On the Blow: how struck iron and cubes deform
Why a light or heavy hammer-blow widens a bar differently, and why a battered cube grows round
Continuing his study of percussion, Leonardo describes how iron of equal faces struck lightly on the anvil widens more at the top, while a great blow makes its greatest thickness fall in the middle. He notes that a bar struck in mid-air bends downward where it is hit while its ends stay up, and that a cube hammered hard on the anvil becomes round, growing at its faces and not at its corners, because every part flees the center of its own position. Marginal sketches at right show blocks, an anvil and a carpenter's plane, numbered 51 to 57.
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Iron widened by light and heavy blows on the anvil
Iron of equal faces struck lightly on top upon the anvil widens more above than below. But struck with a great blow, the top no longer exceeds the bottom in width and the greatest thickness falls in the middle.
A bar struck in mid-air bends at the point of impact
When a body is struck in the air at the middle of its length, the struck part obeys the blow and goes downward while the ends remain up high. Everything struck goes downward, its adjacent parts staying in their first place in the air.
A hammered cube becomes round
If the cubic block is struck upon the anvil with a great blow it becomes round, growing at its faces and not at its corners. It rounds because every part flees the center of its own position.
Striking the plane and the tin plate
Striking the back of a carpenter's plane drives its iron blade out; and a tin plate lying face-up on a board, when struck, flattens out entirely.