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60 such a level that it shall be filled at high tide and nearly emptied at low tide. On the water in this basin floats a huge iron caisson of a circular shape, but flat at top and bottom, like an ordinary gasometer. According to the power required, these caissons are larger or smaller. The largest weigh many hundreds of tons. It is obvious that as the tide flows and ebbs, the caisson will rise and fall with it. To the top of it is attached one arm of a lever, which is hinged on to a strong upright on land; the other arm of the lever being attached to a crank that moves a shaft. As the caisson rises the crank is depressed, and as the caisson falls it is raised.

But it is obvious that at the turn of the tide both ways, there will be a period of rest during which no force will be exerted. In order to keep up the moving power, so as to communicate a continuous and equable rotatory movement to the shaft, there is a second caisson with another lever attached to a crank, so arranged that when it comes to the perpendicular, the other crank is nearly at right angles to it in advance. The period of rest of this second crank occurs some time after that of the first crank. In fact it is the same arrangement as we see in the shaft in double-cylinder steam-engines. The retardation of the second crank is caused by allowing the tidal water to enter a reservoir, before coming into the basin of the second caisson, and so regulating the admission of the water that it shall still raise the second caisson after the first has come to rest. By this arrangement the shaft is turned round with irresistible force once in little more than twelve hours; and by the common arrangement of cog-wheels, as in a clock, or wheels and endless bands, the slow movement can be accelerated to any required