CLASSICAL AND NEW MECHANICS 167 



bodies, Professor Einstein believes, lies in our deter- 

 mination of time, and clear thinking in regard to time 

 is nearly all that is necessary to clear up the trouble. 

 Suppose a body or a material point to be at rest rela- 

 tively to a coordinate system of three rectangular lines, 

 then its position can easily be determined by the ordi- 

 nary geometrical method of measuring its distance 

 from each of the lines by rigid measuring-rods. But 

 if the point is in motion with respect to the reference- 

 system, its position depends on time and cannot be 

 determined by the geometrical method. Our idea of 

 time is usually defined by what we call isochron- 

 ism; we say an event occurs at seven o'clock, 

 when the occurrence of the event and the position of 

 the hour-hand of a clock at seven are simultaneous. 

 But suppose the clock were at some distance from us, 

 then we could observe the hand to be at seven only 

 after it had passed beyond that figure, since it would 

 take some time by any method of transmission for the 

 intelligence to reach us. As the velocity of light is the 

 greatest of all known motions, the least discrepancy 

 would be caused by using light signals as the mode 

 of transmitting such intelligence. Also, if we accept 

 the second postulate, our intelligence will be still more 

 accurate because the velocity of light is unaffected by 

 other motions and we thus avoid the difficulty caused 

 by the question whether any relative motion between 

 us and the clock is an approach or a separation. To 



