I mentally conceive of some moveable [sphere] projected on a horizontal plane, all impediments being put aside. Now it is evident... that equable motion on this plane would be perpetual if the plane were of infinite extent, but if we assume it to be ended, and [situated] on high, the movable, driven to the end of this plane and going on further, adds on to its previous equable and indelible motion, that downward tendency which it has from its heaviness. Thus, there emerges a certain motion, compounded...

Author, Day Four, On the Motion of Projectiles, Stillman Drake translation (1974) p. 268 - Dialogues and Mathematical Demonstrations Concerning Two New Sciences (1638)