174 Transactions of the South African Philosophical Society. 



also the month showing the greatest increase of velocity from the 

 first day to the second. But because the daily velocity of the wind 

 is greatest in the spring months it does not always follow that the 

 wind attains its greatest force at the same time. Stronger winds 

 are, in fact, occasionally experienced in July than in February. 

 Moreover, the average velocity between 2 p.m. and 3 p.m. in July, 

 on the second day, is 13J miles per hour ; the highest average 

 velocity in February is less than 12 J miles per hour, between 5 p.m. 

 and 6 p.m., on the second day. The greater daily velocity attained 

 in the spring and summer months is simply due to the fact that 

 during the winter the wind, however hard it may blow during the 

 day, generally falls at sunset, whereas in the warmer months the 

 high speed is maintained far into the night. 



The diurnal variation in the velocity of the wind is as evident 

 during the passage of a depression as it is at other times, although 

 the shape of the curve is considerably modified. In general the 

 average velocity changes from 5*0 miles an hour just after midnight 

 to 4-4 miles an hour just before sunrise, and to 7*8 miles just after 

 noon. During a depression the maximum comes rather later on 

 all three days, the corresponding velocities changing according to 

 Table 10 :— 





Midnight. 





Before 

 Sunrise. 





After 

 Noon. 



First day, from 



4-4 



to 



4-2 



to 



9-0 



Second day, from 



4-4 



to 



4-4 



to 



13-1 



Third day, from 



6-3 



to 



5-7 



to 



8-0 



We distinguish, then, in this way between general and cyclonic 

 winds at Kimberley, so far as the class we are considering is 

 concerned — that in front of the trough the range of velocity is 

 greater, being lighter by night and stronger by day, while in the 

 rear the range is smaller. In fact, the velocity at the midnight 

 following the passage of the trough is half as great again as that 

 twenty-four hours earlier. Some of the variations in question may 

 be due in part to the fact that the winds observed are, to some 

 extent, resultants compounded of normal winds and winds proper 

 to the cyclone. Thus the normal wind between sunrise and noon is 

 from some northerly direction ; also the cyclonic wind in front of 

 the cyclonic depression is likely, of itself, to be from some similar 

 azimuth. We should expect here, therefore, an accelerated 

 velocity. But the cyclonic wind in the rear of the depression is 

 likely, of itself, to be from some southerly direction, and we should, 

 therefore, expect a retarded velocity. Again the normal wind after 

 sunset is from south-west to south, and this is also the likely 



