144 A YEAR IN SCIENCE 



hear the thunder some time after we have seen the 

 lightning. 



Tornadoes. Like thunderstorms, tornadoes occur in 

 hot weather and generally in a cyclone. They differ 

 from a cyclone because the atmospheric pressure at 

 the center is very low, and the area of low pressure 

 is very small. In some tornadoes the atmospheric 

 pressure has been reduced as much as one-half. As 

 a result, the winds are violent and very destructive. 

 It is probable that in some instances the velocity of 

 the wind is as great as 500 miles an hour. Fortu- 

 nately tornadoes do not occur frequently, and the path 

 over which they travel is very narrow. In tornadoes 

 of recent years in the United States many lives have 

 been lost and much property destroyed. In St. Louis 

 in 1896 the property loss was estimated at $13,000,000, 

 and in Louisville in 1890 at $2,500,000. Tornadoes at 

 sea are called water spouts. 



Effect of winds on rainfall. We can scarcely over- 

 estimate the importance of rainfall to all living things. 

 We know that much of our own western country has 

 remained uninhabited because the annual rainfall is 

 not sufficient to enable plants to grow, or because 

 it does not fall during the growing period of plants. 

 We do not have any extensive deserts in the United 

 States, yet about one-third of the land is too arid to 

 grow crops successfully. By means of irrigation we 

 are succeeding in watering artificially millions oft 

 acres of land. Because of the fertility of the soil and 



