This every-day experience is a good illustration of the much discussed Principle of Relativity, in its simplest form. If there were no jolting, the motion of the train, straight ahead at a uniform speed, would have no effect at all upon the relative motions of objects inside it, nor on the forces required to produce or change these motions. Indeed, the motion of the earth in its orbit, which is free from all jar, but a thousand times faster, does not influence even the most delicate apparatus. We are quite unconscious of it, and would not know that the earth was moving, if we could not see other bodies outside it. This sort of relativity has been recognized for more than two centuries and lies at the bottom of all our ordinary dynamical reasoning, upon which both science and engineering are based. But there are other things in nature besides moving bodies,--above all, light, which is intimately related to electricity and magnetism, and can travel through empty space, between the stars. It moves at the enormous speed of 186,000 miles per second, and behaves exactly like a series of vibrations or "waves." We naturally think of it as travelling through some medium, and call this thing, which carries the light, the "ether." Can we tell whether we are moving through this ether, even though all parts of our apparatus move together, and at the same rate? Suppose that we have two mirrors, $M$ and $N$, at equal distances, $d$, from a point $O$, but in directions at right angles to one another, and send out a flash of light from $O$. If everything is at rest, the reflected flashes will evidently come back to $O$ at the same instant, and the elapsed time will be $2d/c$ seconds if $c$ is the velocity of light. But suppose that $O$, $M$, and $N$ are fastened to a rigid frame work, and all moving in the direction $O M$, with velocity $V$. The light which goes from $O$ toward $M$, at the speed $c$, will overtake it with the difference of their speeds, $c-v$, taking $d/(c-v)$ seconds to reach $M$. On the way back, $O$ will be advancing to meet it, and the return trip will occupy $d/(c+v)$ seconds. The elapsed time for the round trip comes out $2cd/(c^2-v^2)$ seconds, which is longer than when the system was at rest--the loss of time in the "stern chase" exceeding the saving on the return. The light which is reflected from N has a different history. When it starts, $O$ and $N$ have certain positions in the ether, $O_1$ and $N_1$. By the time it reaches the mirror, this is at $N_2$, and $O$ is at $O_2$, and when it returns, it finds $O$ at $O_3$. The distances for the outward and inward journeys are now equal, but (as is obvious from the figure), each of them is greater than $d$, or $O_2N_2$, and the time for the round trip will be correspondingly increased. A simple calculation shows that it is $2d/\sqrt{c^2-v^2}$.