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Aubrey O'Callaghan writes-

>I find the comment about using lunars interesting.
>On one of Cooks voyages he took a copy of Harrison's chronometer with him
>(was this the same voyage as they are reenacting on the BBC ?).
>In the book "From Satellites to Sails" by J.E.D. Williams, Oxford
>University Press 1994. Page 102 with note reference to  page 106, referring
>to Harrison's chronometer no. 4
>"...There was not the remotest possibility that a copy of No. 4 could be
>put aboard every ship. The Board had to pay the enormous sum of 450 pounds
>to Larcum Kendall to make an exact copy (ref. 21) of No. 4."
>
>Ref. 21: This was the instrument that gave Captain James Cook such
>excellent results. On the conclusion of his second voyage of exploration,
>Cook wrote in the log that on making land about Plymouth on 29 July 1775,
>the error in longitude by Kendal's watch was only 7' 45". how Cook sights
>with such accuracy he does not say; but there is the entry in his own hand.
>While commanding Endeavour five years earlier Cook had remarked 'an
>observation (of longitude) to within 30' of accuracy sufficient for all
>nautical purposes'.
>
>I note the slight difference in the spelling of Kendall in the text and the
>reference - only 1 "L" in the reference.
>
>My copy of "The True Story of a Lone Genius Who Solved the Greatest
>Scientific Problem of His Time -- Dava Sobel;"  is in London (I'm in
>Venezuela), I am sure that some of our readers have this book; it would be
>interesting to cross check and see if  there is any reference to Captain
>Cook in this book.
>
>Aubrey.

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

Here are some comments in reply.

Cook did not have a chronometer for his first voyage. For longitudes, he
had to rely on astronomical measurements: lunar distance and (from land)
timings of Jupiter's satellites.

It is a leg of that first voyage, Northern Queensland to Jakarta
(=Batavia), that the BBC are planning, in some aspects, to replicate.

On later voyages, when he did carry chronometers, the astronomical methods
of determining longitude remained equally important. One of the aims of the
voyages was to confirm the usefulness of the chronometer to a mariner, and
to this end Cook made astronomical cross-checks to establish the "going" of
his chronometer whenever possible. Subsequently, time was not taken at
face-value from the chronometer but adjustments were made to observations
to allow for any time errors that were deduced. This was important when a
vessel was away from locations with known longitudes for years at a time.
So the often-expressed view that from the second voyage on, Cook became
completely reliant on his chronometer for longitude, is to some extent
mistaken.

Toward the end of the third and final voyage, failure or interruption of
all the chronometers meant that navigation had to revert entirely to the
earlier methods for finding longitude.

To avoid confusion for book-hunters, I should point out that the excellent
and wide-ranging book by JED Williams, referred to by Aubrey, is "From
Sails to Satellites", NOT satellites-to-sails.

Aubrey's quote (via Williams) about Cook's reference to a watch error of 7
minutes 45 seconds in longitude on return from his second voyage is
correct. However, Cook subsequently revised those pages somewhat, and the
final version as printed became-

"Error of the watch on our arrival in Portsmouth, 16 minutes 26 1/2 sec"

This was the error in long., corresponding to just over one minute of time.

For details see J C Beaglehole (ed.), the Journals of Captain James Cook,
vol 2, 1961, pages 678 - 681.

However, the above creditable figures for the performance of the
chronometer (Kendall's watch) should be taken with a large pinch of salt.
By the time Resolution had reached the south seas, daily gains of the watch
of 6 to 9 seconds of time a day were being measured. When Tahiti was
reached, an error in longitudes-by-the-watch had built up to 2 degrees 38
minutes of arc, a watch-error of over 10 minutes of time. The watch was
found to lose again in cold climates.

In Tonga, by some carelessness, the watch was allowed to run down and had
to be reset by using Sun apparent time.

In the year from leaving Ship Cove in New Zealand to return to the same
spot, the watch had accumulated an error of nearly 20 minutes of time, and
was then gaining at nearly 12 seconds a day.

The above information can all be found in Beaglehole.

If, then, the overall error at the end of the second voyage turned out to
be no more than a minute of time, this was more by good luck than anything
else. It's clear that if Cook had not had lunars available to back up his
chronometer, his second voyage would have been in deep trouble.

I do not wish to belittle the value of a chronometer. It enabled Cook to
establish Greenwich time wherever he was (and therefore his longitude), by
interpolating with the watch between his lunar observations.

As for the Sobel bookling on "Longitude", entertaining though it might be,
I think it should be read more as a romance than as a record of fact.

George Huxtable.

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george@huxtable.u-net.com
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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