of Physical Knowledge. 65 



asserts that the velocity of the propagation of light in open 

 space is independent of the motion of the source of light. 

 If, therefore, we assume that Relativity is experimentally 

 established, we must sacrifice either the principle of the 

 Constancy of the Velocity of Light or the Mutual Inde- 

 pendence of Space and Time. 



For let us consider a simple example. Let a time signal 

 be given out by Wireless Telegraphy from a central station, — 

 say the Eiffel Tower, — as provided by the International Time 

 Service already projected. Then all stations in the vicinity 

 which are at the same distance from the central station 

 receive the signal at the same time, and can set their clocks 

 accordingly. But this kind of time regulation becomes 

 theoretically faulty, if accepting the relativity of all motion 

 we transfer our standpoint from the earth to the sun, whence 

 we must regard the earth as moving. For, according to the 

 principle of the Constancy of the Velocity of Light, it is 

 clear that those stations which, seen from the central 

 station, lie in the direction of the earth's motion, receive the 

 signal later than those lying in the opposite direction, be- 

 cause the former stations are moving on in advance of the 

 light waves which they have to receive, and must be 

 overtaken by them, whereas the latter stations travel to meet 

 the waves. Thus the principle of the Constancy of the 

 Velocity of Light renders impossible an absolute determin- 

 ation of time which shall be independent of the motion of the 

 observer. The two are incompatible. So far as the conflict 

 has proceeded, the principle of the Constancy of the Velocity 

 of Light has been decidedly victorious, and in spite of many 

 doubts which have latterly been raised, it is not at all 

 probable that any abandonment of that position will occur. 



The third of the above theories concerns the Continuity 

 of all Dynamical Effects. This was formerly taken for 

 granted as the basis of all physical theories, and, in close 

 correspondence with Aristotle, was condensed into the well- 

 known dogma — Natura non facit saltus. But even in this 

 venerable stronghold of Physical Science present-day in- 

 vestigation has made a considerable breach. This time it is 

 the principles of Thermodynamics with which that theorem 

 has been brought into collision by new facts, and unless all 

 signs are misleading, the days of its validity are numbered. 

 Nature does indeed seem to make jumps — and very extra- 

 ordinary ones. As an illustration, let me make an instructive 

 comparison : — 



Let us imagine a sheet of water in which strong winds 

 have produced high waves. Even after the total cessation 



Phil. Mag. S. 6. Vol. 28. No. 163. July 1914. F 



