I agree, I cannot make sense of the dial described by Frans. The article Patrick links does not describe this particular arrangement of hour lines. Looking closely at the image, it seems that all of the odd numbered hours are labeled on the same large sheet of material (this material may or may-not be polarizing). The even numbered hours each have their own sliver of material, and are spaced 15 degrees apart like hours on an equatorial dial. * It is possible that the dial is properly constructed, but improperly labeled.* I think a correct labelling would be if each of the even-numbered strips had been labeled consecutively (from 6:00AM to 6:00 PM for the semicircle).
-Bill Gottesman On Tue, May 21, 2019 at 7:00 PM Patrick Vyvyan <patrickvyv...@gmail.com> wrote: > Wheatstone's Polarizing Dial > > An explanation of Wheatstone's invention is to be found in the Report of > the meeting of the British Association for the Advancement of Science in > Swansea held in 1848, although to be honest it's scientific foundation has > me a little challenged! The report is available online at > https://www.biodiversitylibrary.org/item/46637#page/188/mode/1up > Should you have access to an original paper copy, the description of the > Polarizing Dial begins on page 10. The numeral scale represents one > complete day of 24 hours - it seems the device can work even without direct > sunlight. > > Best wishes, > Patrick Vyvyan > > > > On Tue, 21 May 2019 at 16:07, Maes, F.W. <f.w.m...@rug.nl> wrote: > >> Recently the Tesseract Catalogue 109 was announced on this list. Item nr. >> 13 is a polarizing sundial by Charles Wheatstone. A virtually identical >> dial is in the collection of the Greenwich museums, see: >> https://collections.rmg.co.uk/collections/objects/265579.html. >> I have never understood how this sundial works. The hour scale shows 2 x >> 12 hour numbers in a semicircle. So whatever pattern is observed in the >> black glass reflector, it is obviously supposed to rotate over 180° in 24 >> hours, which is half the angular velocity of the sun itself. How does this >> frequency division-by-two come out? Can anybody explain? >> >> Thanks! >> Frans Maes >> --------------------------------------------------- >> https://lists.uni-koeln.de/mailman/listinfo/sundial >> >> --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > >
--------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
