GI4 TILE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



formed, which interferes with the transmission. It is, therefore, 

 preferred, if the amount of correspondence requires it, to lay several 

 cables between the extreme stations, and for this further reason, that 

 any serious accident happening to one of them generally leaves the 

 other still available. 



On the same line the -cable differs generally in size according to 



FIG. 402. Transatlantic cable from Brest to St. Peter's, laid in 1867 (sections of natural size). 



the part of the route it is to lie in. Near the coasts, where the sea is 

 shallow and the cable is exposed to accidents arising from the agitation 

 of the sea during. storms, the size of the cable is greatest. The metallic 

 element is formed of wires of large diameter, covered with a siliceous 

 compound, for the purpose of increasing its resistance to wearing by 

 friction against the rocks. This is the shore end. For medium depths, 

 a smaller diameter is adopted, both for the cable as a whole and for 



