Weight hung by cords versus weight on rollers
On a perfect plane rollers keep the load constant; a suspended weight gains weight when swung aside
Two diagrams contrast a heavy 200-pound box suspended from two cords (m, o) with the same box resting on rollers (a, n). Leonardo argues that a cord-hung weight makes degrees of weight in every degree of motion — pulled sideways it swings in a circular path, rising away from the earth's centre and so pressing harder on its mover — whereas a load rolled on balls or rollers over a perfect plane changes its weight in no degree of motion. He raises the doubt that both are sustained upon a point, invoking the fifteenth proposition of the first book on the centre of gravity below the cord.
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A suspended weight gains weight when pulled aside
The centre of gravity of a weight hung on a cord lies below the centre of that cord and seeks the centre of the world. Pull the weight sideways and it makes a circular motion, rising upward and moving away from that centre, whence its weight increases to its mover.
On rollers over a perfect plane the weight never changes
A flat weight placed upon balls or rollers on a perfect plane changes no quantity of weight in any degree of motion, for it is sustained upon a point just as the hung weight is.
The same load hung by two cords and set on rollers
The upper sketch shows a 200-pound box suspended from two cords marked m and o; the lower shows the same box resting on rollers marked a and n, ready to be moved along a plane.