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CHAPTER VII. 



MATERIALS. 



SECTION I. 



General remarks Erosion Eroding agents Power of moving shingle Sand-blast 

 Danger of open joints in structures. 



IN selecting materials for sea-works, the two principal qualities 

 to be sought after are durability and high specific gravity. The 

 latter of these has more especial reference to the various 

 descriptions of stone and concrete. 



Durability is affected by erosion, as well as by changes of 

 temperature and chemical action, which latter may be taken to 

 include ordinary decay. 



The chief eroding agents are sand and shingle, set in motion 

 by the waves of the sea and by currents ; but rain and frost, and 

 the acids contained in the atmosphere and in sea-water, often 

 play an important part in the process of erosion by disintegrating 

 and loosening the surface particles. The same may be said of 

 excessive heat, especially when acting upon surfaces which are 

 intermittently wetted. 



In some localities floating ice is a destructive agent; so also 

 is the ice which at times accumulates in large masses upon the 

 face of piers situated in cold regions. I have known such ice, in 

 falling away from the face of a pier, take with it a slab of con- 

 crete several inches in thickness, which it had detached from the 

 main mass. 



The amount of work performed by waves, in rolling about 

 the stones upon a shingly beach and rubbing them against each 

 other, is indicated in some degree by the familiar grinding noise 

 which is made. The power of such a " mill " can, however, only 

 be appreciated by those who have carefully noted it. It is 



