.S7A' WILLIAM SII-.MKXS, 



119 



perfectly smooth, the strains produced by the suspended portions 

 of the cable must ultimately lead to fracture, if the iron wires 

 became laid open to the action of rusting. So long as a cable 

 ivtiiined its full strength, it was not indeed necessary that it should 

 be perfectly supported throughout its entire length ; but unless 

 it were proved that the bottom of the Atlantic was so perfectly 

 even that no portion of the cable would be in suspension, the 

 continuance of the cable in working order must be dependent he 

 considered upon the durability of the hemp covering protecting 

 the iron wires from corrosion, which in the absence of marine 

 animals might be for many years. 



In addition to preserving the iron wires from rust, it must be 

 borne in mind that the durability of the hemp covering was also 

 essential to the strength of the cable, in consequence of the hemp 

 acting as a packing to keep the wires at the proper distance 

 apart. If the hemp were eaten away, the wires would be left like 

 a loose cage round the core, and the latter would stretch and 

 become faulty in consequence. 



ON THE CONVERSION OF DYNAMICAL INTO 



ELECTRICAL FORCE WITHOUT THE AID 



OF PERMANENT MAGNETISM. 



By C. W. SIEMENS, F.R.S.* 



SINCE the great discovery of magnetic electricity by Faraday in 

 1830 electricians have had recourse to mechanical force for the 

 production of their most powerful effects ; but the power of the 

 magneto-electrical machine seems to depend in an equal measure 

 upon the force expended on the one hand, and upon permanent 

 magnetism on the other. 



An experiment, however, has been lately suggested to me by my 

 brother, Dr. Werner Siemens of Berlin, which proves that perma- 

 nent magnetism is not requisite in order to convert mechanical 



* Excerpt from the Proceedings of the Royal Society, Vol. XV. 1867, pp. 367-369 



