b a - d c

The friction of clean and flat bodies resists their mover with a force equal to a quarter of their weight.

Let a be the rubbed body, that is, the one dragged by the mover b; let c d be the polished plane over which this body is rubbed.

The frictions of bodies are of 4 kinds, of which the first is when two bodies are polished and flat, as is proposed here. The second is when the dragged body and the plane over which it moves are rough. The third is when the dragged body is rough and the plane over which it moves is polished. The fourth mode is when the dragged body and the plane over which it is dragged are both rough.

b – d – a - c

Experience shows that a polished thing, dragged over a polished plane, resists in its motion the mover with a force equal to the fourth part of its weight. And of the other 2 following kinds, to move a rough thing over a polished plane is as much as to move a polished thing over a rough plane. The fourth species is when the rough body is moved over a rough plane.

The friction of polished bodies will lose so much more resistance and gain in weight, the less oblique is the site over which it moves, the mover being above its movable body. The friction of polished bodies will lose so much more resistance and weight, the less oblique is the site over which it moves, the mover being lower than its movable body.

d-a - b - c

When the polished incline disposes the polished heavy body to weigh in the line of motion by a fourth part of its gravity, then the heavy body is of itself disposed to the motion of descent.

d - a - c

The heavy body that moves and does not depart by a revolving motion, the contact of the friction being polished, will always have the force of friction as a fourth part of its weight.