Why clocks run slow in damp weather; force along the line of motion
A note on humidity and timekeeping, with a key-and-bar lever example
The top of folio 48v asks why the hours told by clocks are sometimes long and sometimes short, arguing that in foul, damp weather the thickened air offers more resistance and the counterweight cords swell, moving their centre away from the roller, so the hours run long. A general principle is then stated: everything that moves bodily exerts force and weight along the line of its motion. A wheel-and-lever diagram (o, a b, n, m f) shows that a key m n turning in a circle cannot push the little bar a b straight but drives it along the line f o, making the motion hard for its mover. A lever weighted with a bell is drawn at the foot of the page, called an experiment of the same nature.
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Why clock-hours lengthen in humid weather
Leonardo argues that damp air thickens and resists the clock's movement, and that the counterweight cords grow thicker in humid weather so their centre lies farther from the roller. Although a thicker cord ought to shorten the hour, the hours are found longer, which he attributes to the weather's effect on clocks and no other cause.
Force and weight act along the line of motion
A stated proposition: everything that moves bodily exerts its force and weight along the line of its motion.
A key cannot push a bar straight when it turns in a circle
As an example of the proposition, the key m n pushing the little bar a b toward a turns in a circle and so cannot push straight, exerting force only between angles. It therefore drives the bar along the line f o rather than the required line a b, making the motion difficult for the mover.