Since the time of Laplace (1799), scientists had been puzzled as to why pressure variations measured at the Earth's surface associated with the semi-diurnal solar tide dominate those of the diurnal tide in amplitude, when intuitively one would expect the diurnal (daily) passage of the sun to dominate. Lord Kelvin (1882) had proposed the so-called "resonance" theory, wherein the semi-diurnal tide would be "selected" over the diurnal oscillation if the atmosphere was somehow able to oscillate freely at a period of very close to 12 hours, in the same way that overtones are selected on a vibrating string. By the second half of the twentieth century, however, observations had failed to confirm this hypothesis, and an alternative hypothesis was proposed that something must instead suppress the diurnal tide. In 1961, Manfred Siebert suggested that absorption of solar insolation by tropospheric water vapour might account for the reduction of the diurnal tide. However, he failed to include a role for stratospheric ozone. This was rectified in 1963 by the Australian physicist Stuart Thomas Butler and his student K.A. Small who showed that stratospheric ozone absorbs an even greater part of the solar insolation.