128 INCREASE OF INERTIA [CH. xn. 



travels with a sufficiently rapid bullet, and is demon- 

 strated in Mr. Boys' bullet photographs. 



No known speed which exists in ordinary matter 

 is sufficient to bring any variation of inertia into 

 prominence. The quickest available carriage is the 

 earth in its journey round the sun, 19 miles a second, 

 or 60 times faster than a cannon ball ; but the 

 earth's velocity is only the 10000 of the speed of 

 light, and consequently any spurious inertia due 

 to its orbital motion is only 1 part in a hundred 

 million ; and even the accuracy of astronomy could 

 not display an effect of that order of magnitude. 



There are a very few stars which move 200 miles 

 a second, but even these have only one-tenth per 

 cent, of the speed of light, and the excess inertia 

 will be only 1 part in a million. The only known 

 place where charges or charged atoms were known, 

 prior to 1903, to move at speeds greater than this, 

 was in a vacuum tube. There the cathode-propelled 

 particles are flying 20,000 miles a second or T ] ^ n 

 the speed of light, and they may have 1 per cent, 

 excess inertia ; or more if they can be persuaded to 

 go still faster. 



But higher speeds are now known, being obtained 

 in the spontaneous emission of electrons and atoms 

 by radio-active materials ; so it becomes of the 

 greatest interest to determine the constants, and 

 especially the inertia, for rays of this kind. 



