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Since ancient times and across cultures, devices for marking mandkind's hours have proliferated. Timekeeping devices, clockwise from top left, include an American sundial and the mold it was cast from, 1779; a terra-cotta water clock from classical Athens; a sandglass. used in a schoolroom in 19th century Sterling.Massachusetts; a German chamber clock; c. 1500; and a French 19th-century portable sundial.