266 POPULAR SCIENCE MONTHLY 



regarded it merely as an extremely sensitive galvanometer without 

 knowing how it came into existence. It was devised by Thomson to 

 enable him to utilize his mathematical solution of signaling through a 

 long submarine cable, and was regarded of such national importance 

 that a private act of Parliament was sanctioned by the Privy Council 

 to extend the normal life of fourteen years for the patent of this cable 

 " speaking instrument/' as it was originally called. The invention of 

 this instrument marks the theoretical solution of a most important 

 problem, which solution Thomson found great difficulty in getting the 

 electrician of that day to accept. 



As early as 1855 — before any long submarine cable had been con- 

 structed — Thomson published in the Proceedings of the Royal Society 

 the theory of the propagation of signals through a cable based on a 

 correspondence which he had had with the late Sir Gabriel Stokes, and 

 he showed that the book " Fourier de la Chaleur " — that " mathemat- 

 ical poem," as he used to call it — contained, in Fourier's mathematical 

 equations of the flow of heat, the entire mathematical solution of the 

 propagation of electric waves through a cable. From " Fourier's 

 series" he deduced that, whereas on a short overhead telegraph line 

 the signal reaches its full strength at the distant end practically as 

 soon as the signaler at the near end of the line begins to send it, with 

 a submarine cable it is retarded, spreads out, and blurs the next signal. 

 There is a past history effect as in politics and in many natural phe- 

 nomena. The passing of an act of Parliament can not suddenly change 

 a people; indeed, it is well known that the actual effect of an act of 

 Parliament promoted by well-wishers is often gradually found to be 

 most harmful, and has to be repealed or curbed in its action. 



Herbert Spencer in his " Sociology " strongly advocated legislators 

 to study the science of politics. Thomson would perhaps have said, 

 " Study Fourier's mathematical poem." 



If it were attempted to send a series of electric signals through an 

 Atlantic cable with the same apparatus and at the same speed as mes- 

 sages are sent between London and Brighton, the signaler at the far 

 end would not have the slightest knowledge that the signaler at this 

 end was trying to send a message, whatever were the strength of the 

 current sent into the cable. To work a long submarine cable, either 

 time must be allowed for each signal to grow at the distant end, or, 

 as this would make the sending of messages very slow, the receiving 

 instrument and the signaler receiving the message must, like a clever 

 doctor diagnosing a disease, be able to interpret mere indications. 

 Sending the letter " e," for example, produces at the other end of a 

 long cable a totally different result, depending on what has preceded it. 

 In no case, at a speed of, say, thirty words a minute with a 3,000-mile 

 cable, will it be more than a suggestion, even at the beginning of a 



