176 Transactions of the South African Philosophical Society. 



continuously, and continues so to do until sunrise of the third day. 

 It then backs by pretty well a right angle until 2 p.m., after which 

 it approximates to a normal position. 



The intensity and angular velocity of the resultant are displayed 

 graphically in Fig. 1. In this diagram O is the origin of co-ordinates ; 

 and the resultant wind at any time is supposed to move from a given 

 point on the curve towards O. Thus the line joining XX to O will 

 represent the position and magnitude of the resultant wind direction 

 (exclusive of velocity) between 7 p.m. and 8 p.m. It is pretty clear 

 that the backing indicated by Table 12 between 4 p.m. and midnight 

 of the first day, and between 6 a.m. and 3 p.m. of the third day, is 

 only apparent, and that the extremity of the resultant steadily veers 

 hour after hour. There are one or two minor irregularities which 

 would most likely be smoothed out in a longer series. The whole 

 portion of the curve, however, of the day before lies in a loop outside 

 the origin to the north, and that of the day after is also wholly 

 outside to the south-south-west. In the normal curve of Kimberley 

 wind the veering, both in tabular and diagrammatic form, is complete, 

 the origin being inside the curve. The orientations of the major 

 axis of the normal curve, and of those in our diagram, are pretty 

 nearly the same. Hence it seems that one of the most important, 

 variations introduced by the passage of a depression is to transfer 

 the normal resultant-direction wind curve bodily to the north of the 

 origin as the depression approaches, and to the south as the depres- 

 sion recedes. The vane tends strongly to the north-west of the 

 normal direction during the morning of the day before, and still 

 further westerly during the morning of the day of lowest pressure ;. 

 it also tends strongly south-west of its normal position during the 

 afternoon of the day after. At 8.30 a.m., e.g., the normal resultant, 

 direction makes an angle of 62 degrees with the east and west line, 

 whereas 31 hours before the passage of the centre it is 69 degrees, 

 and seven hours before it is 100 degrees. An effect of this is that 

 pronounced easterly directions are eliminated in cyclonic weather, 

 although they are not, as it happens, replaced by very decided 

 westerly directions. 



In 1894-5 I made some comparisons between the barometric 

 pressures of Kimberley, Durban, and Cape Town (as published 

 casually in the Gape Times). It seemed from this that, on the 

 whole, low pressures were experienced during the same day at 

 Durban and Kimberley,* but from 20 to 40 hours earlier at Cape 



* The annual depression of the middle of July is experienced, on the whole, a 

 day earlier at Durban than at Kimberley. See " Some Pressure and Temperature 

 Kesults," &c, in Trans. S. A. Phil. Soc, vol. xi., pt. 4. 



