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On 25 Apr 2006 at 8:38, Alexandre E Eremenko wrote:

> But let me cite William Wales, Cooke's companion:
>
> "It must be owned there is yet something
> in the constitution of this Quadrant very disagreable,
> and not easily to be accounted for. Sometimes, many months
> together, the longitudes deduced from observations made about the same time
> with my two sextants would not differ more than 10 or 15 miles, and very
> seldom so much; after which the longitudes, so deduced, would begin to
> differ, and the difference would gradually increase, sometimes more than a
> degree and an half: In little time it would again decrease, and soon after
> the observations would agree as well as ever. It will be readily supposed,
> that no means were left untried by me to discover the ause of this strange
> aberration; but all my endeavours were ineffectual; and I mention the
> circumstance to induce some person, more skilful in mechanics, to attempt
> it".

Does he say what kinds of sextants (quadrants) he is using? If one was
made of wood and the other metal, then changes in humidity would change
the dimensions of the wooden instrument. Alternatively, two sextants made
from different metals would expand and contract differently with
temperature. These would be small changes, but they might be enough to
show up when making arc measurements to a part in 10,000 or better.

Ken Muldrew.

   

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