THE ELECTRIC TELEGRAPH. 



tells me that lie has sometimes found a longitudinal incision 

 measuring ten feet in length, made in the gutta-percha, by the 

 lightning, and cut as clean as if it had been done with a razor. 

 At other times he has^ found the gutta-percha swelled, rough and 

 porous, and sometimes pierced with countless numbers of openings 

 like pinholes. 



These appearances are supposed by Mr. Newall to arise from 

 imperfection in the covering of the wire. The slit, he thinks, is 

 caused, by air getting in behind the arm, which holds the mandril 

 through which the copper wire passes before leaving the cylinder, 

 and the porous covering arises from air mixed with the gutta- 

 percha. Mr. Newall has ascertained that a wet hair, or a hole of 

 equal size is sufficient to destroy the insulation of the wire. 



89. Some eminent scientific authorities express doubts as to the 

 durability of the submarine cables. In the case of the Dover and 

 Calais cable it has been observed that the bottom of the channel at 

 that part of the strait is proved by the soundings to be subject to 

 undulations, so considerable that the summits of some of its elevated 

 points rise to such a height that the water which covers them is 

 not deep enough to secure them from the effects of the tumultuous 

 agitation of the surface in violent storms. It is here well to remind 

 the reader that the agitation of the ocean, which seems so awful in 

 great tempests, has been found to extend to a very limited depth, 

 below which the waters are in a state of the most profound repose. 

 The objection we now advert to is, therefore, founded upon the 

 supposition that the crests of some of the elevations upon which 

 the submarine cable rests are so elevated as to be within that limit 

 of depth, and it is feared that such being the case, the violence of 

 the water in great tempests may so move the cable against the 

 ground on which it is deposited with a motion to and fro, as to 

 wear away by frequent friction its metallic armour, and thus 

 expose the conducting wires within it to the contact of the water, 

 and destroy their insulation. 



But it has been most satisfactorily proved by a part of the 

 experimental wire which was laid down between Dover and Calais, 

 in 1850, and which was picked up two years afterwards in as 

 perfect a state as when laid down, that the action of the waves 

 does not affect the bottom of the Channel there. The greatest 

 depth is 30 fathoms, and the bottom shelves regularly from Dover 

 to near Cape Grinez, where there is a ledge of rocks rising 

 suddenly from the bottom. 



It has been also feared that, notwithstanding the effect of the 



galvanisation of the surface of the surrounding wires, the corrosive 



action of the sea water may in time destroy them ; and it has been 



suggested that some better expedient for protection against this 



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