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
Jewish views of leather is often used to designate relaxation (Yer. Bik. i. 64a, et passim). It is very probable that sandals generally were made of thick hide; for wooden sandals are indicated as such (Yeb. 101a, et passim). Besides shoes, the Talmud speaks of leather hose ("anpilia"), and of a kind of glove and foot-wear of skin for a cripple who was compelled to use his hands in order to move from place to place (ib. 102b). The Mishnah, Kelim xxvi. 5, enumerates the following articles made of leather: a covering for the mule or ass; aprons worn by muleteers and by surgeons