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 corresponding 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 received. 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.