Two blown bulbs joined at a narrow waist, filled with graded sand and set in a turned wooden frame. The glassworker’s difficulty is the waist: too wide and the glass runs fast and unevenly, too narrow and damp sand stops it altogether. A good hourglass is calibrated against a sundial over many turnings, and is worth more than most of what stands in the room with it. Watch-keepers, navigators, and preachers all rely on one.
Hourglass
Paired glass bulbs and graded sand in a wooden frame; measures a fixed interval.