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MR JOHN AITKEN ON 



drive all the moist hot air to its circumference, keeping it near the ground. Suppose, 

 now, that the hot air begins to form a cyclone at the outside of the anticyclone, as 

 shown in fig. 6, it will be evident it has a supply of hot air collected for it by the 

 anticyclone. But not only has the anticyclone collected the material for making 

 the cyclone, but it also supplies the cyclone with the tangential force necessary for 

 producing the spiral circulation so well known in cyclones. 



The explanation generally given of the spiral movement in cyclones is, that the 

 air drawn in from the south has a greater amount of eastward motion than the centre 

 of the cyclone, whilst the air from the north has less, and that these oppositely moving 

 airs from the two directions cause the whole system to circulate round the centre. 

 This, no doubt, is one cause of the circular movement in cyclones, but it is far from 



^"-^N 



/ ^ 



Fig. 5. 



Fig. 6. 



being the only one. This tangential action — due to the earth's rotation — only comes 

 into play after the air has travelled a considerable distance towards the centre, whereas 

 the air from the anticyclone already has a considerable tangential movement over the 

 area between the cyclone and the anticyclone. The true explanation would rather 

 appear to be that the cyclone and the anticyclone form one system ; the anticyclone 

 forces its air tangentially into the cyclone, whilst the cj^clone draws air tangentially 

 from the anticyclone. The earth's rotation is the original source of the rotatory 

 movements, and starts the machine, but both intensify the initial motion ; they are, so 

 to speak, geared into each other and kept in motion by the earth's rotation, but both 

 capable, when conditions are favourable, of developing energy on their own account and 

 increasing their rate of rotation. 



We saw in the experimental part that a cyclone in the conditions represented in 

 fig. 6 — that is, with strong winds blowing tangentially on one side — cannot remain in 

 the same place, but must move in the direction of the strongest winds. It would be 

 fatal for a cyclone to remain long in one place ; it would soon use up all the supplies, 

 and when all the hot moist air had been drawn in, it would be starved and get weaker, 



