34 Report S.A.A. Advancement of Science. 



That is to say, if I interpret this quotation aright, the semi- 

 diurnal oscillation of the barometer is to be regarded as mainly a 

 forced wave of pressure of 12 hours period due to the harmonic 

 component wave of temperature of the same period. There have 

 been many adverse criticisms of this view. Hann himself notices one 

 to the effect that, — 



" There is, in reality, no daily variation of temperature with 

 two maxima and minima ; and if we, in spite of this, obtain a 

 double daily temperature wave, because we insist on representing 

 the daily march of temperature by a series of sines, this forced 

 methematical form can never serve to explain an observed 

 phenomenon, whereas for this some real natural process must be 

 sought for as a cause." 



Such an objection would only be valid if the diurnal variation 

 of temperature were a simple impulse of 24 hours' period. In reality 

 it is anything but that. It is itself the result of a train of acting and 

 reacting processes, due largely to the presence of great water areas 

 over the earth's surface, and to the presence of aqueous vapour in 

 the atmosphere. Before the final result of a temperature impulse 

 upon the air can be considered, we have to take into account the fact 

 that the temperature impulse itself is largely modified by the train 

 of operations set agoing by itself. Thus, for example, the diurnal 

 variation of temperature gives rise to a diurnal variation of aqueous 

 vapour. This aqueous vapour absorbs and radiates heat, thus 

 disturbing the otherwise simple curve of temperature. Then, again, 

 clouds are formed, and these shew a tendency to two, or, perhaps, three 

 maxima in the day. These also regulate the temperature. Then, as 

 Hann has observed, insolation by day and radiation by night combine 

 to made the diurnal curve of temperature asymmetrical, and composed 

 of two parts which do not follow the same law of variation. For all 

 these reasons it seems clear that the various harmonics are not 

 necessarily figments of the mathematical imagination. The strongest 

 objection, to my mind, against the second harmonic wave of 

 temperature being the ultimate cause of the semi-diurnal oscillation of 

 pressure, is that for all altitudes known to us the variation of the one 

 month by month differs considerably from that of the other. Taking 

 Kimberley as an example, we find that the maximum phase of the 

 second harmonic wave of pressure comes earliest in October and latest 

 in February, the difference in time being almost exactly one hour ; 

 whereas the same epoch in the second harmonic wave of temperature is 

 earliest in October and latest in July, the difference in time being more 

 than two hours. And nearly the same rule holds up to altitudes of 

 15,000 feet, if not more. A second objection takes the form of a 

 query : Have we exhausted every other possible source having an 

 indubitable 12-hour period? There is, for example, the evaporation 

 of water from the surface of the earth, which, as I have tried to 

 explain before to-day,- and as you will also see from the harmonic 

 constants of vapour tension given above, shews strong barometric 

 affinities. Is the semi-diurnal curve of vapour tension over the land 

 due, as Bigelow says, to the displacement of the lower damp air by 



