On Friction: Thin Media and Rollers That Ease Rubbing
Any thin substance or cylindrical roller placed between rubbing bodies lessens the resistance of their motion.
A full page of mirror-script under the heading 'On friction' argues that anything interposed, however thin, between two bodies rubbed together reduces the difficulty of their friction. Leonardo cites the dragging of great weights and refers back to an earlier proposition, noting that the larger the wheel set beneath a load, the easier the motion; conversely, motion is less easy the thinner the medium, as with a film of oil. He ranks the aids to sliding, from millet-like granules up to wooden balls and cylindrical rollers, adding that the larger such rollers are, the more easily the weight moves.
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Interposed matter lessens friction
Leonardo states that whatever is placed between two bodies rubbed together, however thin, diminishes the difficulty of their friction. He observes that a thinner medium, such as a light oiliness, makes the motion less easy than a thicker interposed body does. The claim is presented as a general law of frictional resistance.
Rollers and cylindrical wheels under heavy loads
The note ranks the means of easing the motion of great weights, from granules the size of millet, to balls of wood, to rollers, that is cylindrical wheels. Leonardo adds that the larger such rollers are, the easier the movement becomes, echoing an earlier proposition that a larger interposed wheel renders the motion easier.