250 PLEASANT WAYS IN SCIENCE. 



acts precisely as an enormous Leyden jar ; or rather, Fara- 

 day showed that such a cable, without the wire sheathing, 

 would act when submerged as a Leyden jar, the conducting 

 wire acting as the interior metallic coating of such a jar, 

 the gutta-percha as the glass of the jar (the insulating 

 medium), and the water acting as the exterior metallic 

 coating. Wheatstone showed further that such a cable, 

 with a wire sheathing, would act as a Leyden jar, even 

 though not submerged, the metal sheathing taking the part 

 of the exterior coating of the jar. Now, regarding the cable 

 thus as a condenser, we see that the transmission of a 

 current along it may in effect be compared with the passage 

 of a fluid along a pipe of considerable capacity, into which 

 and from which it is conveyed by pipes of small capacity. 

 There will be a retardation of the flow of water correspond- 

 ing to the time necessary to fill up the large part of the 

 pipe ; the water may indeed begin to flow through as 

 quickly as though there were no enlargement of the bore of 

 the pipe, but the full flow from the further end will be 

 delayed. Just so it is with a current transmitted through a 

 submarine cable. The current travels instantly (or with the 

 velocity of freest electrical transmission) along the entire 

 line; but it does not attain a sufficient intensity to be 

 recognized for some time, nor its full intensity till a still 

 longer interval has elapsed. The more delicate the means 

 of recognizing its flow, the more quickly is the signal re- 

 ceived. The time intervals in question are not, indeed, 

 very great With Thomson's mirror galvanometer, in which 

 the slightest motion of the needle is indicated by a beam 

 of light (reflected from a small mirror moving with the 

 needle), the Atlantic cable conveys its signal from Valentia 

 to Newfoundland in about one second, while with the less 

 sensitive galvanometer before used the time would be rather 

 more than two seconds. 



Now, in duplex telegraphy the artificial current must be 

 equal to the chief current in intensity all the time ; so that, 

 since in submarine telegraphy the current rises gradually to 



