28 Report S.A.A. Advancement of Science. 



circuit of the globe. Tlie whole of the diurnal phenomenon of 

 the atmospheric tides is therefore rapidly propagated over the 

 surface of the earth from east to west, the movement being most 

 rapid in equatorial regions, and there the amplitude of the 

 oscillations is greater than in higher latitudes under similar 

 atmospheric, astronomical, and geographical conditions. Owing 

 to the rapidity of the diurnal heating of the atmosphere by the 

 sun through its whole height, some time elapses before the higher 

 expansive force called into play by the increase of temperature 

 can counteract the vertical and lateral resistance it meets from 

 the inertia and viscosity of the air. Till this resistance is 

 overcome, the barometer continues to rise, not because the mass 

 of atmosphere overhead is increased, but because a higher 

 temperature has increased the tension or pressure. When the 

 resistance has been overcome, an ascending current of the warm 

 air sets in, the tension begins to be reduced, and the barometer 

 falls and continues to fall till the afternoon minimum is reached. 

 Thus the forenoon maximum and afternoon minimum are simply 

 a temperature effect, the amplitude of the oscillation being 

 determined by latitude, the quantity of aqueous vapour overhead 

 and the sun's place in the sky 



" When the daily maximum temperature is past, and the 

 temperature has begun to fall, the air becomes more condensed 

 in the lower strata, and pressure consequently at great heights is 

 lowered. Owing to this lower pressure in the upper regions of 

 the air, the ascending current which rises from the longitudes 

 where at the time the afternoon pressure is low flows back to 

 eastward, thus increasing the pressure over those longitudes where 

 the temperature is now falling. This atmospheric quasi-tidal 

 movement occasions the PM. increase of pressure, which reaches 

 the maximum from 9 p.m. to midnight according to latitude and 

 geographical position. This maximum is therefore caused by 

 accessions to the mass of the atmosphere overhead contributed by 

 . the ascending currents from the longitudes of the afternoon low 

 pressure immediately to the westward. 



" As midnight and the early hours of morning advance, these 

 contributions become less and less, and at length cease altogether, 

 and pressure continues steadily to fall. But between the time 

 when the increase of pressure from the overflow through the upper 

 regions of the atmosphere ceases and the time when pressure 

 increases from the heat ravs, direct or indirect, of the returning 

 sun, or during the hours of the night when the effects of nocturnal 

 radiation are at the maximum, pressure is still further reduced 

 from another cause. Radiation towards the cold regions of space 

 takes place, not only from the surface of the globe, but also 

 directly from the molecules of the air and its aqueous vapour. 

 The effect of this simultaneous cooling of the atmosphere through 

 its whole height is necessarily a diminution of its tension. Since 

 this takes place at a more rapid rate than can be compensated for 

 by any mechanical or tidal movement of the atmosphere from the 



