A fine example of the clockmaker's ingenuity. Two sets of Roman numerals, from one to twelve, were engraved, usually in the dial arch alongside the 29 1/2 days of the moon's phase. By adjusting an indicator it was possible to forecast the approximate time of high tide at any given port. This was of particular value to mariners when ship departures depended on the tide. John Flamsteed, the first Astronomer Royal, is said to have drawn up tidal tables for use in the Port of London, the calculations being worked out in conjunction with the changes in the moon's phases.