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Historical view of a graduation tower - a structure used in the production of salt which removes water from a saline solution by evaporation, increasing its concentration of mineral salts. The tower consists of a wooden wall-like frame stuffed with bundles of brushwood. The salt water runs down the tower and partly evaporates; at the same time, some minerals from the solution are left behind on the brushwood twigs. Graduation towers can be found in a number of spa towns, primarily in Europe. The mineral-rich water droplets in the air are regarded as having beneficial health effects similar to that of breathing in sea air. Wood engraving, published in 1894.