Gravity, Geoid and Height Systems In this book we shall go through all three stages of the development, devoting one chapter to Galileo’s pioneering work, six chapters to Newton’s ideas and their subsequent development, one chapter to Einstein, and one chapter to post-Einsteinian speculations concerning the relation between gravity and other physical phenomena. The emphasis on the “classics” in this outline grows from the fact that the theory of universal gravity is a classical theory. It is very probable that there is a hidden relation between gravity on the one hand and the electromagnetic field and material particles on the other, but nobody is prepared today to say what kind of relation it is. And there is no way of foretelling how soon any further important progress will be made in this direction.