1887.] Storms of South Africa. 237 



disagree with me. They will point out numerous winds blowing, 

 where no storm is, as they assume, notably the trade winds. But i 

 will answer them, that no wind whatever can blow without a distur- 

 bance of equilibrium, and that the trade winds blow constantly in 

 one direction because of the existence of a permanent storm along 

 the equator, the so-called belt of calms being in fact the line of 

 constant disturbance. 



To understand the nature of storms correctly we must study the 

 distribution of the atmospheric pressure generally over the whole 

 earth. First, let us assume the earth at rest and no disturbing 

 influence whatever at work ; what state would the atmosphere be 

 in ? The whole atmosphere would be in a state of perfect calm. 

 Now, let us assume the sun to shine vertically over one spot on the 

 earth, what would ensue ? In this case the air would rise perpen- 

 dicularly at that one spot, and a lower current would set in towards 

 it, while a higher one would flow from it, for a limited distance 

 around that spot. And now let us assume that the earth turns on 

 its axis, the sun being vertical over the equator. Here the sun's 

 effect would travel completely round the earth, the result being a 

 belt of rising air, with a lower indraught and upper overflow for a 

 limited distance on either side. We will confine ourselves entirely 

 to the atmospheric movements in the southern hemisphere. Before 

 the upper current had reached the surface of the earth, a great 

 quantity of air would have been drawn from the south towards the: 

 equator, the indraw extending almost to the pole. Consequentljr 

 there would then be a vacancy to fill, and the upper current on 

 reaching the ground would continue its course towards the pole, 

 to fill this vacancy, till meeting the cold banks of high pressure 

 round the pole, it would momentarily force it back ; but as the polar 

 pressure increased, this current would in its turn drive the equatorial 

 one back towards the equator, until having penetrated further than 

 necessary to restore equilibrium, a return oscillation would take place 

 and the polar current recede. 



We thus see that the atmosphere would be divided into three, 

 low pressure belts and four high pressure ones, that is to say, — at 

 low pressure belt round the earth at the equator ; two high pressure 

 ones over the north and south tropics ; and two low pressure ones in the 

 temperate zones, while two high pressure areas would rest over the 

 poles. Every outburst of solar energy would deepen the equatorial, 

 belt, and cause the air to be drawn in greater quantities from eithec- 



H2 



