Ship's time was divided into half hours, and the helmsman used a thirty minute hourglass to keep track. One half hour was called a 'glass'. Our museum replica solid brass-housed 'glass' is cleverly gimbaled and tooled. A great desk accessory for people who like to keep their meetings short and snappy! Comes with wall mounting hardware, and is attractively packaged, complete with full color booklet telling about the glass, and the ships they travelled on.