Hi Fabio As you say, this is not really a sundial, but a sundial recorder of the Campbell-Stokes type. See http://en.wikipedia.org/wiki/Campbell-Stokes_recorder.
If you get the paper strip - which you must change each day - correctly aligned, it can of course be used as a sundial. Also by adjusting the strip according to the EoT it can be a mean time dial. Before electronics took over, every meteorological station had one of these recorders. Every day the paper strip - which was made of special card so that it would only blacken and not completely burn - was changed and the length of blackened line was measured to give the hours of sunshine on the previous day. The paper was designed to burn right through if there was full sunshine, but just to leave a mark if the sun was only hazy. (I did this every day for three years at school when around 14 years old - it was a very boring job) Because the focal length of a glass sphere is so short, such instruments are not great as sundials. A greatly improved version can be made with a long focal length sphere as invented by Prof Julian Chen of Columbia University. See NASS Compendium 14(4) pp 5-8 (Dec 2007). This uses a hollow lucite sphere filled with a copper sulphate solution which gives a cool non-burning blue image. This is very difficult to fabricate. Best regards Kevin Karney On 8 Jun 2010, at 17:01, Fabio Savian wrote: > hi all, > > today I find a new sundial on Sundial Atlas like the one you described > www.sundialatlas.eu/atlas.php?so=IT1611 > > Riccardo Anselmi, a diallist of Aosta Valley in Northern Italy, registered it > with some info. The sundial was in the balcony of a thermae in Saint-Vincent. > The glass sphere focused the Sun beam on a paper strip placed on the > equatorial ring; The beam burned the paper so that it was possible to > evaluate the period of sunstroke and the local hour. The photo was taken in > the eighties before the sundial was stolen. > > SA is able to classify this sundial as 'stolen' to contribute to its finding > or to discourage these events. > > ciao Fabio > > > Fabio Savian > fabio.sav...@nonvedolora.it > Paderno Dugnano, Milan, Italy > 45° 34' 10'' N 9° 10' 9'' E > GMT +1 (DST +2) > ----- Original Message ----- > From: John Carmichael > To: 'Fred Sawyer' > Cc: sundial@uni-koeln.de > Sent: Monday, June 07, 2010 6:17 PM > Subject: The Jacobs Glass Sphere Dial > > > Hello Fred (cc Sundial List): > > I enjoyed reading about the Jacobs glass sphere dial in the recent issue of > the Compendium as I have not been up to the mountain and have not seen it in > person. I had forgotten that I had discovered the Jacobs glass sphere dial at > Kitt Peak several months ago while searching for dials on Flickr. A Flickr > non-dialist photographer took two EXCELLENT photographs of it and gave me > permission to post them at: > http://www.advanceassociates.com/Sundials/Stained_Glass/sundials_EGP.html > (Dial # 435 at the bottom of the page) I knew it looked familiar! There are > several similar glass sphere dials on our webpage, but this is one of the > best. Here are the two photos taken by the photographer, Matt Anderson. © > All rights reserved. Unlike the cloudy overcast photo given to you by L. & > B. Graver, these photos clearly show the focused spot of light. > > See: > http://www.advanceassociates.com/Sundials/Stained_Glass/sundials_files/New%20Files/Originals/SGS_435a.jpg > and > http://www.advanceassociates.com/Sundials/Stained_Glass/sundials_files/New%20Files/Originals/SGS_435b.jpg > > I must go visit this dial in person because I have several questions about > it. > > First, if you look closely at the spot of light it is NOT a small point of > light as seen in Campbell-Stokes recorders. It looks wider (about ½ inch). > I’m wondering if professor Jacobs did this on purpose by placing the > projection screen closer to the glass sphere than the focal point distance. > As we all know, the sunlight focused by a glass ball can get VERY HOT! It > burns paper and fingers (Once I accidentally placed a glass sphere gnomon > counterweight from one of my cable dials on an outdoor sofa and it burned a > hole in the cushion!) If Jacob’s projection screen is of Plexiglas or > acrylic plastic, then a sharply focused point of light would melt it or might > even catch it on fire. So perhaps Jacobs intentionally placed it closer to > the sphere to spread out the light and avoid the hot temperatures. > > If the projection screen is made from glass, it wouldn’t melt or catch on > fire, but it might crack from the high temperature differential. So placing > the screen out of focus would prevent this. > > In any case, since it appears that the focused spot of light is out of focus, > this would affect the precision of the time readings. > > So the big questions are: > What is the projection screen made from? > And why does the focused light seem too big and out of focus? > > Jim Tallman and I have discussed this dial and Jim also wonders of there is a > water drainage hole at the bottom of the cup-shaped projection screen. > > Do you know the answers to any of these questions? > > John > > p.s. Would you give me permission to post a pdf copy of your article on the > SGS website? I can get it from the digital copy of the Compendium that I have. > > > John L. Carmichael > Sundial Sculptures > 925 E. Foothills Dr. > Tucson AZ 85718-4716 > USA > Tel: 520-6961709 > Email: jlcarmich...@comcast.net > > My Websites: > (business) Sundial Sculptures: http://www.sundialsculptures.com > (educational) Chinook Trail Sundial: > http://advanceassociates.com/Sundials/COSprings/ > (educational) Earth & Sky Equatorial Sundial: > http://advanceassociates.com/Sundials/Earth-Sky_Dial/ > (educational) Gnomons, Styles & Nodi: > http://www.flickr.com/groups/1207...@n23/ > (educational) My Painted Wall Sundial: > http://www.advanceassociates.com/WallDial > (educational) Painted Wall Sundials: > http://advanceassociates.com/WallDial/PWS_Home.html > (educational) Stained Glass Sundials: http://www.stainedglasssundials.com > (educational) Sundial Cupolas, Towers & Chimneys: > http://StainedGlassSundials.com/CupolaSundial/index.html > > > > > > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial > > --------------------------------------------------- > https://lists.uni-koeln.de/mailman/listinfo/sundial >
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