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Heinrich Blasius developed a similarity solution for the thin boundary layer along a flat plate. Even when viscosity is small, a fluid must satisfy the no-slip boundary condition at the surface, so an inviscid flow approximation cannot describe the whole region. Balancing downstream advection against transverse diffusion of momentum gives the boundary layer scale $\delta (x)\sim \sqrt{\nu x/U}$, where $\nu$ is kinematic viscosity, $U$ the external speed, and $x$ the downstream distance from the plate's leading edge. The similarity solution reduces the developing boundary layer profile to a function of one scaled coordinate.^[ref]