 One of Galileo’s problems was that the Archimedian simple machines he was using as his models of intelligibility, especially the balance, are not easily conceived of in a dynamic way (but see Machamer and Woody 1994). Since they generally work by establishing static equilibrium, time is not a feature of their action one would normally attend to. In discussing a balance, for instance, one does not normally think about how fast an arm of the balance descends, nor how fast a body on the opposite arm is rising (though Galileo does in his Postils to Rocco circa 1634–45; see Palmieri 2005). The converse is also true. It is difficult to model dynamic phenomena that involve rates of change as balance arms moving upwards or downwards because of differential weights. So it was that Galileo’s puzzle about how to describe time and the force of percussion (the force of body’s impact) would remain unsolved. Throughout his life, he could not find systematic relations among specific gravities, heights of fall, and percussive forces. Even while the Two New Sciences was going to press in 1638, Galileo was laboring on an additional “Fifth Day” (not published until 1718) that presciently explored the concept of the force of percussion, which would become, after his death, one of the most fecund ways to think about matter and its motion.