disc disk divisions gnomon light measure metal shadow stone sundial sunlight time pedestal clock England garden hour old ornamental plinth point traditional architecture elements english Great Britain historical Sussex wall ancient Cornish cornwall Michael mount national trust quality saint sea St. Michael's Mount water horizon aligned alignment aligns cast-metal circle circular decorated decorative device dial distinctive flat-plate garden-object hour-lines inaccurate indicator lines local-solar-line nodus ornament pointed shadow-edge shadow-line style sun’s-position time-telling-device