THE OCEAN AND ITS WORK 201 



coast varies from about 600 to 800 pounds a square foot. The force 

 of the impact of waves 10 feet high on certain harbor walls and piers 

 was determined to be 1.36 tons a square foot. The average wave 

 pressure on the coast of Scotland for the five summer months'is 611 

 pounds a square foot and for the six winter months is 2086 pounds. 

 At Dunbar in the North Sea the pressure is sometimes three and a 

 half tons a square foot. 



Height of Storm Waves. — On an islet off the coast of Oregon (Tillamook Rock) 

 which is exposed to the sweep of the ocean, the waves of a storm in 1912 dashed against 

 the lighthouse with such force and to such a height as to break the heavy glass of the 

 lantern 132 feet above the sea. During another storm on the same islet a mass of 

 concrete, weighing half a ton, was thrown to a point 88 feet above sea level. During 

 the construction of the breakwater at Plymouth, England, blocks of stone weighing 

 from 7 to 9 tons were removed from the seaward side of the breakwater at low-water 

 level, carried over the top, a distance of 138 feet, and piled upon the inside. During 

 a heavy gale thre' 3 and three quarters million tons of shingle are estimated to have 

 been taken from ' " ^land, and carried seaward by the waves. 



The height t > of storm waves is thrown is often very great. At 



Alderney breai a and, the spray from the breaking waves was thrown 



upward to a he 1 200 feet. At Hastings, England, water was thrown as high as 



the top of a krge hotel, and pebbles were lifted from the beach and carried across the 

 wide promenade into the bedroom windows of the houses fronting the sea. It is 

 stated that windows in the Dunnet lighthouse, Scotland, were broken at a height of 

 300 feet above high-water mark, by stones swept up the cliff by sheets of sea water 

 during heavy gales. 



Tides. — Tides must be considered in a discussion of the work 

 of the ocean, since they are an important, though usually inconspicu- 

 ous agent. Tides are produced by a combination of the attraction 

 of the sun and moon, and of the rotation of the earth ; and almost 

 every part of the ocean experiences two high and two low tides each 

 day. Although the tide in mid-ocean is only about three feet high, its 

 height becomes greatly increased when it approaches shallow shores or 

 enters funnel-shaped bays or estuaries. In the Bay of Fundy, Nova 

 Scotia, for example, the difference in height between low and high tide 

 is sometimes greater than 50 feet. Because of its effect on the level 

 of the water, the tide permits a wide vertical range for the work of 

 waves on shores. 



Tidal Currents. — Tidal races or currents, such as that at Hell Gate, in the City of 

 New York, are not infrequent in narrow straits, and are often effective in erosion. 

 The race at Hell Gate is due to the fact that the tide rises higher in Long Island Sound 

 than in the bay of New York harbor, and to the further circumstance that the time of 

 the high tide is different on the two sides of the strait. The inlets of barrier islands 



