SUBMARINE TELEGRAPHY 217 



lay of the hemp round the steel, it was repeatedly found that the 

 strength of the two combined exceeded the sum of the strengths 

 of the two separately, and this strange result has been fully con- 

 firmed by independent experiments conducted by Mr. Fairbairn 

 and others for the Atlantic and Telegraph Construction Com- 

 panies. The explanation is simple enough. Neither material 

 is really homogeneous : each has its weak places ; it is ex- 

 tremely unlikely that the weak places of both should coincide. 

 When, therefore, the two are combined, we obtain the sum of 

 the average strengths of each material ; when they are tested 

 separately, we get the sum of the strengths of the two at their 

 weakest points. 



This form of cable was first used in 1 860 for a cable between 

 France and Algiers, Messrs. Gisborne and Forde being the 

 engineers, and Messrs. Glass and Elliot the contractors. The 

 cable, after some misadventures, was successfully laid, and 

 behaved well during submersion, but the form fell into some 

 discredit, owing to the discovery that even in 1,500 fathoms the 

 hemp was eaten away by a species of teredo after a few months 

 of submersion. This left a mere cage of loose iron or steel 

 wires, unfit to be lifted, or relaid if lifted. Fortunately it 

 appears that these animals, which in the Mediterranean fasten 

 on every inch of exposed hemp, do not exist in the Atlantic. 

 Where they have eaten the hemp the gutta-percha appears as 

 if marked with the small-pox ; but no instance has yet occurred 

 where they have actually penetrated the gutta-percha to any 

 serious depth. 



The form has other defects. Many persons think that the 

 two injuries which the 1865 cable received during submersion 

 were not due to malice, but to short pieces of broken wire, 

 which would penetrate the soft sheathing of hemp with much 

 greater ease than the hard mail of the common iron-covered 

 cable. The arguments used in favour of this view are as fol- 

 lows : The hemp conceals a break in the wire which it en- 

 closes ; a broken wire may be bent out when being coiled, and 

 penetrate the neighbouring coil ; the injury may not occur, 

 or not be fully completed, until the coils are disturbed by the 

 trampling of the large number of men engaged on the coil 

 when it is being paid out. Pieces of broken wire were found 



