellers could claim to measure a compass azimuth to that
>precision is beyond >me. Discounting the 00" part of it" how did they even
>measure the 47'? Also, that entry >gives no clue as to which sector of the
>compass it's to be found in.
>
>Hans Heynau has suggested that this figure may be the result of a
>mistranscription from >Lewis's original manuscript entry, and without
>seeing that manuscript (in his "Eastern >Journal"), I tend to agree. The
>editor would be accustomed to transcribing sextant >altitudes, given in
>minutes and seconds, and coming across this entry for azimuth, which
>>would be in degrees and minutes, and no seconds, but with a compass
>sector given with a >cardinal point (in this case presumably "W"), might
>have been tempted to trancribe a >badly -written "W" as 00". However,
>wihout a sight of the Eastern Journal manuscript, >this can be no more
>than intelligent supposition.
>
>For the time of observation, we can readily compute the azimuth of Polaris
>to be 0deg 56' >(clockwise from North, by modern convention), which
>results in a magnetic variation of >8.7deg East. Again, L&C make no
>attempt to deduce a figure for the variation, though they >could easily
>have made at least a rough guess by presuming Polaris to be due North. We
>>have to remember, though, that Polaris in those days was much farther
>from the true Pole than it is now, 1deg 40' rather than about 0deg 40' as
>at present.
>
>================

It's the last paragraph that's wrong, in which I said "...we can readily
compute the azimuth of Polaris to be 0deg 56' ..." In fact, that figure
should have been 359deg 48', which results in a revised magnetic variation
of 7.6deg East, not 8.7deg East.

To review their magnetic variations, then, over the Mississippi part of
their journey, the following values were obtained, which now show a very
satisfactory reduction in the scatter-

Near Cairo, 7.2deg East.

At Kaskasia, 7.6deg East, and 7.4deg East.

We can take the magnetic variation in that part of the journey, then, to be
7.4 degrees East, roughly speaking. We can apply that variation to all
their magnetic courses, recorded over the Mississippi part of the journey.

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

As one who has often been critical of the carelessness of Lewis & Clark,
and their  tendency to commit errors, I should be setting a better example.
Sorry about that, and thanks again to Hans Heynau.

George.




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