Why a heavy hammer drives a nail in one blow
A ten-pound and a one-pound hammer, and the proportion of percussive force.
Two sketches show a nail being driven into wood: a hammer marked 'ten pounds' sinks it home in a single blow, while a 'one-pound' hammer leaves it only part-way. Leonardo argues that the light hammer will not finish the nail even in ten blows, because the hardness of the wood does not lessen in proportion as the blows are divided. To treat such penetration properly, he says, one must weigh the nature of the striking weight, of the struck object, and of the place into which it is driven.
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The ten-pound hammer sinks the nail at one blow
A hammer marked 'ten pounds' drives the nail completely into the wood with a single stroke. Its concentrated weight carries the nail home at once.
The one-pound hammer fails even in ten blows
The hammer marked 'one pound' will not drive the same nail fully home in ten blows, because the wood's resistance does not diminish in proportion to the divided force; it stays as hard as before. To judge such penetrations one must consider the striking weight, the object struck, and where it is driven.