RAINFALL AND ITS MEASUREMENT 



231 



toward the earth. In the center of the whirl the air pres- 

 sure is much diminished and the velocity of the inrushing 

 whirling wind is tremendous, being often sufficient to de- 

 molish all obstacles 

 in its path. 



The length of the 

 path swept over by 

 a tornado is rarely 

 over thirty or forty 

 miles and the width 

 generally less than 

 a quarter of a mile. 

 The rate of progress 

 in the Mississippi 

 valley is from twenty 

 to fifty miles an 

 hour, usually in a 

 northeasterly direc- 

 tion. These storms 

 are often wrongly 

 called cyclones. 

 When storms of this 

 kind occur at sea, a 

 water column is formed in the funnel-shaped part of the 

 storm and they then receive the name of waterspouts. 



Rainfall and Its Measurement. Experiment 79. Place a 



dish with vertical sides in a large open space so that the rim is hori- 

 zontal and at a height of about one foot above the ground. Fasten 

 the dish so that it cannot be overturned by the wind. After a 

 rain, measure the water that has collected in the dish to the smallest 

 fraction of an inch possible. This will be the amount of rainfall 

 for this storm. 



A TORNADO 

 Notice the funnel-shaped cloud. 



