  We report on measurements of the critical temperature of a harmonically
trapped, weakly interacting Bose gas as a function of atom number. Our results
exclude ideal-gas behavior by more than two standard deviations, and agree
quantitatively with mean-field theory. At our level of sensitivity, we find no
additional shift due to critical fluctuations. In the course of this
measurement, the onset of hydrodynamic expansion in the thermal component has
been observed. Our thermometry method takes this feature into account.
