Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Legend of the Leather Bridge enough to span the five hundred metres between them. A leather bridge is supposed to have linked the two castles of Stolzenburg and Pielstein over the Urft. According to local legend, their knights lived in the lap of luxury, demanding socage from the farmers, yet allowing their children to play skittles along the bridge with loaves of bread, whilst the subjects were dying of hunger. God himself is said to have destroyed both castles by unleashing a natural disaster. Near Arnsberg legend has it that the devil made a leather bridge between the castles of Rüdenburg and Arnsberg in order