MA TERIALS. 109 



explosion was almost sufficient to stun them ; and diving, whilst 

 blasting was going on, had to be discontinued. 



From what has been said, it will be seen of what vital 

 importance it is to prevent wave-stroke being transmitted in 

 structures, in the manner described. 



In Chapter V., p. 83, it was mentioned how a block weighing 

 7 tons was started out of its place by water-pressure or com- 

 pressed air. On another occasion several adjoining blocks were 

 forced outward on the harbour side of the breakwater at East 

 London (Cape Colony), presumably from the same cause. Too 

 much care cannot, therefore, be taken to ensure close and well- 

 filled joints and beds in sea- works. 



It is impossible to say where the mischief, originating in the 

 withdrawal by the sea of a badly set face-stone, may end. 

 When once a breach has been made in the face of a breakwater, 

 it is amazing with what rapidity the work of destruction some- 

 times proceeds. 



The large fronds of " sea- wrack " or " tangle " (Laminaria 

 digitata and L. saccharina), with their thick and tough stems, 

 when swayed to and fro by the waves, often tear out the pieces of 

 rock to which they are attached, and then float away with them, 

 or materially assist the waves and currents in transporting them. 



It is curious to notice, after a storm, what a large proportion 

 of the stems of the wrack which have been thrown upon the 

 beach, have stones or pieces of rock usually about the size of 

 one's fist attached to their roots. Often, however, much larger 

 pieces of rock are detached and transported by seaweed. 



Mr. Kinahan, in his communication on the "Carriage of 

 Stones in Deep Water" (Min. Proc. Inst. C.E., vol. Iviii. p. 

 284), says 



" At the end of the Tacumshin ^Eolins sand-ridge, there is, below 

 the sloping shingle beach, a nearly flat sandy strand, usually free 

 from blocks (of stone), which, on April 4, 1876, during low water, 

 after a heavy gale from the south-west, was covered with blocks, 

 having deep-sea weed attached, while on the slope there were other 

 similar blocks. As the tide rose, these blocks began to drift land- 

 ward, and in twenty-four hours nearly all the blocks were collected 

 in horizontal lines." 



Mr. Kinahan goes on to state that some of the stones weighed 

 from 2 to 3 cwt. apiece. 



