320 Lord Kelvin on the Motions of Ether 



rear, wholly within the cone, an ever growing disturbance of 

 ether, and therefore requires the application of a continual pull 

 forward to keep it moving uniformly at any constant velocity 

 exceeding the velocity of light. In 1888, Oliver Heaviside* 

 arrived at a corresponding conclusion by purely mathematical 

 work, from Maxwell's electromagnetic formulas, without any 

 dynamical foundation : and in 1897 f, still without assuming 

 any dynamical or chemical properties of ether and atoms, 

 he corrected an erroneous hypothesis, that no force however 

 great could give an atom a velocity equal to the velocity of 

 light, which has been somewhat extensively adopted within 

 the last ten years in speculations and reckonings regarding 

 radioactivity. 



§ 11. Purely dynamical reasoning J on our physical 

 assumptions of §§ 1, 6, 8, and 9, teaches us further that : — 



(a) No force is required to keep an atom moving uniformly 

 through ether, at any velocity less than the velocity of light. 



(6) To start an atom suddenly into motion from rest, causes 

 a spherical pulse to travel outwards with the velocity of light, 

 from the place in which the atom was when it was receiving 

 the supposed velocity. 



(c) The magnitude of this spherical pulse is a maximum in 

 the plane through the centre perpendicular to the line of 

 motion, and is zero at the two points in which the spherical 

 surface is cut by that line§. 



(d) This spherical pulse carries outwards through infinite 

 space a finite quantity of energy, /, due to a part of the 

 work, io, done by the force which was applied to the atom 

 to start it in motion. The sharper the suddenness of the 

 stopping, the greater is I. 



(e) If at any time a resisting force suddenly stops the atom, 

 work is done on the ether, in virtue of which another pulse 

 carries away an amount of energy, V ; and work is done on 

 the stopping agent amounting to w — I — /'. 



(/) If the suddenness of the stopping is equal and similar 

 to the suddenness of the starting, the second pulse is equal 

 and similar to the first, and V is equal to I. 



§ 12. To understand clearly the meaning of (e), take an 

 example. Let three equal and similar ideal non-electric atoms, 

 A, B, C, be given in a straight line ; B at rest, A moving 

 with velocity q towards B, and C moving towards B in the 

 contrary direction with a velocity just great enough that B is 



* Heaviside's ' Electrical Papers/ vol. ii. pp. 494,-516. 



t Heaviside's ' Electromagnetic Theory,' vol. ii., Appendix G. 



t ' Baltimore Lectures,' Appendix B, §§ 4.. .7. 



§ l Baltimore Lectures,' Lee. vin., p. 88 ; Lee. xiv., p. 197. 



