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Alex posted this intriguing message-

>While studying the pictures
>of Hadley quadrants to answer Omar's
>questions, I made an unexpected discovery:
>
>Hadley's quadrant can be used as a dipmeter!
>To see this, read carefully the beginning
>of p. 178 in Norie:
>(go to
>http://www.mysticseaport.org/library/initiative/
>ImPage.cfm?BibID=13617&ChapterId=1
>Then click on "description and use of Hadley's quadrant"
>on the left, then go to page 6, and read
>IV. To set the Back Horizon Glass perpendicular to the
>plane of the Index Glass, 0 on the Nonius being at 0 on the Arc.)
>
>Once this is done, the Back Horizon Glass is set
>perpendicularly, you can use the VERY SAME procedure
>to measure the dip!
>
>This shows how one can attach ONE extra horizon mirror
>(and a sighting
>vane or a second telescope mounting bracket) to a
>sextant
>to convert it to a dipmeter:-)
>
>Unfortunately, modern sextants are just a little
>bit too short to do this conveniently: you won't
>be able to wear a hat, and some maybe will have to shave
>their hairs:-)
>
>Alex.
>
>P.S. Norie never mentions this possibility.
>But it is hard to believe that I noticed this first:-)

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

My own copy of Norie's dates from 1900, so doesn't refer to back-sights.

At the webpage that Alex refers to, where he says

>Then click on "description and use of Hadley's quadrant"
>on the left,

On my screen there's no such box or such an option to click on. Instead, I
get a nice engraving of the Hadley quadrant, etc., but otherwise am stuck
within the 8 pages of the preface, with no apparent means of escape.

I am quite prepared to accept that these problems are due to deficiencies
in my ancient computer system, or in my navigation of that website.

It would be worthwhile, perhaps, if Alex would kindly summarise what Norie
says in-

>IV. To set the Back Horizon Glass perpendicular to the
>plane of the Index Glass, 0 on the Nonius being at 0 on the Arc.

I am especially interested in this matter because Lewis & Clark used a
quadrant with a backsight, for their reflected altitudes in Summer, when
the Sun was higher than 60 deg, because then the doubled angle exceeded the
range of their sextant. It has never been clear to me how the index error
in backsight mode was determined by them on land or, for that matter, by
mariners at sea. The index error in normal (foresight) mode gives no clue
to what it will be in backsight mode.

On land, the following procedure seems plausible-

First, check the index error in normal foresight mode, using a very distant
landmark or a heavenly body. Then choose a pair of landmarks, such as
distant tree-trunks, with a horizontal angle between them of roughly 90 deg
in normal mode. Then advance or retire, until the angle becomes exactly 90
degrees, having allowed for the known (forward) index error. Then switch to
backsight mode and measure again the angle between them, from the same
spot, which should also give a 90-degree horizontal angle: any departure
from that value would be a measure of the index error in backsight mode.
Whether that was what Lewis and Clark did, or whether they checked their
index error in backsight mode at all, I just don't know.

At sea, it seems to me to be rather more difficult, because the angle
between the fore and aft horizons gets so tangled up with the effects of
dip. Perhaps it involves measuring altitude of a star as it crosses the
observer's zenith, in the two modes.

Alex adds-

>Unfortunately, modern sextants are just a little
>bit too short to do this conveniently: you won't
>be able to wear a hat, and some maybe will have to shave
>their hairs:-)

I suspect that for most modern sextants there is insufficient headroom even
for that. More drastic treatment would be called for; such as drilling a
suitable hole from the observer's forehead, emerging at the back of his
skull, to let the light pass.

George.




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