I HAVE read the remarks of Prof. Horace Lamb in NATURE, November 5, 1908, p. 24, and November 12, 1908, p. 47, where, although mentioning difficulties, he apparently accepts the suggestions of Kelvin, Margules, and Hann that the semi-diurnal wave of pressure can be explained by the fact that “the daily variation of temperature is not harmonic, and when analysed there is a definite component with a half-day period,” and “on a rotating earth the period of free oscillation of the atmosphere lies very near to twelve hours.”