144 Transactions oj the South African Philosophical Society. 



could it come about that, e.g., at Batavia, where the diurnal range of 

 temperature is 5° 9' C, that of pressure is 2'7 mm., whereas at Vienna, 

 where the temperature range is 8°, that of pressure is only about 

 9 mm. We had better deal with the action of the sun on the upper 

 strata of the atmosphere." Buchan goes a little further, ascribing 

 the generation of the barometric tides directly to the solar and 

 terrestrial radiations of the regions where they occur. " The baro- 

 metric oscillations," he adds, "are independent of any change of 

 temperature of the floor on which the atmosphere rests." 



In Tables 17 and 18 will be found the hourly pressures, and the 

 hourly changes of pressure, month by month, under clear skies. 

 The first point to be noticed is the secular increase of pressure, 

 amounting at the end of a day to '006 inch. It is curious that a 

 clear day should force up both pressure and temperature. The 

 different phases, with the exception of the morning minimum, come 

 a little later, although the difference does not run into many minutes. 

 The greater average pressure under clear skies is mainly due to the 

 high barometer characterising anticyclone weather. It is important 

 to notice, however, that the pressure during the first quarter of the 

 year falls to less than the mean when the sky is clear, just as the 

 collateral temperature rises. Upon referring to Table 19 it will be 

 found that the pressure increases relatively to the true mean without 

 a break from midnight to XVL, differing in this respect from the 

 corresponding temperature, which only begins to increase from V. 

 It follows of course that the pressures and temperatures of the same 

 hours, on cloudy days, must fall. 



Table 20 contains the hourly pressures under cloudy skies. The 

 numbers in the difference column are all minus, and therefore appear 

 in ordinary type. 



The differences in the ranges of pressure on clear, and on cloudy 

 days, are of interest. They are : — 



1. From midnight to first minimum : — 



Observed. Reduced. 



Inch. Inch. 



Clear skies -006 -006 



Cloudy skies -014 



2, From first minimum to first maximum : — 



Clear skies -051 -049 



Cloudy skies -044 



