OCTOBEE 3, 1913] 



SCIENCE 



467 



respect to the measurement of time should 

 crystallize, and a new time concept be found. 

 Any regular process of nature may serve as 

 a measure of time; for example, the fall of 

 sand in the hour-glass, the swing of the pen- 

 dulum, the sun dial, or to he more modern, 

 an ideal watch which is regulated by a per- 

 fect spring and balance wheel. Let us 

 imagine we have two perfect watches, one in 

 San Francisco, the other in New York. How 

 can we synchronize or set them so that both 

 will indicate the same instant of time? To 

 synchronize them both at the factory, and 

 send one to New York and the other to San 

 Francisco, will not do, as we shall see later. 

 Since experiment appears to justify the as- 

 sumption that the velocity of light through 

 interstellar space is always the same, let us 

 use a light flash to synchronize the watch at 

 San Francisco with the one at New York. 



are then in synchronism for observers at these 

 two stations. The simultaneity of an occur- 

 rence at New York with one at San Francisco 

 can then be established by the two synchronized 

 watches. The connotation of the word simul- 

 taneity thus becomes very definite. 



In order to bring out nature's facts with 

 regard to time and space, which Einstein has 

 so clearly presented in mathematical form, we 

 have built a model, constructed briefly, as 

 follows : A triple lead-screw, eight feet long, 

 gives motion to the upper or moving system 

 when the crank at the right of the model is 

 in motion. By throwing in the proper gear- 

 ing at the crank shaft, a second lead-screw 

 supplies motion (toward right or left) to a 

 light particle {L), in the model, a pocket elec- 

 tric lamp resting upon a traveling nut. Two 

 worm wheels meshing with the first lead-screw 

 operate the hands of the lower or stationary 



\c> 



At twelve o'clock the observer in New York 

 sends a light flash to San Francisco, where a 

 mirror immediately reflects it back to him; he 

 finds it took thirty thousandths of a second 

 for the light signal to travel to San Francisco 

 and baclt; he reasons, therefore, that it took 

 fifteen thousandths of a second to travel one 

 way; he then writes the observer in San Fran- 

 cisco to set his watch at twelve o'clock plus 

 fifteen thousandths of a second, as soon as the 

 light flash again sent from New York at 

 twelve o'clock reaches him; the two watches 



clocks, while a pair of spur gear acting as 

 pinions upon a stationary rack move the clock 

 hands of the system in translation when the 

 latter is in motion. 



Following the method of Emil Cohn, of 

 Strassburg,° we shall speak of the stationary 

 system as the sun; the two sun clocks are 

 flxed to the sun and are sixty sun miles apart; 

 at each clock station is an observer, sun-man 

 A at the zero station, and sun-man B at the 

 sixty-mile station. 



'"Himmel imd Erde," 23: 117, 1911. 



