1886.] illustrating the value of Cape Point as a Warning Station. 207 



.atmospheric j07'e55^/re is least at the centre, but the air is not the most 

 rarilied there. 



One of the laws of atmospheric equilibrium is, that dense air 

 always flows towards that which is most rarified and as the air in the 

 advance half of the cyclone is more rarified than that at the centre, it 

 is easy to see how this centre moves from west to east, and as the 

 whole system of winds travels with the centre, the front will always 

 draw the centre onwards. 



When a hot stream of water is encountered the cyclone travels along 

 it, because the equatorial air current is more heated there than 

 elsewhere. The Mozambique current is one of this sort, our winter 

 storms generally travelling along it. 



But there is another peculiarity as being the result of the opposite 

 winds which affect stations of medium height, and isolated like Cape 

 Point, which will best be described by an example. 



Let us once more return to our cyclone which has been patiently 

 waiting over the South Atlantic Ocean. No, not patiently waiting 

 for while we have been considering its physical conditions the 

 advance winds have reached Cape Point. 



Simultaneous readings are taken at the various stations, reduced to 

 32° and to sea level, and lo ! Cape Point reads too high by about a 

 half a tenth of an inch. 



Why is this ? The reading was correct and so was the reduction. 

 Then why this discrepancy ? 



If you consider for a moment you will see that such must be the case. 

 Assume the atmosphere greatly rarified, with a sea pressure of 30 

 inches, then the height of the air column must have been considerably 

 increased, and a greater amount of air must exist above Cape Point 

 -than would be the case if the air were more dense, but as the reduction 

 additions to both are the same, providing of course that the tempera- 

 tures are the same, this will make the reduced reading for rarified air 

 higher than it would be if the air were dense. The converse of all 

 this will be the case on the other side of the cyclone. 



Let us now retrogress a little and imagine our cyclone just outside 

 the colony and advancing on to it. Owing to the wedge-shaped form 

 of the advance disturbed portion, isolated peaks will be the first to 

 feel it. Cape Point being such a station we will imagine ourselves 

 there and follow the sequence of the weather. A low pressure lies 

 over the centre of the Colony, the wind at Cape Point is consequently 

 south-east. Below, at sea level, it is south. 



