the earth in its orbit, which is about 19 miles per second, the shortening of the earth's diameter due to this velocity as seen by an observer at rest relative to the earth would be approximately a couple of inches only. Similarly for the relative motion of the earth and the sun, the shortening of the time unit would be approximately one second in five years. Even if this were the highest relative velocity known, the results would still be of importance, but the earth is by no means the most rapid in its movement of the heavenly bodies, while the velocity of the radium discharge is some thousand times the velocity of the most rapidly moving planet.
In addition to space and time there is a third fundamental concept of mechanics, though the physicists have not yet settled to the satisfaction of everybody whether it is force or mass. But in any case, the one taken as the fundamental motion, mass say, is, in the classical mechanics, independent of the velocity. Mass is usually defined in physics as the quantity of matter in a body, which means simply that there 13 associated with every body a certain indestructible something, apart from its size and shape, independent of its position or motion with respect to the observer, or with respect to other masses. But in the relativity mechanics this primary concept fares no better than the other ones, space and time. Without going into the details of the argument by means of which the new results are obtained, and this argument, and the experiment underlying it, are by no means simple, it may suffice to say that the mass of a body must also be looked upon as depending on the velocity of the body. This result would seem at first glance to introduce an unnecessary and almost impossible complication in all the considerations of mechanics, but as a matter of fact exactly the opposite is true. It has been known for some time, that electrons moving with the great velocity of the electric discharge, have suffered an apparent increase of mass or inertia due to this velocity, so that physicists for some time have been accustomed to speak of material mass and electromagnetic mass. But now in the light of the principles of relativity, this distinction between material mass and electromagnetic mass is lost, and a great gain in generality is made. All masses depend on velocity and it is only because the velocity of the electric discharge approaches that of light, that the change in mass becomes striking. This perhaps may be looked upon as one of the most important of the consequences of the theory of relativity in that it subjects electromagnetic phenomena to those laws which underlie the motions of ordinary bodies.
In consequence of this revision of our notions of space, time and mass, there result changes in the derived concepts of mechanics, and in the relations between them. In fact the whole subject of mechanics has had to be rewritten on this new basis, and a large part of the work of those interested in the relativity theory has been the building up of the
