destructdve, after which waves of decreasing height follow one 

 another at intervals of ten to twenty minutes for some hours. 



There have been many highly destructive tsunamis throughout 

 history. When a series of violent shocks shook the west coast of 

 South America in 1868, the sea withdrew ominously from the coast, 

 but shortly afterward returned as a great wave that swept boats a 

 quarter of a mile inland. When the volcanic island of Krakatoa 

 exploded in 1883, the resulting tsunamis swept a gunboat inland 

 for two miles. Tsunamis announce themselves indirectly, their 

 messengers being the earthquakes that touch them off. As soon as 

 the seismologist has detected and fixed the position of a submarine 

 earthquake, he can send out a tsunami warning to people in the 

 path of the wave, usually several hours in advance. 



Whether we are concerned with surges or simply with normal 

 tides, there are enormous amounts of energy locked up in the 

 movement of the water. Tidal energy enters the western end of 

 the English Channel from the Atlantic Ocean at the rate of about 

 240 million horsepower, but eighty-seven per cent of this energy is 

 dissipated by bottom friction with the channel floor. On a global 

 scale this tidal friction acts as a kind of brake that is slowly but 

 decisively slowing down the rate of the Earth's rotation, with the 

 result that the length of our days is increasing at a rate of about 

 j^ second in a hundred years. At the same time, the tidal friction 

 is slowly pushing the Moon farther away from the Earth, which 

 means that the Moon is taking longer and longer to circle the 

 Earth — which in turn means that the length of our months is also 

 increasing. Small as these effects are, their consequences in the 

 course of many millions of years are considerable. 



In its early life the Moon rotated rapidly on its own axis, but 

 tidal friction on its surface gradually slowed it down until it became 

 "fro2en." Today we see only one side of the Moon as it revolves about 

 us. Its far-side face is forever turned away. Astronomers tell us 

 that, if other conditions continue as at present, a similar fate awaits 

 the Earth 50,000 million years from now. Our days will continue 

 increasing in length and the Moon will continue spiraling away 

 from the Earth until the length of a day and a month will eventually 

 be the same — equal to about forty-seven of our present days. At 

 that time, tides as we presently know them would have ceased to 

 exist, but the astrophysicists tell us that long before this state of 

 affairs could come about the Sun will become so much brighter and 

 radiate such intense heat that all our oceans will be boiled dry. 



Among many proposals to harness the tides 

 for power generation the French plan now being 

 developed at the Ranee Estuary at St. Mala is 

 the most promising. A system of two-way 

 turbines will be operated by both the incoming 

 and outgoing tides. Twenty-four hydroelectric 

 generators are expected to produce an average 

 of 567.5 million kilowatt hours per year. 



pump turbine 



control room 



sluice gates 



