NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The backstaff. was: Re: The Shovell disaster
From: Michael Daly
Date: 2007 Nov 03, 14:13 -0400
From: Michael Daly
Date: 2007 Nov 03, 14:13 -0400
Nicol�s de Hilster wrote: > George Huxtable wrote: >> In all cases the 30-degree arc carries a transversal scale of the type used >> by Tycho Brahe, adapted to the backstaff from the first half of the >> seventeenth century. > > 'In all cases' means in 'all surviving instruments with 30 degree arcs'. > Not only are there two types of surviving Davis Quadrants, those with 25 > degree arcs and those with 30 degree arcs, but through my research I > found proof that there was a time that the Davis Quadrant (if we may > call it that way already) did not have diagonal scales at all. The > earlier instruments were divided in a similar way as the hoekboog in > half or quarter degrees. The introduction of the diagonal scales was > most probably not before 1650. I'd be interested in knowing whether the transversals were always linear or whether they (in later instruments) used circular transversals. The former could be used on a circular arc with some error (error decreasing with increasing arc radius). Circular arcs require circular transversals to be accurate. However, it was difficult to figure out how to construct them (one technique was determined by Philippe de La Hire and used by Nicolas Bion) and very tedious to construct (I have created a diagram of linear transversals on Wikipedia but have yet to tackle the effort of making a circular transversal diagram - yet I have the advantage of a CAD program, not a sheet of brass with dividers and compasses). Mike --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---