Collectable because of their combined historical and practical value. Some glasses measured up to two hours, used at a time when church pews were designed by the Inquisition to torture the human back. Ships hourglasses were calibrated to the half hours of the ships watch, and if the helmsman didnt turn the glass too early it actually doubled in defining longitude. Navigators tried to keep count of the hours; the galleons wrecked on the Western Australian reefs remind us of the perils of incorrectly kept time. To appreciate the important symbolism of the hourglass observe, Renaissance painted still lives, also called Vanitas, filled with symbolism and morals. They very often depict hourglasses.