A sphere at rest on a plane: gravity and the central line
The condition for equilibrium, and gravity directed toward the centre of the world
Leonardo studies when a heavy sphere resting on a plane will stay still, illustrated by pairs of tangent circles with lines drawn to the centre of the world. His rule: the sphere rests only when the 'central line' from the earth's centre to the sphere's centre passes through the point of contact, since the sphere's parts are then of equal weight about that line. Repeated variant phrasings define the central line as the straight line between a body's centre of gravity and the centre of the world, and hold that a body settles at the point of the plane nearest that centre. He adds that gravity and levity arise from one element being drawn into another.
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When a sphere comes to rest on a plane
The spherical body will not stay at rest on the plane unless the line running between the centre of the world and the centre of the sphere passes through the point where the sphere touches the plane. It rests because the parts of the sphere are of equal weight about that central line, and equal things do not overcome one another.
Definition of the central line
The central line is the straight, potential line imagined between the centre of gravity of bodies and the centre of the world. A heavy sphere settles at whatever part of the plane lies nearest to that centre, and the power of every heavy body resides in its central line.
Gravity as one element carried into a lighter
At the foot of the sheet: gravity is caused by one heavy element being carried into a lighter element, and the power of every gravity extends toward the centre of the world. This ties the resting condition above to a physical account of what weight is.