NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Navigational instruments at the Naval Museum in Lisbon
From: George Huxtable
Date: 2010 Feb 26, 00:12 -0000
From: George Huxtable
Date: 2010 Feb 26, 00:12 -0000
Joergen Hoefeld's comments about the supposed Elton quadrant replica are perceptive. I agree that it's a replica of an ordinary Davis quadrant. Until the "Flamsteed" lens was introduced, to throw a sharp shadow of the Sun, the penumbra of the Sun's shadow was so wide (~half a degree) that minutes on the large arc were largely irrelevant anyway. It was not a precise instrument, which was why the Hadley was taken up so quickly to replace it. The Elton version, supposedly an improvement, was so impractical for use at sea, with its spirit levels, that it's no surprise that it never got anywhere. George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "J�rgen Hoefeld"To: Sent: Thursday, February 25, 2010 10:34 PM Subject: [NavList] Re: Navigational instruments at the Naval Museum in Lisbon By the way, the Davis quadrant http://www.fer3.com/arc/imgx/IMGP5954.JPG labelled: 'Elton's quadrant replica' seems to be no Elton quadrant at all but an ordinary Davis quadrant. I can't see any bubble or levelling attachment. This brings me to some more questions on diagonal scales on Davis quadrants: - The sight vane on the 30� scale of a Davis quadrant has to be proper aligned with the horizon vane to get a meaningful reading on the transversal scale on the large arc. Otherwise the reading of minutes on the diagonal scale is meaningless. - How did the manufacturers and/or users ensure this alignement? - What is the oldest existing Davis quadrant? I only found examples which are not much older than the first octants from the 1730s, compared to Davis publication of 1595. - Does anyone remember having seen in a museum or collection an existing Elton Quadrant like that by made by Sisson in the Phil. Trans. 1731-1732, vol. 37, 273-279, depicted in http://en.wikipedia.org/wiki/File:Elton-quadrant.jpg or on John Vanderbanks painting http://www.nmm.ac.uk/mag/pages/mnuExplore/PaintingDetail.cfm?ID=BHC3128 in the National Maritime Museum? I am looking forward to your hints. Best regards, Juergen Hoefeld Aachen, Germany 2010/2/25 Douglas Denny > Ah! but which came first the chicken or the egg? > > Or, in this case: is 'Gunwhale Circle' a simple error by whoever wrote the > label due to the fact a 'Borda Circle' (so called because of the inventor) > has the same name in Portugese? It could be as simple as that. > > I think your extrapolation needs further evidence of the use of a term > like > 'Gunwhale Circle' being in actual use by Portugese sailors > contemporaneously > with the use of the circle itself in the nineteenth century. > > I have come across simple blunders in labelling in museums before where it > is obvious the person who wrote the label did not 'know his onions'. > > Douglas Denny. > Chichester. England. > > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- >