1887.] 



Storms of South Africa. 



241 



E is the equatorial current and D the polar one. When the two 

 currents come into contact with one another, as E is a rising wind 

 and D a falling one, the rising one will naturally flow up and over 

 the falling one, the line of meeting being shewn at A B, Both 

 currents flow in almost parallel lines at different heights, one having 

 a downward and the other an upward tendency, and wherever either 

 current meets the line A B, the particles of air are carried upwards 

 towards B. At F they reach a point of equilibrium, but being forced 

 upwards by the particles below them, they flow outwards on either 

 side and compensate for the loss sustained by the lower currents. 



Now it is evident that if there is an increase of atmosphere at N 

 it will press unduly on D and the whole system will move towards 

 the south. A vacancy will be caused at A, by the upward current 

 from A to B, and the surrounding surface winds will rush in to 

 compensate for the loss, the general direction of the winds being 

 as shewn on Fig. 2. 



■i- 



(Fig. 2.) 



Suppose the initial banking up of E (Fig. 1), which caused the 

 •motion, to cease ; as the motion is now towards the south, a point 

 would be ultimately reached where the weights at D and N would 

 be balanced. 



But the depression will not stop here. Dynamic force has to be 

 overcome, and so the disturbance will still travel to the south, till 

 the increased banking up of D will not allow of its further motion, 

 and it will consequently come to a standstill. 



But the greatest weight of air being now at D, the whole system 

 will be forced back and be pushed against E in a similar manner, to 

 be in its turn beaten back to D, and so the oscillation will be kept 

 up till equilibrium is restored. 



Sometimes the two currents will bank up simultaneously. The 

 depression will then be motionless, but the winds will be very strong 



