632 EEPORT— 1892. 



cannot exceed ten metres a second for a current density of one ampere per square 

 centimetre. This, then, is the upper limit for a possible velocity of the medium. 

 Can we iind a lower limit ? The answer to that question depends on the inter- 

 pretation of a well-known experiment of Fizeau's, who found that the speed of light 

 is increased if it travels through water which moves in the same direction as the 

 light. If this experiment implies that the water carries the ether with it, and if a 

 motion of the ether means an electric current, we should be led to the conclusion 

 that a current of water should deflect a magnet in its neighbourhood. An experi- 

 ment made to that effect would almost certainly give a negative result, and would 

 give us a lower limit for the velocity of the medium corresponding to a given current. 

 iSuch an experiment, together with that of Eayleigh, would probably dispose of 

 the theory that an electric current is due to a translatory velocity of the medium. 

 This would be an important step, and it would be worth while to arrive at a 

 final settlement of the matter.^ The whole question of the relation between tlie 

 motion of matter and motion of the medium is a vital one, and we shall probably 

 not make any serious advances until experiment has found a new opening. But we 

 must expect many negative results before some clue is discovered. Nor can we 

 attach much importance to negative results unless they are made by someone in 

 whose care and judgment we place full reliance. We should all the more, there- 

 fore, recognise the courage and perseverance of those who spend theii" valuable 

 time in such investigations as Professor Lodge has recently undertaken. That 

 ultimately some relation will be found between moving matter and electrical 

 action there is no reasonable doubt. 



One of the most hopeful openings for new investigations has always been found 

 in the pursuing of a theory to its logical conclusions, and there is one result of 

 the electromagnetic theory of light which has not, in my opinion, received the 

 share of attention which it deserves. 



When sound passes through air it is propagated more quickly with the wind 

 than against it, and we may easily find the velocity relative to the earth by com- 

 bining the ordinary sound velocity with the velocity of the wind. Similarly, when 

 any waves pass through a medium moving with uniform velocity, the waves being 

 due to internal stresses in the medium, we may treat of the velocity of the waves 

 independently of that of the medium, and say that the wave-velocity in the direc- 

 tion of motion of the medium, and relative to a fixed body, is the sum of the wave- 

 velocity calculated on the supposition that the medium is at rest and the velocity 

 of the medium. Professor J. J. Thomson,'^ applying Maxwell's equations, has 

 arrived at a different result for electromagnetic waves, and has come to the conclu- 

 sion that in order to get the velocity of light along a stream of flowing water we 

 have to add to the velocity of light only half the velocity of water. The follow- 

 ing considerations suggest themselves to me with respect to this result. Maxwell's 

 theory is founded on certain observed effects, which all depend on the relative 

 motion of matter. A result such as the one referred to implies actions depending 

 on absolute motion, and appears therefore to point to something which has been 

 introduced into the equations for which there is no experimental evidence. The 

 only assumption clearly put down by Maxwell is that electromagnetic actions are 

 transmitted through the medium, and it is possible that that assumption necessarily 

 carries Professor J, J. Thomson's result with it. If a careful examination of the 

 subject should show that this is the case, we are brought face to face with a serious 

 difficulty. It is .said, with justice, to be one of the great advantages of Maxwell's 

 theory that it does away with action at a distance ; but what do we gain if we 

 replace action at a distance by something infinitely more diflicult to conceive, 

 namely, internal stresses of a medium depending on the velocity of the medium 



' Fizeau's result must either be due to the motion of matter through the medium 

 or to the fact that moving matter carries the ether with it. If it is due to the 

 former cause, and matter does not carry the ether with it, may we not consider that 

 matter moving through the ether, that is a relative motion of matter and ether, must 

 produce effects equal and opposite to those of ether moving through matter ? In 

 that case the reasoning in the text would, mutatis mutandis, hold good. 

 ^ Phil. Mag., vol. ix. 1880, p. 284. 



