The invention relates to preserving the height level of the wetting liquid pressed upward by air pressure through the inlet opening in an evacuated hollow body, which comprises an outlet opening, by completely filling the same, even the inside edges of the outlet opening, with capillaries, the inner ends of which are completely surrounded by the wetting fluid and the capillarity of which is so large that the capillary acts counter to the air pressure and thereby keeps the capillary away from the wetting fluid in the hollow body; producing potential energy through flexible capillaries, the one end of each protruding into the outlet opening or capillary opening of the evacuated hollow body, said ends being completely surrounded by the wetting fluid, the capillary thereof being at least large enough to transport the wetting fluid over the entire length of said flexible capillaries and at least as large as the flexible capillaries used to preserve the height level are long so that they extend to an object located outside thereof and dropping off with the largest gradient possible, on which the wetting fluid exits the capillary and constitutes potential energy.
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