  The dependence of Tc on hydrostatic (He-gas) pressure has been determined to
0.7 GPa. We find that Tc decreases linearly and reversibly under pressure at
the rate dTc/dP = -1.11 +- 0.02 K/GPa. These studies were carried out on the
same sample used in earlier structural studies under He-gas pressure which
yielded the bulk modulus B = 147.2 +- 0.7 GPa. The value of the logarithmic
volume derivative of Tc is thus accurately determined, dlnTc/dlnV = +4.16 +-
0.08, allowing quantitative comparison with theory. The present results support
the emerging picture that MgB2 is a BCS superconductor with electron-phonon
pairing interaction.
