158 Royal Society : — 



pose. This has enabled me to make the estimate, and so has con- 

 vinced me that a kind of experiment which I proposed in a paper on 

 Transient Electric Currents in the Philosophical Magazine for June 

 1853, and which I hope to be able before long to put in practice, will 

 be successful in giving a tolerably accurate comparison of the electro- 

 statical and electro- dynamic units ; and, with a further investiga- 

 tion of the specific inductive capacity of gutta percha which will 

 present no difficulty, will enable me to give all the data required for 

 estimating telegraph retardations, without any data from telegraphic 

 operations. This experiment is simply to put two plane- conducting 

 discs in communication with the two poles of a Daniell's battery (or 

 any other battery of which the electromotive force is known in 

 electro- magnetic units), and to weigh the attraction between 

 them. I now find that 100 cells of Daniell's so applied would 

 give a force of not less than four grains between two discs each 

 a square foot in area, and placed y^th of a foot apart. As the 

 force varies inversely as the square of the distance between the discs, 

 the weighing will be rather troublesome in consequence of instabi- 

 lity, but I think with a good balance it will be quite practi- 

 cable. 



In making this estimate, I suppose the retardation observed between 

 Greenwich and Brussels to be chiefly due to the subterranean part 

 of the wire, and I have taken it as if it were actually observed in 180 

 miles of coated copper wire. Not having worked out the theoretical 

 problem in the case of a number of insulated wires under the same 

 sheathing, I have considered the cases of a single wire excentrically 

 placed in the iron sheathing, and insulated from it by gutta percha, 

 and a single wire in its own gutta percha tube with the others re- 

 moved, and itself symmetrically sheathed with tow and iron wire in 

 the usual manner. In the former case the electro -statical capacity of 



the wire would be =^^ — 7^ approximately*, if R, the inner radius 



^' RR' 



of the conducting sheath, be a considerable multiple of R' the radius 

 of the copper wire, / denoting the distance between their axes. In 



the latter case it is , where R, is the inner radius of the 



^H§ 



sheath. These become and , if we take 1=2 fas it pro- 



2-45 1-35 ^ ^ 



bably is for gutta percha, nearly enough), and R="5, R^=_' 



8 



R'='0325, as the information given me by the Astronomer Royal 

 indicates. Whatever the theory may show for the influence of the 

 other wires, the result as regards retardation must be intermediate 



* The rigorous expression, which is very easily found by the method of 

 " electrical images," need not be given here. 



