Definitions and Hydrostatics

Abstract

From the viewpoint of a man living on the surface of the earth, the world appears to be a ball of rock covered with an envelope of air, with water forming the oceans. This superficial view might lead him to believe that, similarly, the universe consists of solid bodies. But this is of course not so. The earth has a solid crust but its core probably consists of molten iron, while the stars are balls of very hot plasma and the galaxies appear to rotate as vortices. Where galaxies collide the pattern of star streams observed closely resembles turbulent fluid flow. Solid bodies are found only on the surface of planets, in asteroids and in the small-scale debris of planetary space.