103 



2. They only occur during 1 the warmest part of the year and 

 day, and in calm weather. 



8. They are due to the unstable equilibrium of the air, the hot 

 lower portion acquiring a high state of tension by being unable to 

 rise according to the laws of heat, being prevented by some rapidly 

 moving current of cold air. 



4. They are produced by the hot air obtaining exit through a 

 narrow vent forced through the upper current. 



5. Their locomotion is produced by the vent and its cross air 

 current being carried along with the cold current. 



6. Their spiral motion is produced by the shifting of the centre of 

 attraction (the vortex) of the hot air particles and their infringement 

 upon each other, and the unaffected volume of the adjacent air, assisted 

 by friction. In fact they are produced by the same causes, only inversely, 

 as the rotating motions of fluids escaping through a narrow vent below 

 them. 



7. They are terminated by the closing of the vent by the onward 

 pressure of the cold current. 



8. The retention of light articles in the upper regions (even, 

 perhaps, their ascension) may be greatly facilitated by electricity. 



9. Stable equilibrium is produced by the escape of the over- 

 heated air, and its replacement by cold air derived from the upper 

 current or the S.W. surface wind. 



