Why Thinner Axles and Larger Wheels Turn More Easily
Proof with paired wheels and an ox pulling nine cords tied to an upright
The sheet proves two related propositions about wheels. First, of two wheels of equal weight the one with the thinner pole (axle) is more easily moved: wheel A with a thick pole clears the ground by only 2 once, while thin-poled wheel B rises to 3 once. Second, a large diagram sets an ox pulling nine cords tied at rising heights to an upright, showing that the higher the point struck or pulled, the more readily a body is toppled, and hence the higher a wheel's centre stands the more conveniently it turns. Concentric circles at the right represent cart wheels of increasing size.
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A thinner axle moves more easily
Comparing two wheels of equal weight (B and A), Leonardo shows the one with the thinner pole is moved with greater ease. Wheel A, having a thick pole, rises only 2 once from the ground; wheel B, with a thin pole, rises to 3 once, and the demonstration of shaft and wheels below confirms the thinner-poled wheel turns more readily.
The higher the centre, the easier it turns
Any corporeal thing is the more easily thrown or drawn to the ground the higher it is struck or pulled, because the farther it moves from its origin the weaker it becomes. In the large figure a single ox pulls an upright to the ground by a tie fixed high up, while its lower part would not be moved by nine; so the greater the wheel, the higher its centre and the more conveniently it does its work.
Cart wheels of increasing size
At the right the cords converge on a stack of concentric circles, drawn as the wheels of carts numbered 1 through 9. They illustrate how progressively larger wheels raise the centre and ease the pull.
A single draught ox
At the left a lone ox is drawn straining at the cords, its body harnessed to the demonstration. It stands for the single beast the text says can pull the upright down by the high tie.