Einstein found, then, that by taking the properties of space, as given by measurement, to be modified in the neighborhood of masses of matter, he could devise general laws according to which gravitational effects would be produced, and there would be no absolute rotation. All forces will be the same whether a body rotates with everything outside it fixed, or the body is fixed, and everything rotates round it. All motion is then relative, and the theory is one of "general relativity." The velocity of light is, however, no longer constant, and its path is not a straight line, if it is passing near gravitating matter. This does not contradict the special theory, which did not allow for gravitation. Rather, the special theory is a particular case to which the generalised theory reduces when there is no matter about, just as the Newtonian dynamics is a special case of the special theory, which we obtain when all velocities are small compared to that of light. XXII THE GENERAL THEORY Fragments of Particular Merit on This Phase of the Subject BY VARIOUS CONTRIBUTORS When Dorothy was carried by the cyclone from her home in Kansas to the land of Oz, together with her uncle's house and her little dog Toto, she neglected to lower the trap door over the hole in the floor which formerly led to the cyclone cellar and Toto stepped through. Dorothy rushed to the opening expecting to see him dashed onto the rocks below but found him floating just below the floor. She drew him back into the room and closed the trap. The author of the chronicle of Dorothy's adventures explains that the same force which held up the house held up Toto but this explanation is not necessary. Dorothy was now floating through space and house and dog were subject to the same forces of gravitation which gave them identical motions. Dorothy must have pushed the dog down onto the floor and in doing so must herself have floated to the ceiling whence she might have pushed herself back to the floor. In fact gravitation was apparently suspended and Dorothy was in a position to have tried certain experiments which Einstein has never tried because he was never in Dorothy's unique position.]188 The Principle of Equivalence, of which Einstein's suspended cage experiment is the usual illustration, and upon which the generalized theory of relativity is built, is thus stated by Prof. Eddington: "A gravitational field of force is precisely equivalent to an artificial field of force, so that in any small region it is impossible, by any conceivable experiment, to distinguish between them. In other words, force is purely relative."