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Dear George,
I also reply several of your messages on related subjects:

On Sun Nov 21 2004 - 12:05:39 EST, George Huxtable wrote:

> This was a sales-pitch for Peter Dollond's new sextant, with
> modifications
> that had been suggested by Maskelyne.

Nevil Maskelyne was of different opinion.
In the paper which immediately follows Dollond's letter,
he praises Dollond's suggestions high:

XV. Remarks on the Hadley's Quadrant, tending principally to
remove the difficultiues which have hitherto attended the use of
the back observations... Phil Trans. May 28, 1772.

On Tue, 23 Nov 2004, George Huxtable wrote:

> Alex will find a picture of the Gavrisheff dipmeter

Already. Thanks Frank who sent me a copy of the paper.

> I'm grateful to Alex for pointing me to
> the Mystic facsimile of Halley's

This is not Mystic faximile and not Halley.
This is a paper of Wollaston
in Phil Transactions, Nov. 11, 1802.

He had several very interesting papers on refraction and
anomalous dip.

> Alex and I seem to agree that by making two observations,
> one inverted,
> both the dip and the index error can be discovered.

I think it is more important that dip can be measured.
Instead of having a separate divice, dipmeter,
it seems to be enough to make a very simple addition
to an ordinary sextant.

> Alex says that back observations never worked well
> I can suggest one possible reason why.
> There was no telescope
> fitting for the back-observation,

In the paper of Maskelyne I mentioned above in this message,
he recommends fitting such a telescope.
This paper is also interesting because it touches another
subject much discussed on this list: the wires in the telescope.

Now I found a lot of discussion of wires in the XVIII century papers,
beginning from the first communication of Hadley himself
where he talks a lot about wires.

Alex.

   

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