Temporarily communicating vessel isolated hydroelectric power plant with swimming piston, driven through hydrostatic pressure One fixes to the bottom of a pool filled with water a cylinder also filled with water. On the bottom of the cylinder there is an initially closed valve, referred to as “A”. Through the closed valve “A” the cylinder is isolated from the pool, they constitute no communicating vessels. On the surface of the water of the cylinder a hollow piston can move, which is initially located in the upper side of the cylinder and which has a passing borehole. An optionally extendable tube connects to the borehole on the side of the pistol which is not in contact with water. The piston, initially, is in contact with the wall of the cylinder through a valve, referred to as “B”, which, while the piston is going down, prevents the water flowing up, but does not break the movement of the piston. The piston is in this phase no swimming body, but an object, that is weighing on the surface of the water. The mass of the piston is such that it best results the water underneath to pump upwards, however, its density is equal or smaller to the density of the water. When letting go the piston from its upper position, due to its weight, it starts to go down. Meanwhile the water penetrates the tube through the borehole. In the course of the sinking of the piston, the hydrostatic pressure of the rising water can increase such that it could be in balance with the pressure exerted on the water by the weight of the piston. In order to avoid this situation, the length of the tube has to be made suitably short. Given this circumstances, the water squeezed out of the cylinder, due to the sinking of the piston, will continuously leave to the outside pool. At this time one opens valve “B”, so that the water can unhindered flow next to the piston. One also opens valve “A” (down part of the attached illustration). Through the open valve “A” the cylinder is no more isolated from the pool, they constitute communicating vessels. Through the open valve the water from the pool starts to flow to the cylinder. The piston, due to the effect of the inflowing water, starts to rise, up until the level of the water in the pool and in the cylinder is even. The piston is in this phase trough the open valve “B” a swimming body. By so doing the piston arrived back to its starting position. By closing valves “A” and “B” the circle restarts. The practical benefit of the apparatus is that the water flowing in the tube connected to the piston as well as the water flowing in through the bottom of the cylinder, with the help of one generator for each can be used to produce electrical energy. To those, who insist on that only a pistol with higher density than water can make the water flow upwards from underneath the pistol, I suggest the following idea. One places a piston made of plastic (disc) and able to swim on the top of a cylinder filled with water, which is connected to an optionally extendable tube. It is easy to calculate the weight of the solid lead object which fixed to the piston can pump upwards, due to its weight, the body of water from underneath the piston. What changes in the functional mechanism, if the solid lead body is replaced by a lead body that is hollow, able to swim (thus its density is smaller or equal to that of the water), but of the same weight. Nothing, so it seems. This is similar to the ebb and flow power plant, where the complex effect mechanism of gravitation of the Earth is temporarily isolated (with valve “A” being closed) from the communicating vessel system created by cylinder and the pool. In this case, the interior of the cylinder is also conceivable as if it did not stand in water, but on lend, when it pump out the body of water from underneath itself. In the ebb and flow power plant comparison this is the body of water “close to the Moon”, while the body of water in the pool is the one “far from the Moon”. By opening valve “A” the body of water in the pool becomes the body of water “close to the Moon”, the communicating vessel system fulfills its duty. If according to some coincidence, this system works, it will probably to a certain extent consume the gravitation of the Earth, the same way the ebb and flow phenomenon contributes to the decrease of the height of the Moon. According to this, there is no contradiction with the conservation of energy law.