CH. xu.] DUE TO RAPID MOTION 127 



ever we can afford to neglect quantities of this order,, 

 the field and therefore the inertia of the moving 

 charge will continue practically constant. 



But when its speed of motion begins to approach 

 the velocity of light, say even no more than T Vth 

 of that speed, then a perceptible disturbance is 

 to be expected, and something like a 1 per cent, 

 increase of inertia must occur. 



The complete investigation makes the inertia 

 infinite when the speed reaches that of light (see 

 Appendix K), but there is probably no need to 

 press this to extremes, unless the charge were an 

 absolute point ; clearly, however, the inertia will 

 ! then be very great, and possibly therefore it may 

 always be impossible to make matter, or at least 

 charged matter, move with a speed greater than 

 that of light. There may be ways out of this, 

 j however, just as it is possible for a bullet to move 

 through air with a velocity greater than that of 

 sound. This is managed by the violent adiabatic 

 condensation of the air in front of such a bullet, 

 the effect being to raise the appropriate velocity of 

 sound to the required value ; and by the ridge behind 

 it where discontinuity makes its appearance. It seems 

 unlikely that the ether can adjust itself to excessive 

 speed beyond the speed of light without a change of 

 structure akin to what would be rupture in the case 

 of a material medium. 



It has been shown both by Mr. Heaviside and 

 by Prof. J. J. Thomson that if the speed of motion 

 is ever greater than that of light, the fan or radial 

 plane of lines of force bends backwards and becomes 

 a conical surface, gradually closing up as the speed 

 further increases : in accordance with the analogy of 

 the conical surface of discontinuity aforesaid, which 



