  We scan the collision energy of two clouds of cesium atoms between 12 and 50
$\mu$K in atomic fountain clock. By directly detecting the difference of s-wave
scattering phase shifts, we observe a rapid variation of a scattering phase
shift through a series of Feshbach resonances. At the energies we use,
resonances that overlap at threshold become resolved. Our statistical phase
uncertainty of 8 mrad can be improved in future precision measurements of
Feshbach resonances to accurately determine the Cs-Cs interactions, which may
provide stringent limits on the time variation of fundamental constants.
