A vane balanced against the wind; equal powers and resistances
How a pivoted rule turns under the wind, and the law of powers against resistances
On blue paper Leonardo studies the behaviour of a rule or vane pivoted like a balance and struck by the wind. He argues that things turning about a centre obey the wind at their most distant end, that equal wind on equal arms leaves the balance still, and that unequal wind or unequal arm-shape makes the vane swing round to align with the wind, its smaller arm meeting the oncoming air. Two lettered figures (c b; b, c) restate the rule that equal powers against equal resistances make no motion, while equal powers against unequal resistances move the weaker. Small mechanical sketches, a turned column and thin rods, appear in the margins, and an inverted heading reads 'On percussion.'
On this page
A pivoted rule set as a balance against the wind
If the rule is pivoted with equal arms and its breadth set against the wind like a balance, equal wind leaves it motionless; but unequal wind makes the arms turn a half-circle and align with the wind's course, driving the smaller arm into the oncoming air. If the arms are equal in length but unequal in shape, the balance likewise turns to face the wind.
Equal powers against equal and unequal resistances
A figure of a rule with unequal arms struck by the wind carries the rule that equal powers against equal resistances generate no motion. An analogous figure labelled b, c adds that equal powers against unequal resistances move the weaker one.
Bodies turned by the wind about their centre
Leonardo notes that the effect arises from beating the wings against still air, as motionless wings receive the percussion of the wind. Of bodies revolving about a centre, the one whose end is most distant from that centre obeys the course of the wind and moves along its line, and no such body turns except through differences in its parts or inequalities of the wind.