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Thanks to Alex for countering my speculation, that Simms' book might have
been referring to a different sort of dip. Clearly, it wasn't.

He sais-

>I site the history of the dipmeter from Simms book:
>
>"When the late professor Vince was engaged in making
>observations upon extraordinary refraction in Ramsgate,
>Mr Throughton contrived and constructed for his
>use an instrument which he
>called a Refraction-Sector.
>
>About five years afterwards, when preparations were making for
>the first of the late North Polar Expeditions,
>Mr Troughton was applied to by the late Dr Wollaston,
>to make him an instrument on the principle of back observation
>with the quadrant, to send with the expedition, to measure
>the dip of the horizon; but upon Mr Troughton's producing his
>Refraction-Sector, which was as well adapted to Dr. Wollaston's
>purpose as that for which it was devised,
>the Doctor immediately ordered
>one to be made for him, and named it a Dip-Sector.....

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

That's interesting. Barrow, at the Admiralty, in the mid-19th century, was
sending expeditions (especially naval ones) off around the World,
particularly to Polar areas.

Wollaston was a physicist specialising in optics. I remember the name, but
not the purpose, unfortunately (birefraction?) of the "Wollaston prism",
from my schooldays.
Barrow put the provision of instruments into Wollaston's hands.

As a result there are many natural geographic features, particularly Arctic
/ Antarctic ones, named after Wollaston. Perhaps the explorers were
particularly grateful for well-chosen instruments. More likely, on such
voyages of discovery, they simply ran out of names, after using up their
wives' names, and the Admiralty Commisioners' names, and perhaps a sponsor
or two, Yes, even then, sponsorship paid off: for example, Boothia
peninsula, in the Arctic, was closely related to Booth's Dry Gin. But I
digress.

As I understand it, temperature gradients above the horizon can be
particularly strong and stable in the Arctic / Antarctic. I have read an
account of a sledge expedition, trudging over what turned out to be a
perfectly flat plain, in which it appeared that they were forever in the
bottom of a shallow bowl, with the horizon appearing to slope upwards in
all directions; very apparent to the naked eye, and rather disheartening.
So a dipmeter could well have been an important item to Arctic travellers,
and perhaps to the Soviet Arctic fleet.

It will be interesting to learn about the Troughton refraction-sector.
Rather then measure front and back horizons, as is done with a quadrant
fitted for back-observation, and capable of looking over the observer's
head, I wonder if instead it measured the vertical angle between two views
of the horizon, to the observer's left and right.

George.

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