MAKING AND LAYING SUBMARINE CABLES. 171 
damaged or destroyed by icebergs. The depth of an iceberg 
below the water is at least about double its elevation above, 
and the stones carried by the iceberg often make the difference 
still greater. An iceberg rising 100 yards above the surface of 
the water may extend to 600 yards below it. This constitutes 
a great danger for the Northern Atlantic Cables. The lower 
parts of the icebergs, thawing as they travel southwards, come 
to present nothing but sharp edges and angles under the 
water ; and these, scraping along the bottom of the sea, scoop 
Fig. 11 i 
out deep trenches, and cut as with strong chisels any cable that 
may unfortunately lie in their way (fig. 112). 
Rulhing against rocks .—It often happens, especially near the 
shore, that the bottom of the sea rises abruptly, so that the 
cable cannot lie uniformly along it. Hence a portion of the 
cable swings suspended, and the end resting on the rock has to 
bear the strain. A continuous backwards and forwards move¬ 
ment, caused by the motion of the sea, slowly but surely wears 
through, first the outside wires, and when these fail, the con¬ 
ducting wires soon break, or are worn through to the core, and 
thus all communication is interrupted. 
