Source, in full
The quote is widely misattributed to Galilei, but is actually from two French scholars, Antoine-Augustin Cournot and Thomas-Henri Martin. See "Der messende Luchs: Zwei verbreitete Fehler in der Galilei-Literatur" by Andreas Kleinert in "NTM Zeitschrift für Geschichte der Wissenschaften, Technik und Medizin" May 2009, Volume 17, Issue 2, pp 199–206.