  We report a new method to measure the velocity of a fluid in the vicinity of
a wall. The method, that we call Particle-Shadow Tracking (PST), simply
consists in seeding the fluid with a small number of fine tracer particles of
density close to that of the fluid. The position of each particle and of its
shadow on the wall are then tracked simultaneously, allowing one to accurately
determine the distance separating tracers from the wall and therefore to
extract the velocity field. We present an application of the method to the
determination of the velocity profile inside a laminar density current flowing
along an inclined plane.

Fluid Dynamics;Classical Physics