FETCH, EXPOSURE, AND WAVE-POWER. 83 



no other agency than that of the sea, the high- water margin of which 

 is 74 feet below it." 1 



Lest the reader should be inclined to underrate the power of 

 sea-waves, in consequence of what has been said in regard to the 

 raising of land and glacial action, it may be well now to give a 

 few examples of wave-power as actually witnessed during the 

 construction of sea-works. 



The Ymuiden breakwaters (Holland) are vertical, or nearly 

 vertical, structures, with mounds of concrete blocks, termed 

 " wave-breakers," on the seaward side. These mounds are com- 

 posed of blocks weighing from 5 to 10 tons each, thrown pell- 

 mell into the sea. The mound has a slope of about 1 \ horizontal 

 to 1 vertical, and is surmounted by a row of 20-ton blocks. 



During one gale, one of these 20-ton blocks was lifted by a 

 wave to a height of 12 feet (vertically up the face of the pier) 

 and landed upon the top of the pier, which was 4 feet 10 inches 

 above high water. The following waves knocked it over into 

 the harbour. A similar block was lifted 10 feet, and landed on 

 the top of another one. 



During another gale at the same place, a " header " block, in 

 the seaward face of the pier, measuring 7 feet in length, and 

 presenting a face to the waves of 4 feet by 3 feet 6 inches, was 

 started forward out of its place to the extent of 3 feet by the 

 stroke of a wave compressing the air at the back of it. This 

 block weighed about 7 tons, and the top of it was at the level of 

 low water. It had three courses of concrete blocks, each 3 feet 

 6 inches in thickness, resting upon the top of it. These were all 

 set in Portland cement mortar ; but the course to which the 

 block in question belonged was built dry, i.e. not set in cement, 

 as were also those below it; the joints were, however, fairly 

 close. The length of pier in which this occurred was finished, 

 excepting that the blocks required to form the " wave-breaker " 

 had not been deposited. 



On more than one occasion considerable lengths of courses, 

 the full width of the pier, composed of blocks weighing 5, 7, and 

 9 tons, all of which were set in Portland cement mortar, and 

 tied together by strong wrought-iron cramps formed of 2-inches- 

 square iron, were displaced and washed over into the harbour. 



On other occasions, when the blocks were not disturbed, the 

 1 "The Design and Construction of Harbours," by Thomas Stevenson. 



>F TORO'r 



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