John, You are very perceptive to discern the loss of accuracy around noon, as well as the need for a time scale with two dimensions.
If you request it, I can send you the instructions for making a dial of this type. The instructions are quite detailed and do explain how to create the two-dimensional date scale, as well as the scale for the sliding hole. The instructions are GIF files of pages scanned from Emerson's 1770 sundial book. The english is archaic, but understandable. You'll need an image viewer that lets you zoom in and out as I made the images quite large for easy reading. I like ACDsee as an image viewer. There are five GIF files, four of text pages, and one with drawings. The instructions begin under the heading "Prob II", and only figure 45 on the drawing page is used. I can send you a ZIP file that contains all five files. The file size is 429K, so be forwarned that it will take a while for you to download it from your mail server. consider this before you request it from me. If you have trouble downloading a file that large, let me know and I can send the five files individually (under 100K each). Happy dialling, Charles -----Original Message----- From: [EMAIL PROTECTED] <[EMAIL PROTECTED]> To: [email protected] <[email protected]> Date: Tuesday, June 08, 1999 10:11 Subject: perforated ring dial >Hello, I'm new to the list and don't know much but am interested. >I tried looking in the archive for this list but was unable to >connect. I hope I am not repeating a question which has been >asked 1000 times, but I probably am. 8-) > >I saw in a shop a perforated ring dial with circumferentially-sliding >perforation. I liked the look of it but now can't remember much of >the technical specifics; I should have bought it or sketched it. I >got to thinking about it and I think that near noon, it should be hard >to discern one time from another unless the ring is very large since >the sun's altitude peaks and recedes gently and this dial seems to be >an altitude sensor. Is this a well-known problem of this type of dial? > >Also, I tried to draw the dial and I think even with the sliding ring, >the scale for the time, located on the inside of the ring, must be >two dimensional and not linear as I believe I saw on the product for >sale in the shop. Is this true? The second dimension would be for >different seasons. In drawing the dial, I said that the position of >the perforation for a season would be the average solar altitude for >that season. By average, I mean splitting the difference between >the solar altitude at 8 am (or 4 pm) and noon. Is this the right >way to lay out one of these dials? What scheme does one use? > >Thanks very much, >John B >37.72 N 122.07 W > >PS: I read Waugh's book and that's where I learned the designation, >"perforated ring dial". Is that what this is generally called? > >PPS: I hope you can figure out what I am saying. I am so new to this >that I don't know the correct terminology. >
