216 APPLIED SCIENCE 



but it ceased after a few hours, and was passed over as of insuf- 

 ficient consequence to stop the submersion. 



Much surprise has been expressed at the rupture of a cable 

 estimated as strong enough to bear four tons, when the indicator 

 showed only about one ton. It has frequently been suggested that 

 the instrument gave false indications ; but there is really little 

 reason for supposing this. The cable was covered by 126 small 

 iron wires, spun into eighteen small strands, the whole cable 

 being only 5-8ths of an inch in diameter. The wire was not 

 galvanised, and rusted very readily. It is most probable that in 

 many places its theoretical strength was very much reduced by 

 this cause. 



In 1865 and 1866 the same brake and indicator, or dynamo- 

 meter, as it is sometimes called, were used, but the history of 

 events was widely different. The cable, during submersion, 

 not only escaped fracture, but was not even once strained 

 within a tenth part of its supposed strength. In 1865, the 

 occurrence of a small fault, which would have been far too in- 

 significant to have been detected in 1857 or 1858, caused an 

 attempt to haul back the cable, which was broken by chafing 

 against a projection from the bows of the ' Great Eastern.' 

 The arrangements in 1865 were by no means perfect. The 

 picking-up gear was defective and the system of electrical tests 

 faulty, but the paying-out machinery acted admirably, and the 

 cable hardly admitted of improvement. In 1866 the picking-up 

 gear was good, and the electrical arrangements left nothing to 

 be desired. 



The special form of cable adopted, in which each iron wire 

 is enveloped in hemp, presents various interesting peculiarities. 

 It is actually stronger than the sum of the strengths of the 

 hemp and steel employed to make it. This almost incredible 

 paradox was discovered during experiments made by Messrs. 

 Gisborne, Forde, and Siemens for the Government, with re- 

 ference to a proposed Falmouth and Gibraltar Cable. It seems 

 strange enough that a steel wire can be strengthened by wrapping 

 hemp or rnanilla round it ; but this was soon found to be a fact, 

 and indeed the percentage of elongation undergone by a hempen 

 strand and a steel wire before breaking are by no means so 

 different as most people would imagine. By selecting the best 



