TIME AND MOTION 81 



ness the collapse of a building, the burning of a 

 forest. How ludicrous is a moving picture film 

 which is reeled backward! This dissymmetry 

 enters, not in the pure science of kinematics, 

 but in some further and more complex stage of 

 science, and we must be on the lookout for its 

 first appearance. Finally let me express my 

 opinion that the phrase which is commonly 

 heard to-day, "Time is the fourth dimension of 

 space," is highly misleading. Neither should we 

 say that space is the second, third and fourth 

 dimensions of time. What we mean to say is that 

 the three dimensions of space and the one di- 

 mension of time may be combined, and natu- 

 rally do combine, to form the simple four- 

 dimensional manifold of kinematics. 



It would be out of the question to enter here 

 upon an exhaustive demonstration of the iden- 

 tity between the kinematics of relativity and 

 the geometry of asymptotic rotation, but I may 

 give a few examples. One of the most striking 

 of Einstein's results is the following. If three 

 bodies are moving in the same straight line, and 

 if ^\ is the relative velocity of A and B, and v^ 

 the relative velocity of B and C, then ^3, which 

 is the relative velocity of A and C, will not be 



