440 THE POPULAR SCIENCE MONTHLY 



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 shorten- 

 ing 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 satis- 

 faction 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 i3 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 considera- 

 tions 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 



