How does a ferry work without an engine ?

Ferries without engines are driven by the flow of water. They are attached to a wire cable, which is spanned across the river by means of two short ropes. When the ferryman shortens one of the two short ropes with the help of a device called 'winch', the ferry is tilted at an angle to the current, and the force of  the current propels the ferry across the river

How is energy obtained from high and low tides ?

High and low tides come alternately, that is, one after the other. In high tides, the level of water rises and covers a part of the coast, and in low tides it flows away again. This energy of the tides is sued to run turbines. Special water turbines are used in tidal power plants. Water - rising in case of high tides and receding in case of low tides - can flow through these turbines in both the directions. electricity is generated in tidal power plants due to the rotary movement of the turbines. These power plants need a tidal hub - the difference in the height of water between high and low tides - of more than 5 m to be able to work economically. Such a power plant is int he French Bretagne, in St. Malo.Here, the tidal hub is at 12-15