Since the time of Laplace the periodic oscillations of the atmosphere as a whole have formed a constant subject for dynamical research. The chief interest has centred round the semidiurnal oscillation indicated by the great regularity of the semidiurnal component of the daily variation in barometric pressure. If the semidiurnal oscillation is due to gravitational tides it is very much greater than we should expect unless the tidal force is greatly enhanced by resonance. If it is due to the daily variation in temperature then should expect the diurnal effect to be greater than the semidiurnal unless the semidiurnal oscillation were greatly enhanced by resonance. Both these hypotheses involve the existence of a free period which differs from 12 hours by not more than 4 minutes. The inherent improbability that the appropriate mode of oscillation should chance to have a period so near to 12 hours caused Dr. H. Lamb to reject the tidal explanation of the semidiurnal pressure wave, but, as Prof. Chapman has pointed out in his very complete exposition of the subject, the “temperature variation” explanation requires resonance just as much as the “tidal” one. In fact Prof. Chapman attributes roughly half the effect to tidal forces and half to the daily variation in temperature.