In his , Giuseppe Moletti made the most explicit and thoroughgoing attempt to determine the geometrical principles of Aristotelian mechanics, to establish its Euclidean foundations, and so to realize in fact the subalternation of mechanics to geometry. Having done this in the First Day, he then set out in the Second to extend mechanics generally to explain all motions through the analysis of their forces and resistances. In the process he anticipated Galileo in asserting that all heavy bodies, whatever their weights, fall with equal speeds, and he realized that the same resistance that makes a body hard to move also makes it hard to stop – which is almost the law of inertia. Written in dialogue form in Italian (rather than in Latin) for a courtly and practical audience, the was left unfinished when Moletti quit the Gonzaga court at Mantua to take up the mathematics chair at the University of Padua. Never before published except for brief extracts, the full Italian text is edited from the manuscripts and printed here for the first time, together with a facing-page English translation. The extensive notes that accompany the text cite and quote from a number of Moletti's other, mostly unpublished, works and his numerous sources. In his introduction, W.R. Laird sets the within the historical background of medieval and Renaissance mechanics, sketches the life and works of Moletti, and analyses the arguments and the geometrical theorems of the . offers an unprecedented look at the transformation of Aristotelian mechanics into a mathematical science in the generation before Galileo.