{
  "name": "Volvelle layers: model and Apian 1540 leaf 18r",
  "desc": "Layers of a simple two-disc volvelle model (radius, divisions, window, pointer, pivot), followed by approximate radii measured on the National Library of Poland scan of leaf 18r of Apian's Astronomicum Caesareum (1540). Measured rows give each radius as a share of the scanned leaf's height, with no millimeters, because the size of this copy is not stated. Model rows: paper-mech 1.1.0 (re-issued 2026-11-15, values unchanged; each row gives the full rivet position); measured rows: volvelle-measure 1.1, 2026-11-14.",
  "lic": "CC BY 4.0",
  "lic_url": "https://creativecommons.org/licenses/by/4.0/",
  "by": "Fold, Knot & Bind",
  "url": "https://foldknotbind.com/paper-mechanisms/volvelle/",
  "from": [
    "Model rows: volvelle.design() of the paper-mechanism tool, paper-mech 1.0.0, delivered 2026-11-14; re-issued with paper-mech 1.1.0, 2026-11-15",
    "Measured rows: National Library of Poland scan of Astronomicum Caesareum leaf 18r (Wikimedia Commons, Public Domain Mark), measured 2026-11-14; radii divided by the scanned leaf height in pixels (no millimeter scale)"
  ],
  "how": "Compiled by Fold, Knot & Bind from the data listed"
}