A spherical harmonic analysis of the diurnal surface pressure oscillation shows that its main part can be represented by a wave travelling westward with the sun. The amplitude of this wave is about half of that of the semidiurnal oscillation [eq. (7)]. For the discussion of the excitation of the diurnal oscillation the equivalent depths (Table 4) and Hough functions (Table 5, 6) for the three lowest symmetrical modes are computed. There is no difficulty in accounting for the smallness of the diurnal oscillation compared to the semidiurnal oscillation on the basis of thermal excitation, even if the semidiural oscillation is not greatly magnified by resonance.