Theory of Pressure Effects Due to the Corona Discharge, in Terms of Ionic Mobility.---The characteristic pressure effect is an excess pressure of from 1 to 3 cm. of water inside a tube concentric with the discharge wire. Assuming that the momentum given the ions by the electric field is transferred to the gas, the author shows that the excess pressure is $\frac{i}{4\ensuremath{\pi}u}$, where $i$ is the current per unit length of wire and $u$ is the mobility. Using this expression, the positive mobilities in ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, C${\mathrm{O}}_{2}$, ${\mathrm{O}}_{2}$ and N${\mathrm{H}}_{3}$ are computed from the observed pressure differences, and come out respectively 1.51, 0.50, 0.16, 0.23 and 0.31 times ${10}^{\ensuremath{-}2}$ cm./sec. per volt/cm. The first three values are about half those measured by Hess in the wind of ions from radioactive substances, while the value for ${\mathrm{O}}_{2}$ is only one eighth, the low value being perhaps due to ozone. The results so far refer to the mean mobility. Assuming that there are two types of ions, the percentage of heavy ions comes out about 13 for ${\mathrm{O}}_{2}$. The time for the pressure to rise to its limiting value is computed and comes out in rough agreement with the measurements of Fazel; and an explanation is suggested for the form of the pressure curves obtained by Fazel with A.C. discharges.