He showed that the action of two perpendicular forces upon a point is the same as that denoted by the diagonal of a rectangle, of which they represent the sides.

From this the passage to the proposition of oblique forces was very easy.

This proposition was rediscovered by Stevinus, a century later, and applied by him to the explanation of the mechanical powers.

Da Vinci gave a clear exposition of the theory of forces applied obliquely on a lever, discovered the laws of friction subsequently demonstrated by Amontons, and understood the principle of virtual velocities.

He treated of the conditions of descent of bodies along inclined planes and circular arcs, invented the camera-obscura, discussed correctly several physiological problems, and foreshadowed some of the great conclusions of modern geology, such as the nature of fossil remains, and the elevation of continents.