CURRENTS IN THE OCBAN. 



767 



>n the Scott 'sh coast, stands 112 feet above the rock, and yet it 

 is often covered with the waves and foam, even after the temp- 

 est has ceased to rage. Such light-houses are often washed 

 away ; as that at Minot's Ledge, on the coast of Massachusetts, 

 has often been. In consequence the modern method of build- 

 Lug these structures differs from that formerly in use. The 

 custom was to build them of solid masonry, hoping to make 

 them strong enough to resist the waves. Now they are generally 

 built of iron lattice open work, making the bars as slender as 

 is consistent with the proper strength, so as to offer the least 

 resisting surface to the rushing water. This open frame work 

 ts raised up high enough, if possible, to place the house and 

 lantern above the reach of the body of the wave. 



The force of the water in such positions is prodigious. 

 Stephenson calculated that the sea dashed against the Bell 

 Rock light-house with a force of 17 tons for every square yard. 

 At breakwaters in exposed situations the sea has been known 

 to seize blocks of stone weighing tons, and hurl them as a 

 child would pebbles. At Cherbourg, in France, the heaviest 

 cannon have been displaced ; and at Barra Head, in the Heb- 

 rides, Stephenson states that a block of stone weighing 43 tons 

 was driven by the breakers about two yards. At Plymouth, 

 England, a vessel weighing 200 tons was thrown up on the top 

 of the dike, and left there uninjured. At Dunkirk it has been 

 found that from the dash of the breakers the ground trembles 

 for more than a mile from the shore. Eesults of this kind, to 

 which our attention is specially directed, since they affect man's 

 work, show us what must be the effect produced by the sea, in 

 constantly eating away the shore ; altering the coast lines ; chang- 

 ing continents, and building them up elsewhere ; and suggest 

 h.3w much greater than what we see must have been the effects 

 of the sea upon the land during the countless ages in which it 

 has been at work. 



The c-r rents in the ocean, which constitute the real *r ^'oo 



