Wheels, axles and the perfect sphere at rest
Why a wheel turns faster, how axle friction tires the mover, and rolling on a plane
On folio 177r, four rules headed "Motion" treat wheels and rolling bodies. A wheel moves by itself the faster the farther its ground-contact lies from its central line, and stands still when that line is the center of its whole contact; the friction of a loaded wheel on its axle, over an uneven plane, is what fatigues its mover. A concluding rule states that a perfect sphere on a perfect plane ought never to take motion of itself, and that no body is moved by less power. Marginal diagrams numbered 64, 65 and 66 show a circle on an inclined tangent, a wheel on a line, and a wheel-and-axle pair.
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A wheel turns faster when its contact is off-center
A wheel moves by itself the faster the farther the part touching the ground lies from its central line; and it remains without motion when its central line is the center of its whole contact.
Axle friction under load tires the mover
The friction the wheel makes with its pole (axle) when it is loaded, together with an ill-leveled plane, is what fatigues the mover. The wheel-and-axle sketch at the left illustrates the loaded pole.
A perfect sphere never moves of itself
A perfect sphere upon a perfect plane ought never to take motion by itself, and no body is moved by less power. The circle resting on a straight line at the left figures the ideal case.