curl a small ball is located within the incompressable fluid or gas (the centre of the ball being fixed at a certain point).

http://babel.hathitrust.org/cgi/pt?id=mdp.39015000962285;view=1up;seq=179

Suppose the vector field describes the velocity field of a fluid flow (such as a large tank of liquid or gas) and a small ball is located within the fluid or gas (the centre of the ball being fixed at a certain point). If the ball has a rough surface, the fluid flowing past it will make it rotate. The rotation axis (oriented according to the right hand rule) points in the direction of the curl of the field at the centre of the ball, and the angular speed of the rotation is half the magnitude of the curl at this point.^[5]