slower for you than for him, and if against their direction they would be faster for you than for him. Of course the immense velocity of the light waves, about 200,000 miles a second, would make the difference in a movement of four miles an hour so infinitesimal as to be altogether inappreciable, but it would not be nothing, and you would feel quite confident that if it could be measured the infinitesimal quantity would appear in the result. Now suppose that we could measure it with absolute accuracy, and that the result was the discovery that the supposed difference did not exist at all--and of course, we suppose that there is no doubt whatever about the measurement--what, then, should we be obliged to think? We should be forced to believe that as the velocity of light was the same for the two observers, one moving, one at rest, therefore the space and the time must be different for each. Now, however strange it may seem, such a measurement has been made, and with this surprising result. In consequence there has been formulated a new principle in Physics named the Principle of Relativity. I take this Principle of Relativity for my illustration because it is based on reasoning that practically admits of no doubt, and because {50} it requires us to form new conceptions of space and time which seem to alter fundamentally what we have hitherto considered as the evident and unmistakable nature of those realities. It has always seemed that the distance separating two points, and the interval of time separating two events, were each independent of the other and each absolute. However different the distance and the interval may appear to observers in movement or to observers in different systems of movement in relation to ourselves and to one another, in themselves they are the same distance and the same interval for all. They are the same for the man in the express train as for the man standing on the station platform. The Principle of Relativity requires us to think that this is not so, but that, contrary to all our settled notions, the actual space and time vary--really undergo an alteration, a contraction or expansion--with each different system of movement of translation to which the observer is bound. Events that for an observer belonging to one system of movement happen in the same place, for another observer in a different system of movement happen in different places. Events that for one observer happen simultaneously, for other observers are separated by a time interval according to the movement of translation of the system to which they belong. So that space, which Newton described as rigid, and time which he described as flowing at a constant rate, and which for him was absolute, are for the new theory relative, different for an observer in every different system of movement of translation. Or we may state it in the opposite way, and say that the Principle of Relativity shows us that the reason why