1500 AD 
Many references cite the French mathematician, physicist, and
theologian, Blaise Pascal as being credited with
the invention of the first operational calculating machine called the Arithmetic Machine. However, it now appears that the first mechanical calculator may have been conceived by Leonardo da Vinci almost one hundred and fifty years earlier than Pascal's machine. 
Leonardo da Vinci. Copyright (c) 1997. Maxfield & Montrose Interactive Inc. 
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Da Vinci was a genius: painter, musician, sculptor, architect, engineer, and so on. However, his contributions to mechanical calculation remained hidden until the rediscovery of two of his notebooks in 1967. 
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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. 

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See also:  
The invention of the abacus  Wilhelm Schickard's mechanical calculator 
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These notes are abstracted from the book Bebop BYTES Back 