Conclusions of tlie Theory of Relativity. 847 



a distance is the velocity of a body multiplied by the time 

 taken during an event. If in mechanics static measurements 

 are fundamental and kinetic problems are secondary, now 

 the converse is true and no real advantage is gained. The 

 velocity of light is singularly nnsatisfactory as a standard 

 of measurement, since it must be expressed in terms of an 

 arbitrary static measure. In the next place the light con- 

 sidered is in vacuo and can be determined only by extra- 

 polation from measurements made when moving through 

 matter where it is not constant. Its constancy is simply the 

 fiat of Einstein, who claims that it we agree that the velocity 

 of light in vacuo shall be constaut and the length of the 

 Imperial Yard shall be variable : " There is not the lea^t 

 incompatibility between the principle of relativity and the 

 law of propagation of light, and that by systematically 

 holding fast to both these laws a logically rigid theory could 

 be arrived at." The sacrifice to logic is too great. 



When the velocity of light is determined the distance and 

 time are held to be mechanical. Unless we are considering- 

 abstract motion, the moving something we call light must be 

 an entity. Is this entity of a mechanical nature or is it non- 

 mechanical ? If it is mechanical, how can it differ in its 

 properties from all other bodies of a mechanical nature and be 

 absolute and unchangeable in its momentum and energy ? 

 And if light is of a different nature, how can we use it as a 

 basis for mechanical laws? Lastly, is the velocity of light 

 in any true sense, a velocity, — a transfer of a body from 

 position A to position B ? We usually regard radiation as a 

 periodic wave disturbance in a medium. Now even a material 

 wave in water has no velocity in the sense of the transfer of 

 matter from A to B. It is merely a series of particles 

 moving, one after the other, whose individual motions may 

 not even be in the direction AB. The velocity of a wave is 

 purely an abstract notion which states that in the space AB 

 a series of events occur consecutively. For example, a row 

 of persons may pass a word along from one to the other, and 

 I daresay it would be intelligible to speak of the velocity of 

 speech as the distance AB divided by the time interval. 

 How Einstein will pass from the abstract idea of the velocity 

 of light to the motion of a ponderable body, ho gives 

 no idea. 



Einstein lays great emphasis on the difficulty of measuring 

 the length of a moving object in terms of a static standard, 

 and it is this difficulty which causes him to accept the pos- 

 tulates that the length of a moving body is a function oE its 

 velocity and that time is affected by the motion oi the clock. 



