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On 27 Apr 2006 at 15:55, Fred Hebard wrote:

> On Apr 27, 2006, at 3:38 PM, Ken Muldrew wrote:
>
> > Note: I haven't corrected the data for temperature or pressure (his
> > altitude above sea level was about 3250 ft.).
> >

> Ken,
>
> Depending upon the temperature, pressure can add a fairly significant
> correction, especially at 3250 ft.  Here at 2000 ft one almost drops
> off the bottom of the Nautical Almanac correction chart.  Did
> Thompson record barometric pressure?  I would think he might have
> done this for height above sea level determinations.  If so,  I would
> expect he also would have recorded temperature.

There are a couple of cases in mountain passes where he records the
boiling point of water to measure his altitude (and my recollection is
that he substantially overestimated that altitude) but in general he does
not record barometric pressure. I haven't found a case where he adjusts
for temperature, but when he requested thermometers, his reason was for
correcting for refraction. Most of my recalculations have been for lunars,
which are too involved to be sure of a temperature allowance. When I find
the time I'll look at some meridian altitudes to see if he compensated for
temperature on a regular basis.

Ken Muldrew.

   

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