1 60 Royal Society : — 



two plates, each of area S, at a small distance, a, asunder, when 

 connected with the two poles of a battery of which the electro- 



motive force in electro-magnetic units is F, is —- ( — ) *,orin terms 



OTT \ aaj 



1 ^ / F \2 



of the weight of a grain — ( — ) . If F be the electromotive 



32' 2 Stt Vera/ 



force of 100 cells of Daniell's, which, as I have found from Joule's 

 observations, must be about 250,000,000t, and if a be ^th of a foot, 

 and S a square foot, I conclude from the preceding estimates for o-, 

 that the force of attraction between the plates cannot be less than 

 4'4 grains, nor more than 72 grains. 



It would be easy at any time to make a plan for observing tele- 

 graph indications by means of either Weber's electro-dynamometer, 

 or an instrument constructed on the same principle, or by mea- 

 suring thermal effects of intermittent currents, which could be 

 put in practice by any one somewhat accustomed to make observa- 

 tions, and which would give a tolerably accurate determination of 

 the element of time, even in cases where the observable retarda- 

 tion is considerably less than y^th of a second. A single wire in a 

 submarine cable would, as far as regards the physical deductions to be 

 made from this determination, be to be preferred to one of a number 

 of different wires insulated from one another under the same sheath- 

 ing. I have little doubt but the Varna and Balaklava wire will be 

 the best yet made for the purpose. 



Without knowing exactly what the " retardation " may be in 

 terms of the element of time "a" of the diagrams, we may judge 

 what the retardation, if similarly estimated, would be found to be in 

 other cables of stated dimensions. Thus, if the retardation in 200 

 miles of submarine wire between Greenwich and Brussels be ^i^th of 

 a second, the retardation in a cable of equal and similar transverse 

 section, extending half round the world (14,000 miles), would be 



/14000y J__^gQ seconds, or 81 minutes : 



V 200 / 10 



and in the telegraphic cable (400 miles) between Varna and 

 Balaklava, of which the electro-statical capacity per unit of length 

 may be about one-half greater than in the other, while the conduct- 

 ing power of the wire is probably the same, the retardation may be 

 expected to be 



( — I X-X — =— of a second. 

 V200y 2 10 5 



The rate at which distinct signals could be propagated to the 

 remote end would perhaps be one signal in about a quarter of an hour 

 in the former case, and nearly two signals in a second in the latter. 



* As was shown at the conclusion of a paper " On Transient Electric Currents," 

 published in June 1853, in the Philosophical Magazine. 



t See the paper referred to above, as published in the Phil. Mag., Dec. 1851. 



