Simple and compound motion; the descent of flame
Also the sun's image on a sphere, the eye on a mercury grain, and cutting glass for inlay
The verso of f.140 defines motion as simple or compound: simple motion is a body turning about its pole without changing place (a wheel or millstone) or shifting place without turning, while compound motion combines both, as in a cart wheel. The recto of f.139 turns to optics and fire, noting how the image of the sun grows and shrinks on a spherical body at varying distances, and why a descending flame slows as it consumes and lightens its fuel. A closing note describes cutting thinned glass with scissors to set over gilt bone inlays for a durable lustre. Both sides are written in Leonardo's mirror script.
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Simple and compound motion
Motion is divided into simple and compound. Simple motion is either a body turning about its pole without changing place, like a wheel or millstone, or a body shifting place without revolving on itself; compound motion combines both, as in the wheels of a cart. A complete revolution of a cylinder in compound motion covers as much space as its surface touches.
The sun's image on a sphere and the eye on a mercury grain
Leonardo notes the growing and shrinking of a spherical body at various distances, and the corresponding growing and shrinking of the image of the sun upon it, and of the eye upon the grain of quicksilver.
Why a descending flame slows
The burning matter that feeds a flame slows the further it descends through the air, because during the time of its descent the flame consumes its fuel, and in consuming itself it gains lightness and so becomes slower. The inflamed body turns in a contrary motion near the end of its descent.
Cutting glass for inlay work
Thinned glass, cut with scissors, can be set over inlays of gilded bone or other colours and fitted together; it keeps a lustre that neither breaks nor wears away when rubbed by hand.