Ship's bell clocks were originally used as half-hour sandglasses. With the particular sequence of chimes tolled by a ship´s bell, they set the rhythm for the changing of the watches aboard ship. One bell indicates one half-hour of elapsed time. The seamen standing watch were relieved of their duty at four-hour intervals, i.e. every eight bells. As long ago as his voyages of discovery in 1492, Columbus relied on sandglasses to aid him in navigation and to help time the changes of the ship´s watches. Today too, the distinctive chime of maritime time measurement recalls an era when life on deck was not yet determined by high technology. As was the case centuries ago, the four-hour rhythm of the changing of the ship´s watches continues unchanged today.