Gravitation then is not a force at all; it is the fundamental nature of things. A body free to move through the world must follow some definite path. Euclid says it will stand still; Newton that it will traverse a straight line in three-space at uniform time-rate; Einstein that it will move in a "geodesic" through time-space--in every-day language, that it will fall. The numerical consequences of Einstein's theory are, within the limits of observation, the same as those of Newton's for all bodies save one--Mercury. This planet shows a small deviation from the path predicted by Newton's law; Einstein's theory gives its motion exactly. Again, when modern research showed that light must be affected by gravitation, Einstein's theory, because of the extreme velocity of light, deviates from Newton's, where the speed is less a determining factor; and observations of starlight deflected by the sun during the eclipse were in much better accord with Einstein's theory than Newton's. Moreover, the Special Theory predicts that mass is an observational variable like length and duration. Radioactive emanations have a velocity high enough to give appreciable results here, and the prediction is verified, tending to support the general theory by supporting its limiting case. We like always to unify our science; and seldom, after effecting a unification, are we forced to give it up. Einstein for the first time brings mechanical, electromagnetic and gravitational phenomena within one structure. This is one reason why physicists are so open minded toward his theory--they want it to be true. THE LAYMAN'S LAST DOUBT The final answer to any series of questions is inevitably "because the world is so constructed." The things we are content to leave on that basis are those to which we are accustomed, and which we therefore think we understand; those for which this explanation leaves us unsatisfied are those which are new and unfamiliar. Newton told us that the world of three-dimensional space with one-dimensional time superposed was so constructed that bodies left to themselves would go on forever in a straight line at constant speed. We think we understand this, but our understanding consists merely of the unspoken query, "Why, of course; what is there to prevent?" The Greeks, an intelligent people, looked at this differently; they would have met Newton with the unanimous demand "Why so; what is there to keep them going?" So if, in seeking an explanation of anything, we come sooner than we had expected to the finality "Because the world is so constructed," let us not feel that we have been cheated. IX THE PRINCIPLE OF RELATIVITY A Statement of What it is All About, in Ideas of One Syllable BY HUGH ELLIOT CHISLEHURST, KENT, ENGLAND