1867.] Physics. 287 



replace the paper by a very thin layer of mica. The mica is a 

 better insulator ; it does not absorb damp ; it cannot produce car- 

 bon, since it is of a mineral character ; moreover, its ready cleavage 

 allows of its being split into plates thinner than a sheet of paper. 

 The administration of the French telegraphic lines purposes placing 

 very shortly a great number of these lightning protectors in places 

 subject to most frequent attack by the electric fluid of the atmo- 

 sphere. 



The lecture theatre of the Koyal Institution was crowded on 

 Friday evening, the 15th of February, to hear a discourse, by Mr. 

 Cromwell Varley, " On the Atlantic Telegraph." On the table 

 there were displayed coils of the cable successfully laid last year, 

 and of the cable which had been picked up from a depth of two 

 miles, having lain at the bottom of the sea for twelve months. 

 Mr. Varley did not enter into the consideration of the mechanical 

 construction of the cable, nor of the plan of laying it down, but 

 confined himself to the scientific part of the subject, and to the 

 explanation of the difficulties which arise in attempting to transmit 

 electric signals through great lengths of submerged insulated wire. 

 By means of his artificial cables he demonstrated, for the first time 

 in public, how the electric waves travel through. He also showed 

 what were the limits imposed by nature to rapid signalling. 



A telegraphic cable is a long Leyden jar, one end of which is 

 attached to the earth, whilst the other is attached to a source of 

 electricity each time a signal is to be produced. If the cable be 

 connected to a battery for a long time, the strength of the charge 

 in the different parts of the cable will be nothing at the end next 

 the earth, and equal to the full power of the battery at the other 

 end. 



The lecturer had diagrams showing the rate at which the 

 strength of the electric currents augmented or died away at the 

 distant end of the cable, when a battery was applied or removed 

 from the other end. From these diagrams and experiments it was 

 shown that to get rapid signals the receiving instrument must be 

 very sensitive, and the moment a current begins to appear at the 

 distant end the rest of the electric wave must be neutralized, to 

 permit a second signal to follow. This was illustrated by an 

 artificial line equal to a cable 13,000 miles long. This fine was 

 divided into eleven sections, and ten reflecting galvanometers were 

 inserted at equal distances along it. To familiarize the audience 

 with the relative position of the galvanometers, Mr. Varley called 

 No. 1, Gibraltar ; No. 2, Malta ; 3, Suez ; 4, Aden ; 5, Bombay ; 

 6, Calcutta ; 7, Bangoon ; 8, Singapore ; 9, Java ; and 10, 

 Australia. 



