Theoretically, one of these assumptions is as good as any other. Hence we no longer believe in absolute space and time. This is of great interest philosophically. Practically, it is unimportant, for, unless our choice of an assumption is very wild, our conclusions and measurements will agree substantially with those which are already familiar. Finally, the "general" relativity shows that gravitation and electro-magnetic phenomena--(including light) do not form two independent sides of nature, as we once supposed, but influence one another (though slightly) and are parts of one greater whole. XIX EINSTEIN'S THEORY OF RELATIVITY A Simple Explanation of His Postulates and Their Consequences BY T. ROYDS, KODAIKANAL OBSERVATORY, INDIA Einstein's theory of relativity seeks to represent to us the world as it really is instead of the world of appearances which may be deceiving us. When I was in town last week to buy 5 yards of calico I watched the draper very carefully as he measured the cloth to make sure I was not cheated. Yet experiment can demonstrate, and Einstein's theory can explain, that the draper's yardstick became longer or shorter according to the direction in which it was held. The length of the yardstick did not appear to me to change simply because everything else in the same direction, the store, the draper, the cloth, the retina of my eye, changed length in the same ratio. Einstein's theory points out not only this, but every case where appearances are deceptive, and tries to show us the world of reality. Einstein's theory is based on the principle of relativity and before we try to follow his reasoning we must spend a little time in understanding what he means by "relativity" and in grasping how the idea arises. Suppose I wish to define my motion as I travel along in an automobile. I may be moving at the rate of 25 miles an hour relative to objects fixed on the roadside, but relative to a fellow-passenger I am not moving at all; relative to the sun I am moving with a speed of 18 1/2 miles per second in an elliptical orbit, and again relative to the stars I am moving in the direction of the star Vega at a speed of 12 miles per second. Thus motion can only be defined relative to some object or point of reference. Now this is not satisfactory to the exact scientist. Scientists are not content with knowing, for example, that the temperature of boiling water is +100° C. relative to the temperature of freezing; they have set out to determine absolute temperatures and have found that water boils at 373° C. above absolute zero. Why should I not, therefore, determine the absolute motion of the automobile, not its motion relative to the road, earth, sun or stars, but relative to absolute rest?