Translation (1) T h e criterion which determines whether flow will be vi sc ous or turbulent is the ratio of the inertial force to the viscous force acting on the particle. Thus, the criterion which dete rmi nes whether flow is viscous or turbulent is the quantity , known a s the Reynolds number. (2) Fluid mechanics deals with both incompressible and compressible fluids, that is, with fluids o f either constant or variable density. Liquids are ordinarily considered incompressible fluids. Gases, too, may be considered incompressible when the pressure variation is small compared with the absolute pressure .