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About Lewis and Clark,

On Sat, Sep 18, 2010 at 5:26 PM, Apache Runner 
wrote:

 "Their journal entries on dead reckoning are rather amazing - very 
meticulous."

Indeed, I agree. And their observations for magnetic variation, using their 
"circumferenter" (= surveyor's compass), when analysed later, were 
remarkably precise and consistent, giving variation of 7.3�E at the 
Mississippi-Ohio junction, then 7.4�E and 7.6�E further up-river, at 
Kaskaskia. Those values could have been useful to L&C in working out their 
dead reckoning, if they had been deduced by them en route. Unfortunately, 
they didn't bother to do so.

How they estimated the lengths of the river-reaches so well rather defeats 
me. Their clumsy vessels were propelled by paddles, by poling, or by 
towing, at various points, depending on the state of the river and its 
banks. The current, always adverse over the first part of the journey, 
varied along the river, and with the season. Those distances must have been 
estimated by some form of inspired guesswork.

However, dead reckoning, along such a tortuous path can only work over a 
limited distance, and has to be corrected, at intervals, by celestial 
observations. It was those that let them down.

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

Hewitt Schlereth, on the same day, wrote that he "wasn't holding Lewis & 
Clark up as paragon celestial navigators", but-

"As you say in your analysis they did well with the observations and then 
blundered on the paperwork - which is typical of everyone who has a go at 
celestial. The sextant is the fun part. It's with the almanacs, timepieces 
and trigonometry that the agony commences."

Hew is taking a somewhat rosy view of the capabilities of these American 
heroes. Before getting down to the precision of their readings, several 
layers of blunders have to be stripped away first. Hew might take a look at 
some details of my webpage at-
http://www.hux.me.uk/lewis02.htm
"Lewis and Clark on the Mississippi: commentary on their celestial 
navigation."

Section 1.11 deals with some of the things they misunderstood. Such as-

1.11.2 deals with their index mixup. Without a natural horizon, altitudes 
were all taken by reflection in a liquid surface, in which case the sextant 
reading has to be halved after subtracting index error. L&C, having no 
guidance, did it the other way round. Which gave rise to an error of 4 
minutes odd, in latitudes, when the sextant was used. However, in Summer 
(not the period I've covered) with Sun altitudes over 60�, when the octant 
in back-observation mode had to be used, that instrument was stated to have 
an enormous index error of over 2�! This put Summer latitudes in error by 
over 60'

1.11.3 Limb mixup. Almost invariably the Sun's upper limb was aligned with 
its reflection, but it was often noted, and corrected, as though it was the 
lower limb, (more or less at random, it seems).

1.11.4 Minute-hand mixup. The chronometer readings seem often to have been 
taken with a discrepancy of a minute, presumably due to a misreading of the 
minute-hand.

1.11.5 Declination mixup. The Sun's declination, given in the almanac for 
Greenwich noon of that day, should be interpolated to the actual time of 
the observation, but this tended to be neglected. And at least once, the 
value for the wrong day seems to have been used.

Those confusions relate to ordinary altitude observations, but things get 
much worse when we reach the attempts at a lunar, at Kaskaskia, on the 
night of 2-3 December, 1803. Try as I might, I am quite unable to unravel 
the observations to make sense of them. If anyone else can arrive at any 
understanding, I would be most pleased to hear about it. Details are all 
there on the website, if you follow the journey to that date.

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

Indeed, there's a lot of information available now, for those interested in 
following Lewis and Clark's great journey. Moulton's edition of their 
journal, 13 bulky volumes in the print version, is now freely available at 
http://lewisandclarkjournals.unl.edu/index.html . And now, you can follow 
their path via Google Earth, which wasn't available to me when I did that 
study of the Mississippi part of their journey, but would have made things 
much easier.

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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