Dr. Oliver Lodge on Opacity. 387 



place of 4:7t/jl/o; that is, of electric conductivity; otherwise the 

 heat solution is the same as (2). The 4-7T has come in from 

 an unfortunate convention, but it is remarkable that the con- 

 ductivity term is inverted. The reason of the inversion of 

 this constant is that, whereas the substance conveys the heat 

 waves, and by its conductivity aids their advance, the aether 

 conveys the electric waves, and the substance only screens 

 and opposes, reflects, or dissipates them. 



This is the case applied to sea-water and low frequency by 

 Mr. Whitehead in a paper which he gave to this Society in 

 June 1897, being prompted thereto by the difficulty which 

 Mr. Evershed and the Post Office had found in some trials of 

 induction signalling at the Goodwin Sands between a coil 

 round a ship at the surface and another coil submerged at a 

 depth of 10 or 12 fathoms. It was suspected that the con- 

 ductivity of the water mopped up a considerable proportion 

 of the induced currents, and Mr. Whitehead's calculation 

 tended, or was held to tend, to support that conclusion. 



To the discussion Mr. Heaviside communicated what was 

 apparently, as reported, a brief statement ; but I learn that in 

 reality it w T as a carefully written note of three pages, which 

 recently he has been good enough to lend me a copy of. In 

 that note he calls attention to a theory of the whole subject 

 which in 1887 he had w T orked out and printed in his collected 

 ' Electrical Papers,' but which has very likely been over- 

 looked. It seems to me a pity that a note by Mr. Heaviside 

 should have been so abridged in the reported discussion as 

 to be practically useless ; and I am permitted to quote it here 

 as an appendix (p. 113). 



Meanwhile, taking the diffusion case as applicable to sea- 

 water with moderately low acoustic frequency;, we see that the 

 induction effect decreases geometrically with the thickness of 

 the oceanic layer, and that the logarithmic decrement of the 



amplitude of the oscillation is \/( )> where a is the 



specific resistance of sea-water and pftir is the frequency. 



Mr. Evershed has measured a- and found it 2 x 10 10 C.G.S., 

 that is to say 2 x 10 10 fx square centim. per second ; so putting 

 in this value and taking a frequency of 16 per second, the 

 amplitude is reduced to 1/eth of what its value would have 

 been at the same distance in a perfect insulator, by a depth 



/ a /( 2 x 10 1( y \ _ /1(P_10 5 



V 2*/*p V V2irii, x 2tt x 16 / " V 320 18 centllu - 



= 55 metres. 

 Four or five times this thickness of intervening sea would 



