36 Introduction to the Study of Science 



The tornado resembles a gigantic funnel, the small end of 

 which dips from time to time to the earth, cutting a swath 

 through trees, buildings, or whatever may be in its path, while 

 the larger end is spread out with masses of black cloud. The 

 velocity of the rotary motion of the lower end of the tornado 

 can be estimated only from its effects. But it seems probable 

 that it reaches a velocity of four hundred to six hundred miles 

 an hour. 



The very low pressure within the central part of the whirl or 

 tornado is the important fact in understanding the explosion 

 of closed houses or closed rooms within houses. The air pres- 

 sure may be reduced one third of the normal. Calculate 

 the difference between the suddenly reduced pressure on the 

 outside of a closed house and the normal pressure retained 

 within the house as 4.9 pounds per square inch. The tre- 

 mendous difference in pressure is easily seen to be more than 

 enough to account for the explosion. Buildings which are 

 open may be wrecked, but not by exploding. 



The fact is important in explaining the large breakage of 

 plate glass windows in New York City and other cities within 

 a radius of many miles of the dynamite explosion on July 31, 

 1916, on Black Tom Island. The sudden decrease in pressure 

 on one side of the glass was so great that the glass could not 

 withstand the normal pressure on the other side. In this case 

 the windows fell outwards, or the air in the building expanded 

 and found escape where the resistance was least. 



A heavy fall of hail accompanies tornadoes. The lightning 

 and thunder are often terrible. Where tornadoes occur it is 

 of practical importance that every one know the signs of the 

 approach of such storms, and realize that safety lies to the 

 northwest or southeast of their path, because tornadoes almost 

 invariably move toward the northeast. The rate of progress 

 is usually about forty miles an hour, but it may vary from 

 fifteen to one hundred miles an hour. 



Cloudbursts and waterspouts. Tornadoes with their swiftly 



