We have observed the sharply resonant enhancement of the cross section of the collision process $ns+ns\ensuremath{\rightarrow}np+(n\ensuremath{-}1)p$ between two $\mathrm{Na}ns$ ($16l~nl~27$) atoms which occurs when the levels are shifted by an electric field so that the $ns$ level lies midway between the two $p$ states. The widths of the collisional resonances scale as ${n}^{\ensuremath{-}2}$ and the magnitudes of the cross sections as ${n}^{4}$, with values of 0.5 GHz and 8\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{\AA{}}}^{2}$ for the $20s$ state. These results are in good agreement with calculations based on the long-range dipole-dipole interaction between the two atoms.