XVII.--The Physical Side of Relativity: The Immediate Contacts between Einstein's Theories and Current Physics and Astronomy. By Professor William H. Pickering, Harvard College Observatory, Mandeville, Jamaica 287 XVIII.--The Practical Significance of Relativity: The Best Discussion of the Special Theory Among All the Competing Essays. By Prof. Henry Norris Russell, Princeton University 306 XIX.--Einstein's Theory of Relativity: A Simple Explanation of His Postulates and Their Consequences. By T. Royds, Kodaikanal Observatory, India 318 XX.--Einstein's Theory of Gravitation: The Discussion of the General Theory and Its Most Important Application, from the Essay by Prof. W. F. G. Swann, University of Minnesota, Minneapolis 327 XXI.--The Equivalence Hypothesis: The Discussion of This, With Its Difficulties and the Manner in Which Einstein Has Resolved Them, from the Essay by Prof. E. N. da C. Andrade, Ordnance College, Woolwich, England 334 XXII.--The General Theory: Fragments of Particular Merit on This Phase of the Subject. By Various Contributors 338 I. THE EINSTEIN $5,000 PRIZE HOW THE CONTEST CAME TO BE HELD, AND SOME OF THE DETAILS OF ITS CONDUCT BY THE EDITOR In January, 1909, an anonymous donor who was interested in the spread of correct scientific ideas offered through the Scientific American a prize of $500 for the best essay explaining, in simple non-technical language, that paradise of mathematicians and bugaboo of plain ordinary folk--the fourth dimension. Many essays were submitted in this competition, and in addition to that of the winner some twenty were adjudged worthy of ultimate publication. It was felt that the competition had added distinctly to the popular understanding of this significant subject; that it had done much to clear up popular misconception of just what the mathematician means when he talks of four or even more dimensions; and that it had therefore been as successful as it was unusual in character. In November, 1919, the world was startled by the announcement from London that examination of the photographs taken during the total solar eclipse of May 29th had been concluded, and that predictions based upon the Einstein theories of relativity had been verified. In the reaction from the long surfeit of war news an item of this sort was a thoroughly journalistic one. Long cable dispatches were carried in the news columns all over the world; Einstein and his theories were given a prominent place on the front pages day after day; leading scientists in great number were called upon to tell the public through the reportorial medium just what the excitement was all about, just in what way the classical scientific structure had been overthrown.