THE ATMOSPHERE 



127 



upward away from the earth's surface as an ascending air current, 

 thus ceasing to be a wind. And somewhere off in the general direction 

 from which the wind comes we may suppose that there is a region 

 where cool, dry, and therefore dense air is settling down from the upper 

 atmosphere. 



In tornadoes not only is there an uprush of air having great velocity, 

 but at the same -time this ascending air may have a far swifter whirling 

 motion that causes it to rise with a spiral movement, and to have a 

 twisting effect on objects. 



FIG. 48. The effects of a tornado. 



The central portions of the funnel of the tornado may be considered 

 as more or less a vacuum. This results from the whirling motion 

 which throws the air outward away from the centre of the rising column. 



Buildings more or less tightly closed, and caught in the centre of 

 the track of a tornado, are sometimes found with their walls pushed 

 outward by the air which was confined within them. The air outside 

 them must have been almost instantly and completely withdrawn 

 upward and away from them. 



