152 MR JOHN AITKEN ON 



It seems probable that the small cyclones often seen in nature are produced under 

 similar conditions to those above described. These small natural cyclones are often 

 seen on dusty roads whilst the sun is shining, the whirling column having a core of 

 dusty air, and the centre of motion travelling along the road, tossing up the dust and 

 other light objects as it moves along. On one occasion 1 had the opportunity of 

 seeing one of these small cyclones in a large park in early spring. The wind, which' 

 was very light, was cold and from the north, but the sun was very hot. The move- 

 ments of the cyclone were easily seen as it tossed up the dry leaves in its path, 

 whirling them up to a height of four or five feet and scattering them all round. The 

 path of the cyclone was due south — that is, in the direction of the wind — the distance 

 travelled being more than a hundred yards. As there were a number of large trees in 

 the park, it was easy to see that the necessary conditions were present for starting and 

 carrying on the cyclonic movement. It seems probable that these small cyclones are 

 of more frequent occurrence than we are aware of, but owing to the absence of light 

 bodies — such as dust, leaves, etc. — there is nothing to indicate their presence. 



[Additional Note. Received February 19, and read March 4, 1901.] 



If both the direction of revolution and the direction of movement of cylones as a 

 whole are determined by the anticyclones, it may be asked — Why do cyclones over our 

 area almost always move in an easterly or north-easterly direction, and so seldom in 

 other directions ? and why do these north-easterly moving cyclones generally travel 

 so much quicker than those moving in other directions ? The reason for so many 

 cyclones moving in a north-easterly direction would appear to be due to there being 

 generally a high-pressure area to the south of us, and accompanied with this, there is 

 generally hot moist air coming from the Atlantic and blowing to the north of the high- 

 pressure area, and the cyclones form in this hot moist air and travel in the direction of 

 the winds blowing out of the anticyclone lying to the south. That is, the frequency of 

 these north-easterly moving cyclones is due to the conditions being more generally 

 favourable for the formation of cyclones on the north-west side of the anticyclones, 

 than on any of the other sides. 



Turning now to the question of why these easterly moving cyclones should travel, 

 as a rule, so much more quickly than those moving in other directions. It has been 

 pointed out that the cyclone is formed in the low-pressure area between two anti- 

 cyclones, and that it moves in the direction in which it receives the strongest 

 tangential current. If a cyclone were situated between two equally strong anticyclones, 

 it would receive equally strong winds on opposite sides, and it would not move, as it 

 would receive equal amounts of air from both anticyclones, and as these equal amounts 



