Hour-striking clock mechanism and counterweight balances
An escapement that sounds the hours, with devices that keep a weight in balance
The upper drawings show a striking-clock train in which a toothed wheel n turns on the pivot b m; once an hour the lever K trips a pallet that lets the wheel advance one tooth and drives a lever so the clock unlocks and sounds the hours, with a weight hanging on a cord to drive it. The lower figures show a hinged arm and pairs of wheels forming counterweight devices, labelled 'mobile' (movable) and 'imobile' (immovable). Leonardo notes that this striking motion varies greatly in effort while the wheel's own motion never changes, and that a weight can be kept at rest with its counterweight in every position. The mechanisms are lettered with the marks X, r, e, t, o, n, S, K and a, b, c, f, m.
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How the escapement lets the wheel strike the hours
The toothed wheel n turns on the pivot b m. Once each hour the lever K is pulled forward and a pallet, moving by a contrary motion, pushes the whole of tooth t; the spring S then returns the pallet, which falls on tooth o and holds until the next hour. Meanwhile an inner tooth presses down lever b so its far end rises at c, unlocking the clock to sound the hours.
A weight held in balance at every position
Here it is taught how to give small motion to the counterweight while making great movement in the weight a and b, so that the weight always stays at rest with its counterweight in every position. The instrument f likewise stays fixed at whatever height you leave it, because weight and counterweight are of equal weight and move equally.
Striking effort varies while wheel motion is constant
Leonardo observes that this motion varies greatly in the effort it costs, whereas the motion of the wheel itself never changes.