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Dear George,
today I found in Purdue library a really rare thing:
the book of Simms "Mathematical Instruments", second
edition 1844 which Chauvenet refers to.

Among other things, it contains an excellent picture
of Throughton's Dip-sector (dipmeter) with a brief history
of its invention, and details of its use.
This is the first clear picture of a dipmeter which I encounter.

There is also a description of Troughton's reflecting circle.
It is different from the repeating reflecting circles we discussed.
And it also has a prohibited range of angles.
(at about 30 degrees), and Simms says this explicitly and
explains that for these angles it can be used only as a simple
sextant (the advantages specific to reflecting circles are
thus lost for this prohibited range).

I may try to scan and post the pages of this book concerning
the dipmeter and/or the reflecting circle.
Or can mail you photocopies of these pages.

The book is very interesting, describing a wide range of
astronomical and surveying and plotting instruments.

On Thu, 13 Jan 2005, George Huxtable wrote:

> I haven't yet read the Mendoza paper (but will soon). In that geometry,
> Alex reports that the angle at which light is blocked is from 5 to 10
> degrees.

This is a very rough guess I could make from the pictures.
It can as well be around 20 degrees.

Alex.

   

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