146 Transactions of the South African Philosophical Society. 



through the subsolar point will divide the earth into two hemispheres 

 in which that lying to the west of the subsolar point {i.e., that rising 

 to meet the sun) has the maximum pressure ; and that lying to the 

 east (i.e., that falling away from the sun) the minimum. The 

 position of the great circle varies, apparently, through the months, 

 probably coming earliest in October and latest in February. It 

 cannot be quite accurately located in any month, because the period 

 of four years, with which this discussion deals, is not long enough 

 to eliminate the effects of occasional perturbations, and also because 

 in no month can the true normal curve of pressure be re-entering. 



If we adopt the same process for the clear skies, we shall find the 

 mean pressure for the daylight hours to be 261613 inches, and for 

 the dark hours 26*1542 inches. The daylight pole of the great circle 

 which divides the Kimberley belt of latitude into two halves through- 

 out which the average pressure is the same, will now be found to 

 have shifted from the subsolar point through nearly an hour and a 

 half of time in the direction of the hottest meridian. It follows as a 

 matter of course that the same pole will shift in the opposite 

 direction under clouded skies, and that then the dark hours will have 

 the higher average pressure. Now, as we have seen in the Tables 

 of Temperature, the time of culmination of the heat-meridian 

 depends upon the amount of cloud. Thus the rule is for the pole 

 of symmetrical pressure to precede the heat meridian by some 

 twenty degrees of arc. 



We have seen that the range of pressure at Kimberley, from the 

 morning maximum to the afternoon minimum, is somewhat less 

 under clear than it is under cloudy skies. This is an exception to 

 the rule laid down by Buchan ; for "over the land," says this 

 authority, *' the amplitude of the oscillation from the morning maxi- 

 mum to the afternoon minimum is greatest when the atmosphere is 

 driest and the sky clearest, and least where the atmosphere is highly 

 saturated and the sky more frequently and densely covered with 

 clouds, being thus, generally, the reverse of what is observed to take 

 place over the open sea. ... At Bombay, in April, during the dry 

 atmosphere and clear skies of the north-east monsoon, the oscillation 

 is '118 inch ; but in July, during the humid atmosphere and clouded 

 skies of the south-east monsoon, it falls to -067 inch." '■'' It may be 

 doubted, though, whether one month may be legitimately compared 

 with another in this way, and especially in India, where the con- 



* Ency. Brit., IX. Ed., Art. " Meteorology." In another place the same author 

 remarks that " the Challenrier observations all show that over the ocean, latitude 

 for latitude, the amplitude of the oscillations is larger in an atmosphere highly 

 charged with aqueous vapour, and less in a dry atmosphere." 



