Why the sphere of water has a spherical surface
The element of water: each particle rests, walled by neighbours of equal height
Continuing the proof begun on the facing page, Leonardo argues that no part of the water's surface moves unless it can descend. Since every particle of the sphere of water is ringed by others equidistant from the centre of the world, each sits in a "vessel" whose rims are of equal height and therefore cannot move; the whole surface is thus necessarily spherical, though its underside need not be. He extends the argument to the surface of air bordering the sphere of fire, which may evaporate into cloud-like vapour drawn up by the sun's heat, and holds that fire is hot by its own essence. He concludes that, having proved the flexible elements' spheres to be spherical, he will next treat the nature of each element in turn: first fire, then air, then water.
On this page
No part of the water's surface moves unless it descends
No part of the surface of the water moves of itself unless it descends. Since the sphere of water has no surface anywhere from which it could descend, of necessity it does not move of itself, and its surface must be spherical.
Each particle is walled by equal-height neighbours and stays still
Every least particle of the surface is surrounded by like particles equidistant from the centre of the world, so each is enclosed in a vessel whose rim is of equal height; being thus walled by equal banks, it cannot move of itself, and the surface must be spherical, though the underside need not be.
The air-fire boundary and the evaporation of the elements
What is said of the surface of water bordering air applies to the surface of air bordering fire, which may often vaporize into clouds drawn up by the sun's heat; fire is hot by its own essence. Having proved the spheres of the flexible elements to be spherical, he will treat each element in turn: first fire, then air, then water.