418 Prof. 0. Lodge on Absolute Velocity and 



4. /Etherial Inertia does not Influence Mechanical Actions; 



by which I mean that it does not enter into mechanical 

 equations, that in all attractions and repulsions of matter 

 nothing has to be allowed for in respect of the mass of any 

 connecting mechanism, that everything occurs as if action 

 were really "at a distance." The statement is in fact 

 equivalent to the following alternative form: — 



M . The hypothesis of Action at a Distance accurately 

 accounts for all the results of Astronomical Attractions 

 and Mechanical Collisions. 



Consider two unequal balls driven apart by a spiral spring, 

 or consider a gun and bullet driven apart by exploding 

 powder; it is not true to say that m 1 v 1 + m 2 v 2 = 0, a third term 

 m s v s must be introduced to represent the momentum of the 

 spring or of the powder gases : a statement emphatically 

 obvious in the case of a rocket. 



No such third term is needed to express correctly the 

 results of the impact of elastic particles, nor of the gravitative 

 attractions of two unequal masses : at any rate if such a third 

 term is necessary it has yet to be discovered, it is too minute 

 to have been so far noticed. The only case in which its 

 existence has hitherto been suspected is in Maxwell's u Pres- 

 sure of Light." Assuming that to be a real phenomenon, and 

 it can hardly be doubted on electromagnetic principles, it w T ould 

 seem that any illuminated body, or any non-uniform source 

 of light, is like a rocket reacting on the vibrating sether. 

 It might be urged that the stress really acts between the 

 source and the body on which the light falls, but the finite 

 rate of wave-propagation forbids the universality of this ; and 

 the exceptional character thus required of the wave-impact 

 force is noteworthy. Possibly it is going to constitute a most 

 important exception, the first of a large class of forces for 

 which the usual interpretation of the third law of motion may 

 have to be enlarged. If gravitation were transmitted at a 

 finite rate, a similar phenomenon would occur there ; and the 

 force in either case would be a function of the motion of a 

 body, for a body moving away with the speed of light (or of 

 gravitation respectively) would not feel it. 



We will return to the consideration of this most interesting 

 setherial force, but for the present we will deal with the usual 

 interpretation of the third law, viz that the action and reaction 

 are on material bodies only. 



Now three sufficient reasons may be assigned as to ivhy 

 petheriai inertia should not enter into mechanical equations 

 (assuming for the present that it never does), viz. the following, 

 as alternatives : — 



