  We show how to determine the ratio of the transverse velocity of a source to
the velocity of emitted particles, using split-bin correlation functions. The
technique is to measure $S_2$ and $S_2^{\phi}$, subtract the contributions from
the single-particle distribution, and take the ratio as the bin size goes to
zero. We demonstrate the technique for two cases: each source decays into two
particles, and each source emits a large number of particles.
