In a letter to Leopold de' Medici (1617-1675) dated August 20, 1659, Vincenzo Viviani (1622-1703) gave a detailed account of the invention of the application of the pendulum to the clock by the Pisan scientist: "[…] Galileo started thinking about his timepiece; and one day in 1641, […] I recall that he conceived the idea that one could adapt the pendulum to weight-driven clocks and spring clocks, instead of the usual movement, hoping that the very constant and natural motion of that pendulum would correct all the defects of the mechanism in those clocks. But being deprived of sight he could not draw plans and models […] and since Signor Vincenzio his son was coming one day from Florence to Arcetri, Galileo explained his thoughts to him […] and finally they decided on the way that is shown in the enclosed drawing, and to put it into practice […]. But since Signor Vincenzio wanted to make the instrument with his own hands, so that it was not divulged by the makers before it was presented to his patron the Most Serene Grand Duke and to the States General [of the Netherlands] for longitude finding, he delayed the construction so long that in a few months Galileo, author of all these admirable inventions, fell ill and on January 8, 1641 Ab. Inc.ne, passed away, so that Signor Vincenzio lost all of his enthusiasm and it was not until the month of April 1649 that he started the making of the present clock […]."