SCIENCE OF COMMON THINGS. 83 



Waterspouts. Why winds feel cool. What are clouds? 



535 How are the whirlwinds which we frequently see at the corners of 

 streets in cities produced f 



They are caused by a gust of wind sweeping round 

 a corner of a building, and striking the calm air be- 

 yond it. 



530 What is a waterspout f 



A waterspout is a whirlwind over the surface of 

 water, and differs from a whirlwind on land in the fact 

 that water is subjected to the action of the wind, instead 

 of objects on the surface of the earth. 



530 Why does wind generally feel cold f 



Because a constantly-changing surface comes in con- 

 tact with our body to draw off its heat. 



531 What are the effects of wind noticed in the Arctic regions f 



Arctic explorers inform us that in those regions, when 

 the thermometer ranges from 40 to 60 below zero, 

 the cold of the external air is easily endurable, provided 

 the air is calm and the individual exercises freely / but 

 if a wind arises at this temperature, the severity of the 

 cold becomes too great for human endurance. 



533 If the winds should cease to blow over the ocean, what would be the 

 e/ectf 



The water would undoubtedly become stagnant. 

 Tempests and hurricanes also exercise a beneficial effect 

 by agitating and purifying the atmosphere, and sweep- 

 ing from it the seeds of pestilence and contagion. 



533 What are clouds f 



Moisture evaporated from the earth^ and again par- 

 tially condensed in the upper regions of the air. 



534 What is the difference between a fog and a cloud ? 



Clouds and fogs differ only in one respect. Clouds 

 are elevated above our heads, but fogs come in contact 

 with the surface of the earth. 



635 Why are clouds higher on a fine day f 



Because they are lighter and more buoyant. 



536 Why are clouds lighter on a fine day f 



1. Because the vapor of the clouds is less condensed; 

 and 



