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The Medici’s attempt to link their dynastic glories to Galileo’s discoveries emerged in the field of navigation as well, where the "Medicean planets" offered new hopes for solving the crucial problem of calculating longitude. The discovery of Jupiter’s satellites and the determining of their orbital periods appeared to provide a new and extraordinary ‘compass’ for transoceanic voyages. Galileo (1564-1642) designed a device, consisting of a helmet bearing a small telescope, which allowed navigators to observe the satellites of Jupiter from the deck of a moving ship. To retain the time of the port of departure, the scientist devised a special mechanical clock, fitted with a new type of escapement governed by the isochronous oscillations of a pendulum. Still within the Medici sphere is the proposal of the English Admiral Sir Robert Dudley (1573-1649), at the service of both Cosimo II (1590-1621) and Ferdinando II (1610-1670), for a "mercurial clock" composed of two vessels communicating through a siphon, in which the mercury flowed from one vessel to the other at a uniform rate.