Results are presented for an experimental, impact-tube study of the diffusion of momentum for the isothermal, incompressible, turbulent mixing of a slot jet issuing into a slower moving secondary region. The symmetric spread of the jet into a uniformly flowing secondary stream of low initial turbulence is well correlated by phenomenological expressions based on assumptions suggested by the Engineering Experiment Station of the University of Illinois and extended in this paper to include the case of the slot jet issuing into a moving secondary. The single parameter upon which this correlation is based, the spreading coefficient, is shown to have an interesting interpretation in terms of the diffusion of a stream of fluid “particles” into uniformly flowing fields of homogeneous isotropic turbulence.