The diurnal tide in surface pressure was explained theoretically in 1967, with the help of a framework that had been built up in the preceding six years. A significant contribution to this framework was made by Bernhard Haurwitz, who in 1965 published spherical-harmonic and Hough-function analyses of the diurnal surface-pressure oscillation. The lecture also examines origins of knowledge about negative equivalent depths.

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*A lecture to a general audience at Colorado State University, Fort Collins, Colorado, 11 October 1985, on the occasion of an 80th birthday celebration for Bernhard Haurwitz (1905–1986). The text is unaltered except for a few bracketed insertions and deletions “[. . .]”. (See author's note at end.) A list of references is appended.