Simple and Composite Motion; Impetus as a Wave
The motion of a sphere through air, impetus in air and water, and geometric proofs of equal lines
Two conjugate leaves treat the physics of motion. The main text distinguishes simple motion, in which a spherical body's surface moves as much as its centre, from composite motion, in which a revolving body's surface moves faster than its centre. A second passage defines impetus as nothing but a wave of air or water, each capable of generating the other, and a marginal note asks why a triangular percussion makes a round wave in water. Small diagrams accompany the notes, including overlapping-circle figures with lettered points used to prove, by the sixth axiom and the definition of the circle, that certain lines are equal.
On this page
Simple versus composite motion of a sphere
Simple motion is that in which the surface of the moving body moves as much as its centre; composite motion has much greater motion in the surface than in the centre, the body revolving as it goes. In simple motion no part differs from the whole, whereas in composite motion no part equals the motion of the whole nor moves equally with the others.
Impetus as a wave, not self-generated
Impetus is nothing other than a wave of air or water, generated and moved not by itself but by others. The impetus of falling water can raise a wave in the air, and the wave of the air in turn generates an impetuous wave in the water.
Why triangular percussion makes a round wave
A marginal query asks why a triangular percussion produces a round wave in the water, given that the circular waves penetrate one another where they meet.
Equal lines proved by the sixth axiom
With the lettered points d b a c and f, g, m, n, it is shown that a b equals b d, and that b d and a c are equal to a b; therefore by the sixth conception they are equal among themselves. m f and m n are equal by the definition of the circle.