In this letter, Schickard wrote that he had built a machine that “...immediately computes the given numbers automatically; adds, subtracts, multiplies, and divides”. Unfortunately, no original copies of Schickard’s machine – which he called his Calculating Clock – exist, but working models have been constructed from his notes.
In fact, some people believe that the first mechanical calculator may have been conceived by Leonardo da Vinci (1452-1519) almost one hundred and fifty years earlier than Schickard’s machine.
Da Vinci was a genius: painter, musician, sculptor, architect, engineer, and … the list goes on. However, his contributions to mechanical calculation remained hidden until the rediscovery of two of his notebooks in 1967. These notebooks, which date from sometime around the 1500s, contained drawings of a mechanical calculator, and working models of da Vinci's device have since been constructed.
Actually, it should be noted that some people believe that these sketches represent nothing more than a gear train, and that building a calculator based on them requires a certain amount of wishful thinking and extrapolation.