Exploration of the Inclined-Plane Conception of Projectile Motion

As we have seen in the previous chapter, Harriot conceived of projectile motion as being composed of two motions, a decreasing motion along the line of the shot and an increasing motion vertically downwards. In the notes discussed in this chapter, Harriot assumes both motions to be uniformly difform over time. In its dependence on the elevation angle, the motion along the line of the shot furthermore obeys the law of the inclined plane, the angle of inclination being the elevation angle. In short, according to this conception, projectile motion emerges from the composition of a motion along an inclined plane and the motion of free fall.

We have already encountered trajectories constructed according to this conception on f. G-216v (see 7.3.4). In the notes discussed in this chapter, Harriot makes the inclined-plane conception of projectile motion accessible to an algebraic treatment and thus also to calculation. He does so by representing the motion of a projectile by means of compound diagrams, i.e. diagrams of motion that are composed of diagrams representing the component motions.