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Nigel Gardner wrote, referring to the back sight which was fitted to
certain versions of Hadley's quadrant

>Use of back Sight
>
>In the 1805 edition of Norie's Epitome he says,
>
>[The back sight is used] '... when the observer's back is towards the
>object, and it is brought over to the opposite part of the Horizon, and is
>thence called a back observation. This latter method of observing is very
>seldom used and is required only when the Horizon under the object is
>broken by adjacent shores, or rendered indistinct by fogs or other
>impediments'
>I recollect reading somewhere that it could also be used say for Polaris
>when the Northern horizon was no longer visible but the Southern Horizon
>was still illuminated.
>
>Norie gives four different methods for finding the Lunar Distance but it
>requires the tabulated distances from the Almanac to find Longitude, which
>last included in 1907. The 1967 Nautical Almanac included them to mark its
>bicentenery.
>
>Nigel Gardner

My comment-

By 1805 the sextant was in common use, which could measure directly angles
up to 120 degrees. Lunars frequently required measurement of such large
angles in the sky. The predecessor of the sextant was Hadley's quadrant
(octant was effectively the same thing with another name) and could only
measure up to 90 degrees, so the backward-sight components were intended to
extend its angular range. It's not surprising that by 1805 the backward
view was considered by Norie to be obsolete.

It's interesting to note the instruments that Cook took with him on his
second voyage (1772 to 1775), as described in "The charts and coastal views
of Captain Cook's voyages", ed. Andrew David, Joppien, and Smith, Hakluyt
Society 1992, on page xx (et seq) of the introduction. This was the first
time that Cook had a chronometer with him (the Kendall copy of Harrison's
instrument) and a major object of the voyage was to test the value of
Kendall's "watch". To this end he was equipped with the very best and
latest astro-navigational instruments. These included two 15-inch brass
sextants, one by Ramsden, one by Dollond.

As they were sextants, these could measure up to 120 degrees, but in
addition, the Dollond sextant had an extra sight and horizon mirror facing
in the reverse direction, allowing angles up to 180 degrees to be
measured.. This second mirror could readily be rotated through exactly 90
degrees about a horizontal axis, so that it could be used with a forward or
reverse view with respect to the horizon. The illustration and description
of Dollonds device in the Hakluyt volume are taken from Dollond,
"Directions for using the new improved Hadley's quadrant", 1772, in the
National Maritime Museum, Greenwich.

The Dollond instrument was used by the astronomer on board, Wales, to
observe Sun-Moon angles when they were bigger than 120 degrees. This
allowed him to use Sun-Moon angles over six extra days of each month, and
Wales' largest angle measured was 155 degrees. Lunar distances between Sun
and Moon were omitted from the Almanac if they were over 120 degrees, so in
those cases he had to work out the predicted Lunar distance for himself,
from the individual predictions for the Sun and Moon; quite a feat.

Wales, being a professional astronomer, could manage such calculations,
which would leave a normal ship's officer quite baffled.

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

On that topic, I feel impelled to retell a story fromy J.C.Beaglehole in
his monumental 3-volume work, "The journals of Captain James Cook"

In 1768, the Admiralty had decided to send out a ship to observe the
Transit of Venus in the Pacific under Cook (this was to be his first
voyage) but hadn't decided exactly where he should land to do it. Just at
the right moment, the "Dolphin" returned from her second circumnavigation,
under Wallis. They had found just the right island for the purpose,
shelter, water, food, and the inhabitants were friendly (embarrassingly
so...) It had been named King George's Island; we know it now as Tahiti.
What's more, they were able to state its position, in both latitude and
longitude, so that Cook would be able to find it again.

Knowing the latitude was easy, but how did they manage to be so sure about
the longitude? It came from the purser, a man named John Harrison, by
coincidence a namesake of the inventor of the famous timekeeper.

We know almost nothing about this Harrison. A purser's job is to look after
the ship's cash and accounts and buy the provisions. This was no ordinary
purser. Perhaps he was a mathenmatician, or an expert navigator out of a
job, who had signed on as purser for what seemed likely to be an
interesting voyage. Sometimes, says Beaglehole, a supernumary such as an
astronomer might be borne on the books as a purser.

Before leaving England, Harrison had somehow managed to arm himself with
Maskelyne's predictions of lunar distances, before they were officially
printed. This piece of foresight was to prove most useful.

Wallis, in his log, was able to state a position which was spot on in
latitude (no surprise there) and only 30 minutes out in longitude. He
notes, of Harrison, "... thro whose means we took the Longitude by taking
the Distance of the Sun from the Moon and Working it according to Dr
Masculines Method which we did not understand".

Was this last phrase a generous attribution of the result to his purser, or
was he perhaps distancing himself from it in case it turned out to be
wrong?

Anyway, Matavai Bay in Tahiti was chosen by the Admiralty, and Harrison's
position turned out to be triumphantly correct, Cook being able to go
straight to the spot. Without that clever purser, maritime history might
have been very different. Although history has awarded Harrison a footnote
in Beaglehole, otherwise his feat seems to have gone quite unrecognised.

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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