NOVELTY - Desalination device for desalination of a liquid comprises a water engine for extracting energy from a body of water and a reverse osmosis unit in which the liquid is initially supplied for use as a hydraulic fluid within at least one hydraulic ram (10, 11) of the water engine before being directed under pressure to the reverse osmosis unit for desalination.    USE - The desalination device is used for desalinating a liquid (claimed), e.g. seawater for human consumption. It is particularly incorporated with a body of water that exhibits tides since this provides a renewable source of energy for the operation of the water engine. It can be used for water purification in a run of river situation by installing the device across a weir or small dam in the river to generate the necessary head for operation.    ADVANTAGE - The inventive desalination device operates using tidal energy, thus providing savings in running costs associated with providing the power needed in the prior art systems for pumping the water. As there is no need to install an electricity supply or other generation mechanism to provide the power for desalination, this can lead to cost savings, and the ability to provide water supplies in otherwise impractical conditions. The inventive device employs simple technology and thus provides for low maintenance reliable installations. By using a two pass reverse osmosis system the lifetime of the filter elements can also be extended and thus a lower maintenance more robust reverse osmosis process is produced. It is ideal for installation with an in shore sited barrage that allows for easy access and easy transportation of the output water by pipe to a shore based storage facility. It is fully scaleable such that a sized system can be installed in an existing location, e.g. at the mouth of a disused harbor, or in a short barrage across a narrow inlet. Employing the hydraulic rams to directly pump sea water avoids the use of hydraulic oil, thus reducing the risk of contamination. It allows the desalination device to operate at low heads of water, thus making it suitable for double tide working and providing for a very efficient use of the tidal energy available as it minimizes the losses associated with the prior art water engines in translating tidal energy into electrical power and back again into mechanical power.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method of desalinating a liquid, which comprises deploying a water engine and a reverse osmosis unit within a body of water, supplying the liquid for desalination to at least one hydraulic ram of the water engine, extracting the energy associated with the body of water to power the water engine such that the liquid supplied to the hydraulic rams is pressurized, and supplying the pressurized liquid for desalination by the reverse osmosis unit.    DESCRIPTION OF DRAWING(S) - The figure is a circuit diagram of the desalination device.    Hydraulic ram (10, 11)    Filter (12)    Low pressure pump (12)    Non return valve (13-17)    First reverse osmosis filter (18)    Hydraulic motor (20)    Control valve (24)    Liquid reservoir (29)    High pressure pump (31)